As well as orbiting the Sun, the Earth spins on its own axis, presenting a continuously
changing face to the Sun, hence day and night.
The Earth's axis is inclined, or tilted, at an angle of 66.5 degrees to the orbital plane.
The inclination is sometimes stated as 23.5 degrees to the normal to the Earth's orbital
plane, that is, 90 degrees minus 66.5 degrees equals 23.5 degrees.
The inclination of the Earth's axis is the cause of the seasons and of the changing time
interval between sunrise and sunset throughout the year.
We specify the position of a point on the surface of the Earth using the latitude and
longitude system. An imaginary graticule of lines covers the Earth's surface. The north-south
lines join the north and south poles and are called meridians of longitude.
The meridian passing through Greenwich is known as the prime meridian. The prime meridian
is the datum for defining longitude.
The east-west lines are parallel with the equator and are known as parallels of latitude.
The equator is the datum from which latitude north or south is measured.
A circle on the surface of the Earth whose centre and radius are those of the Earth itself
is called a great circle. It is called great because a disk cut through the Earth in the
plane of the great circle would have the largest area that can be achieved.
A circle on the surface of the Earth whose centre and radius are not those of the Earth
is called a small circle. The main small circles of relevance to position are the parallels
of latitude.
A rum line is a regularly curved line on the surface of the Earth which cuts all meridians
at the same angle, that is, a line of constant direction.
The Earth is divided into hemispheres, the most well known ones being the northern and
southern hemispheres which line north and south of the equator. Less well known are
the eastern and western hemispheres which lie east and west of the prime meridian.
If you fly an aircraft you must be able to navigate, otherwise you'll very quickly get
lost. Whatever type of aircraft you fly you'll use a map or a chart for navigation. You therefore
need to have some understanding of the properties of the map or chart that you are using.
The purpose of this lesson is to explain very briefly some of the general properties of
maps and charts. If you wish to delve into the subject more deeply there are many reference
books available. However, this brief look at the subject will be sufficient for your
PPL studies.
The difference between a map and a chart is essentially one of detail. When driving you
use road maps because you require the detail. However, when flying across the Atlantic you
will use a chart as there is not much detail to be seen in mid-Atlantic. Your IKO one in
five hundred thousand or half million chart is quite detailed, but it's not detailed enough
to be called a map. For example, when navigating you don't necessarily need to know the name
of every small town or the numbers of the roads. On the other hand, when navigating
visually, that is map reading, you might need to use a small town as a checkpoint and to
recognise that a main road runs through it in, say, a north-west to south-east direction.
Therefore the detail on the chart only needs to indicate the presence of a small town and
a major road.
The transfer of information from a globe or reduced earth onto a flat paper chart is achieved
by projection. Originally, the technique involved using a light source within the globe to project
the latitude-longitude graticule onto a flat sheet of paper. Today, the projection is achieved
mostly by using computer models. However, an understanding of the original projection
technique using light is more than adequate for understanding the properties of charts,
whatever actual method of production is used.
There are three general types of projection surfaces. An azimuthal or plane projection
is produced by placing a flat sheet of paper against a point on the reduced earth. A common
use is to provide charts of the north-south polar regions like this one shown here. The
earliest chart projections were produced in the 16th century by a Flemish navigator
called Gerhard Cramer, who used the Latin alias Mercator. His projections used cylinders
of paper wrapped around the reduced earth and touching at the equator. The projection
technique is illustrated here and is known as a cylindrical projection. Conical projections
involve placing a cone of paper over the reduced earth and projecting the graticule onto the
cone. Subsequently the cone is slit along one side when it can be opened to produce
a flat sheet of paper.
Having developed methods of projecting the spherical earth onto a flat sheet of paper
we should now consider what properties we require of the chart. The following is a list
of properties we might like on an ideal or perfect chart. The list is not comprehensive
and does not need to be remembered for your exam.
With regard to the representation of the earth's surface, areas should be represented with
their true shape on the chart and equal areas on the earth's surface should be shown as
equal areas on the chart. Clearly if the shape and area of features on a chart are not accurate
scaled-down versions of the same areas on the earth's surface then the chart will be
of little use for navigation. Similarly angles on the earth's surface should be represented
by the same angles on the chart and scale should be constant and correct. Navigation
would be very difficult if the scale varied as you moved across the chart.
More specific requirements for navigation are as follows. Rum lines should be straight
lines. Great circles should be straight lines. Latitudes, longitudes should be easy to plot.
Adjacent sheets should fit correctly. Coverage should be world-wide. Remember that these
are ideal properties and, like all ideals, they are difficult or impossible to achieve
as we shall now see.
Two of these properties can never be obtained except on the globe. Scale can never be constant
and correct. We can modify charts mathematically to give nearly constant scale in small areas
but not over large areas. The shapes of large areas cannot be represented perfectly. However,
it is possible to represent the shapes of small areas reasonably accurately. All the
other ideal properties can be obtained on charts but unfortunately not together on the
same chart.
Of all the properties just listed the most critical are navigation bearings must be correct.
Angles on the earth must be represented correctly on the chart. This last property is known
as orthomorphism, literally meaning the same shape, or conformality. Charts which have
this property are known as orthomorphic or conformal charts. The next lesson looks in
more detail at the IKO 1 in 500,000 chart which is a conformal orthomorphic projection.
The IKO 1,500,000 chart is a Lambert conformal conic projection. This fact is stated just above
the scale graticules at the bottom of the chart. It is important that you have a basic
understanding of the properties of the 1,500,000 chart as you will use it for most of the time
during your flying in the United Kingdom. This lesson will therefore briefly cover the
following topics.
Construction of the chart covering the projection of the chart, parallel of origin, standard
parallel. We'll then look at the main properties covering meridians, parallels, great circles
and rum lines, convergence of meridians, scale, depiction of height, and finally we'll look
at the various ways in which elevation and height are depicted on the 1,500,000 chart.
As we saw in the previous lesson it is possible to project the graticule of the earth on the
inside of the surface of a cone. This process gives the conical family of projections from
which Lambert's is derived. A cone is placed over a reduced earth in such a way that the
cone is tangential with the reduced earth along a parallel of latitude. The apex of
the cone will lie on the extended line of the earth's axis as shown here. A light source
at the centre of the reduced earth casts shadows of the graticule on the inside surface of
the cone. These shadows are then marked in, the cone removed, cut down its slant side
and rolled out flat to give a simple conical projection as illustrated here. On the simple
conical projection scale is correct on the parallel of tangency, in this example 45°N.
Lambert's conical orthomorphic projection is based on this simple conical projection
and is produced mathematically from it. Shown here is part of a chart which has been produced
to cover a small band of latitude. Firstly the scale is reduced all over the chart. This
mathematical reduction results in two parallels on which scale is correct, one on either side
of the original standard parallel. Then further mathematical modification is applied to produce
an orthomorphic projection. In other words the chart is modified to ensure it represents
accurately the relevant area of the earth's surface. As I have already said this new projection
has two standard parallels on which the scale is correct. Outside the standard parallels
scale is greater, more expanded, than it is on the standard parallels whilst between them
scale is less, contracted, being least of all on the parallel of origin.
As we have just seen scale is least on the parallel of origin. It expands away from the
parallel of origin until it is correct on the standard parallels. Scale is greatest
on the top and bottom parallels of the projection. The chart is orthomorphic by mathematical
construction. In other words, bearings and angles are correct. Meridians are straight
lines radiating from the pole. Parallels of latitude are arcs of circles, all of which
are centred at the pole. Except for the meridians, which appear as straight lines, great circles
appear as curves concave to the parallel of origin. Rum lines are curves concave to the
pole of the projection. That is, they intersect meridians at the same angle. This is the parallel
of latitude on which the projection is based mathematically. For practical purposes it
may be assumed to be halfway between the two standard parallels. This is the angle of inclination
between two meridians on the chart, measured at a given latitude. It is constant on a Lambert's
chart. To conclude this lesson we'll look at the various ways in which elevation, or
altitude and height, are depicted on the half mil chart. You may find it useful to have
a half mil chart with you to refer to as we go through this section.
As is usual on many maps and charts, contours are used to show ground relief. Note that
contours depict elevation, not height, and are shown at 500 foot intervals. To make it
easier for you to see changes in ground relief when map reading, you'll see that coloured
tints are used for the various bands between contours. The legend also states the position
of the highest known elevation, or altitude, on the chart. In this case it's Mount Snowdon
in North Wales. Other elevations showing high ground are referred to as spot elevations.
Note that no contours or elevation tints are shown below 500 feet. Therefore all the white
parts of the chart are below 500 feet above mean sea level. Finally you should note that
the green areas on the chart indicate wooded areas and have nothing to do with contours.
You'll see that the chart is divided into quadrangles bounded by graticule lines every
half degree of latitude and longitude. Maximum elevation figures are shown in blue within
each quadrangle in thousands and hundreds of feet. The one shown here is 1300 feet.
Each maximum elevation figure is based on information available concerning the highest
known feature in each quadrangle, including terrain and obstacles, and allowing for unknown
features. However, you should note that a maximum elevation figure is not a safety altitude.
An air navigation obstacle is a radio or TV mast, a high chimney or any other high
obstacle other than high ground. All such obstacles that are more than 300 feet high
above ground level are shown. Each obstacle has two figures beside it. The figure in italics
shows the elevation of the top of the obstacle. The figure in brackets shows the height of
the top. Regulated airspace refers to all airspace in which the movement of aircraft
is regulated in some way. Examples are airways, control zones, restricted, prohibited and
danger areas. The vertical limits of all regulated airspace on the chart are shown either as
altitudes or flight levels. The following examples will give you an idea of the way
in which vertical limits are indicated. The Linum control zone has vertical limits from
the surface to an altitude of 3500 feet, whereas the Linum control area has limits of 3500
feet to flight level 6.5. Airway Gulf 1 has a base of flight level 7.5. Restricted area
R105 has an upper limit of 2000 feet altitude. The caption C Note 2 refers to notes at the
bottom of the chart giving additional information on certain areas of regulated airspace. If
you have a chart, see if you can find Note 2. The Earth has two magnetic poles which
are not co-located with the geographic poles and have a very small annual movement within
the Earth. If you imagine a giant bar magnet through the middle of the Earth, it would
lie at an angle to the geographic poles as shown here. You can also see that the lines
of force of the Earth's magnetic field run from south to north. Aircraft compasses are
designed to sense the horizontal component of the Earth's magnetic field. This diagram
shows that as the magnetic poles are below the surface and an aircraft is on or above
the surface, the direction of the Earth's magnetic field T at any point may be resolved
into its vertical Z and horizontal H components. The angle between the H and T is known as
the angle of dip. The magnet in an aircraft compass system is hung pendulously so that
the weight of the magnet offsets most of the effect of dip. It is this pendulous arrangement
that causes the well-known turning and acceleration errors which will be covered in the lesson
on aircraft magnetism.
Magnetic north is the direction indicated by a freely suspended magnetic compass influenced
only by the Earth's magnetic field. True or geographic north is shown by the north-south
lines of the graticule on a map or chart. Since aircraft compasses sense magnetic north
but maps and charts are drawn with graticules indicating true north, in order to navigate
successfully we need to be able to determine the difference between true and magnetic north
at any point on the Earth's surface. The difference is called variation and it is the angle between
true north and magnetic north. It is measured in degrees east or west from true north. Because
the magnetic north pole is offset from the geographic north pole, variation will change
as we move across the Earth's surface. If we are in a position where the magnetic and
geographic north poles are directly in line, variation is nil, otherwise magnetic north
may lie east or west of true north.
The following diagrams illustrate the same point in a different and more practical way.
Note that the direction in which the fore and aft axis of the aircraft is pointing is
known as the aircraft's heading. The heading of the aircraft relative to true north is
105 degrees. The heading of the aircraft relative to magnetic north is 122 degrees. Magnetic
north is to the west of true north. Therefore, variation is 17 degrees west. In this diagram,
the heading of the aircraft relative to true north is 105 degrees. The heading of the aircraft
relative to magnetic north is 088 degrees. Magnetic north is to the east of true north.
Therefore, variation is 17 degrees east.
The following rule will help you remember how to convert magnetic direction to true
direction, and vice versa. Make sure you commit it to memory. Variation west, magnetic
best. Variation east, magnetic least. To illustrate this, we'll put the figures from the previous
examples into a table. In the first example, the aircraft true heading was 105 degrees.
The variation was 17 degrees west. Therefore, as variation is west, so magnetic is best.
The magnetic heading of the aircraft is 122 degrees. In the second example, the aircraft
true heading was 105 degrees. The variation was 17 degrees east. Therefore, as variation
is east, so magnetic is least. The magnetic heading of the aircraft is 088 degrees.
We can tell what the variation is at any point because it's shown on maps and charts normally
by dashed lines as shown here. These lines, which connect points of equal magnetic variation,
are called isogonals. On the half-mil topographical chart, an isogonal is printed for every half
degree of variation. The isogonal joining places of zero magnetic variation is known
as the agonic line. Any ferromagnetic material, such as iron or steel or electrical circuits
in an aircraft, will have a magnetic field which will affect the compass so that the
direction indicated by the compass needle will not be magnetic north. Non-ferromagnetic
material, for example brass or aluminium, will not have a magnetic field and so will
not affect the compass. The angle between magnetic north and the direction indicated
by a compass needle is called the angle of deviation and is measured in degrees east
or west from magnetic north. The angle of deviation is more commonly referred to as
compass deviation. Each aircraft will have different deviations on different magnetic
headings. These differences are recorded on a compass deviation card which is displayed
in a cockpit. An example is shown here. As with the section on variation, the following
diagrams illustrate deviation in a more practical way. The heading of the aircraft relative
to magnetic north is 1, 2, 5 degrees. The heading of the aircraft indicated by the compass
is 1, 3, 5 degrees. Therefore deviation is 10 west. In this diagram the heading of the
aircraft relative to magnetic north is 1, 2, 5 degrees. The heading of the aircraft
indicated by the compass is 1, 1, 5 degrees. Therefore deviation is 10 degrees east.
The following rule will help you to remember how to work out compass heading. As with the
variation rule make sure you commit it to memory. Deviation west, compass best. Deviation east,
compass least. As before we'll put the figures in a table. The heading of the aircraft relative
to magnetic north is 1, 2, 5 degrees. Deviation is 10 degrees west. Therefore as deviation
is west, compass is best. The compass heading is 1, 3, 5 degrees. In the second example
the heading of the aircraft relative to magnetic north is 1, 2, 5 degrees. Deviation is 10
degrees east. Therefore as deviation is east, compass is least. The compass heading is 1,
1, 5 degrees.
You
From your everyday experience you'll be aware that in the United Kingdom we use a mixture of
imperial and metric units to measure all sorts of things from weight to temperature. This somewhat
confusing situation applies to the measurement of distance in aviation and so you need to be aware
not only of the various units of distance that are in common use but also how to convert between
them. As pilots we're interested in the larger units of distance such as kilometres, statute
miles and nautical miles. This list shows the principal conversion factors which you will use
when converting between imperial and metric and vice versa. However all of the above conversions
can be made accurately enough for flight planning purposes using the navigation computer.
The proper definition of the kilometre is 1,000 metres when the metre is defined in terms of the
speed of light as the length of path travelled by light in a vacuum during a time interval of
1,300 millionth of a second. However this is not of much practical use for navigation. The simpler
navigation definition of the kilometre is 1,000 of the average distance on the earth between the
equator and either pole. Thus there are 10,000 kilometres between the equator and either pole
and the circumference of the earth is 40,000 kilometres. The statute mile was defined by
Royal Statute in the reign of Queen Elizabeth I and is 5,280 feet. Although the statute mile
is widely used on the ground both in the United Kingdom and America it has limited application
to aviation. The average distance from the equator to either pole is 6,200 statute miles
and the circumference of the earth is 24,800 statute miles. The nautical mile is the most
important large measure of distance used in aviation because it's related directly to the
angular measurements of the latitude and longitude graticule of the earth. The full definition of
the nautical mile is that length of arc of a great circle which subtends an angle of one minute at
the centre of curvature of the earth's surface as shown in this diagram. The IKO definition of the
nautical mile is that it is a distance of 1,852 metres. The standard nautical mile is defined as a
length of 6,080 feet. A complete explanation of why there are these differences is outside the scope of the
syllabus. Suffice to say that the compression of the earth results in slightly varying measurements
for the nautical mile. However, we use the standard nautical mile for air navigation purposes.
To put this in a practical context, a map or chart with a scale of 1 in a million means
that 1 unit
on the map equals 1 million units on the earth's surface. In other words, 1 inch on the map equates
to 1 million inches on the ground. The scale is normally printed with a colon as shown here. So,
for example, if you want to find out what distance on the ground is represented by 11 inches on a
map
with a scale of 1 in a million, you would calculate it like this. As we've already seen, 1 inch
on the
map represents 1 million inches on the ground. This is the same as 83,333.3 feet. We saw earlier
that
1 nautical mile equals 6,080 feet. So, 83,333.3 feet equals 13.7 nautical miles. Therefore, 11 inches
equals 150.8 nautical miles. In lesson 4 you learned that aircraft electrical services such
as radios produce their own magnetic fields which affect the aircraft compass. The combined effect
of these small magnetic fields is recorded when the aircraft compass is calibrated in a process
called swinging the compass, more commonly referred to as a compass swing. During a compass swing,
most of the aircraft's electrical services and sometimes the engines are running.
During a compass swing, compass readings are taken with the aircraft on various headings
and then compared with the readings on a very accurate magnetic compass, frequently mounted
on a tripod, which is located outside the aircraft away from the magnetic influence of aircraft systems.
The differences between these readings is a deviation of that particular aircraft compass
and is recorded on a compass deviation card located in the aircraft cockpit. An example is shown here.
Of course, the aircraft compass can be influenced by a wide variety of other objects in addition to aircraft systems.
Any object such as a headset or a metal clipboard or even a pen can influence a compass if it
is placed close to it.
It is therefore vital that such objects are kept well away from the compass as very large errors can be
induced
in a compass without you realising it. Many people have got lost because of this very simple error.
You can see here the effect of putting your headset next to the compass.
Direct reading compasses are subject to large errors during linear acceleration or deceleration or during a turn.
The following diagrams illustrate the effect on a compass of an acceleration on a heading of 270 degrees.
Because of the dip angle, the end of the magnet which is attracted to the nearest terrestrial magnetic pole hangs
down from the horizontal.
This means that the magnet's pivot point is no longer over the centre of gravity. Instead, it's nearer to the
nearer pole.
The acceleration acts on the pivot point. However, the reaction to the acceleration acts on the centre of gravity.
This produces a turning moment, causing the compass needle to rotate, even though the aircraft is not turning.
This causes an incorrect heading to be shown on the compass card.
The basic theory of turning errors is much the same as that for linear acceleration errors.
Turning errors are maximum when turning through north and south,
and zero when turning through east and west.
Turning errors are usually more significant than acceleration errors for the following reasons.
They are inherently of greater magnitude because greater displacement of the magnet assembly is likely in turns.
Turns occur more often and are likely to be more prolonged than linear accelerations.
Consider an aircraft executing a left-hand turn through the heading of north in the northern hemisphere.
Again, because of dip, the red end of the magnet hangs down from the horizontal, and so the pivot point
will no longer be over the centre of gravity.
Instead, it will be nearer the red end of the magnet. During the turn, the magnet will be dragged to
the left by the pivot point.
However, the inertia will react through the centre of gravity.
As these forces are not in line, there will be a couple, causing a turning effect, moving the red end
of the magnet to the left.
Although the aircraft has turned 90 degrees around the compass, the magnet has been rotated in the same direction by
a smaller amount, say 20 degrees.
The pilot therefore only sees 70 degrees pass beneath the lubber line, and the compass over-reads.
In order to stop on the correct heading, the pilot must stop turning when the compass reads 335 degrees, or
20 degrees early.
Now consider an aircraft executing a right-hand turn through the heading of north in the northern hemisphere.
Again, because of dip, the red end of the magnet hangs down from the horizontal, and as before, the pivot
point will be nearer the red end of the magnet.
During the turn, the magnet will be dragged to the right by the pivot point. However, the inertia will react
through the centre of gravity.
As these forces are not in line, there will be a couple, causing a turning effect, moving the red end
of the magnet to the right.
Although the aircraft has turned 90 degrees around the compass, the magnet has been rotated in the same direction by
a smaller amount, say 20 degrees.
The pilot therefore only sees 70 degrees pass beneath the lubber line, and the compass under-reads.
In order to stop on the correct heading, the pilot must stop turning when the compass reads 025 degrees, or
20 degrees early.
Now consider an aircraft turning right through south in the northern hemisphere.
Again, because of dip, the red end of the magnet hangs down from the horizontal, and as before, the pivot
point will be nearer the red end of the magnet.
During the turn, the magnet will be dragged to the right by the pivot point. However, the inertia will react
through the centre of gravity.
As these forces are not in line, there will be a couple, causing a turning effect, moving the red end
of the magnet to the right.
Although the aircraft has turned 90 degrees around the compass, this time the magnet has been rotated in the opposite
direction by a smaller amount, say 20 degrees.
The pilot therefore sees 110 degrees pass beneath the lubber line, and the compass over-reads.
In order to stop on the correct heading, the pilot must stop turning when the compass reads 0245 degrees, or
20 degrees late.
By now, you should be able to work out what happens when you turn from 225 degrees to 135 degrees
through south.
Click on continue when you have decided what compass heading you must stop the turn on.
You should roll out of the turn when the compass reads approximately 115 degrees, that is, you roll out 20
degrees late.
To conclude the lesson, this table summarises turning errors in the northern hemisphere.
If you ever fly in the southern hemisphere, you'll find that the errors are the exact opposite.
The term sluggish, which appears in the right-hand column of the table, denotes that the compass heading is lagging
behind the aircraft heading.
Conversely, when the term lively is used, the compass heading is leading the aircraft around the turn.
The following rules of thumb apply. During a turn through the north pole, which is physically nearer to the aircraft,
the compass will be sluggish.
It is therefore necessary to roll out early on the indications given by the direct reading compass.
During a turn through the pole which is physically further from the aircraft, the compass will be lively.
It is therefore necessary to roll out late on the indication given by the direct reading compass.
From the previous statements, it can be seen that at the magnetic equator there is no turning error because there
is no dip.
Remember that it is a displacement of the magnets in a clockwise direction when viewed from above, which causes the
compass to under-read.
A displacement in an anti-clockwise direction causes the compass to over-read.
There's a world of difference between learning how to navigate when sitting in front of a computer screen and learning
how to do it in real time in the air.
Nevertheless, you will acquire the skills required in the air a lot more quickly if you've mastered the basics on
the ground beforehand.
Although not essential for the lesson, you may find it helpful to have with you a half million topographical chart,
a navigation protractor and an air navigation ruler.
Whenever you fly you must be aware of your position at all times.
This is self-evidently of prime importance when you're navigating as you'll not be able to get from point A
to point B if you don't know where you are.
There are three principal symbols which are used to denote particular types of positions.
The air position is where the aircraft would be if there was no wind.
It's denoted by a cross, as shown here.
It's not normally used in visual pilot navigation.
The aircraft will always be blown by the wind to a ground position.
It's denoted by a small circle with a dot in the middle, as shown here.
We use this symbol when we pinpoint our position by reference to features on the ground.
This is known as getting a fix, and in this context fix means position.
There will be times, however, when it's not possible to get a positive fix from ground features.
On these occasions we would plot our dead reckoning position, or DR position.
It's denoted by a small triangle with a dot in the middle, as shown here.
We determine this by plotting our estimated track and distance flown since the last fix based on the known wind
and aircraft heading.
Latitude and longitude are used to identify positions on the map accurately.
The larger the scale, the more accurately any given position can be identified.
The latitude and longitude graticule is clearly marked on the map.
Parallels of latitude are marked in one degree intervals.
In the United Kingdom we're north of the equator, so the parallel of latitude shown is referred to as 52
degrees north, or just 52 north.
The latitude numbers are marked on both edges of the map.
And at intervals along each parallel.
Meridians of longitude are marked in a similar way.
Note that those indicated are east of the prime meridian, which runs through Greenwich in London.
So are referred to as one degree east and two degree east, or one east and two east.
Each degree of both latitude and longitude is divided into 60 minutes, with each 5 and 10 minute mark slightly
larger than the 1 minute marks.
Note also that the graticule divides the map into rectangles of 30 minutes, or half a degree of latitude and
longitude.
However, the minute marks are only shown on the graticule lines for whole degrees.
When we define a position using latitude and longitude, we give the latitude first, and then the longitude.
For example, the position of Clacton Aerodrome shown here is 51 degrees 47 minutes north, and 01 degrees 08 minutes
east.
The line between where you're going from and where you're going to, in other words your departure point and your
destination, is called the track, and each track is called a leg.
When planning a navigation flight, you'll draw your track or tracks on your map, measure the direction of each one,
and then enter the number on your flight plan.
For example, let's say that you wish to fly a cross country flight from Oxford Aerodrome to Wellsbourne Mountford Aerodrome
via Ledbury.
To measure the tracks, you use a navigation protractor shown here. The protractor has a graticule of lines on it,
which allow you to align it accurately on the map.
You now place the centre of the protractor over the approximate midpoint of the track from Oxford to Ledbury, and
align the graticule with the graticule on the map, making sure that the north pointer is pointing north.
You then read off the track from the markings at the edge of the protractor. As you intend to fly
from Oxford to Ledbury, which is in a north-westerly direction, the track is 286 degrees.
Remember that the track you have just measured is the true track, because you have aligned the protractor with the
map graticule, which is aligned with true north.
Therefore your track from Oxford to Ledbury is 286 degrees true.
You would follow the same procedure for the next track. You may find that when you place the protractor over
the midpoint of the track, the edges of the protractor extend beyond the ends of the track,
so making it difficult to read the track direction correctly. In this case, you would move the protractor along the
track until the track appears from under the edge of the protractor as shown here.
The track is 072 degrees true.
Measuring the distance between two points is rather more straightforward, because most aviation rulers are marked in statute miles, nautical
miles, and kilometres, and often both for half-million and quarter-million scales.
To measure a distance with such a ruler, you select the scale and the distance marks you require, lay the
ruler along the track with zero at one end, and read off the distance.
We use nautical miles in aviation, so the distance from Oxford to Ledbury is 42 nautical miles.
Finally, remember that a minute of latitude equals one nautical mile. Therefore, you can use the latitude minute marks on
the map if you don't have a ruler available.
In this lesson, we'll look at the types of information on the topographical chart which can help you when you're
navigating.
We'll then put all this information into context by introducing you to the art of map reading.
One of the first things you do when flight planning for a visual cross-country flight is to decide the
route.
Having made this decision, as part of the flight planning process you must study the route to decide what features
along your route you'll use to aid your navigation.
It's therefore important that you are familiar with what is printed on the chart.
A quick glance at the chart will show you that it contains a bewildering array of information.
You must be able to sift this information so that you take note of what is relevant to your particular
flight and what is not.
To show you how you would do this, we will examine the route from Chichester Airdrome to Wickham Air Park
as shown here.
As we move along the track, you'll be shown where to find information about each significant feature.
There are three main blocks of information, or legends, on the chart.
Block 1 is the largest and is in the bottom left corner. It gives details of aeronautical symbols and pertinent
aeronautical information.
Block 2 is in the bottom right corner and contains cultural information, in other words, information on roads, railways, built
-up areas and general features as well as relief information.
The latter was covered in Lesson 3.
Block 3 can be found on the left side of the chart and gives information on aerodrome traffic zones and
RT frequencies.
We'll refer to these blocks as we fly the route to help you find the relevant information.
There's a lot of useful information in the vicinity of Chichester Airdrome.
For example, the blue figures under the names show that the aerodrome is 100 feet above mean sea level
and lies at the centre of its own aerodrome traffic zone.
There is a VOR DME located on the airfield.
To the north-north-east there's a civil heliport within the confines of Goodwood Racecourse with an elevation of 510
feet above mean sea level.
As we fly north from the aerodrome you'll see that we will be flying over lightly wooded areas.
Goodwood Racecourse heliport is on our right.
With rising ground indicated by the change in relief colour from white to brown.
We pass a small town or village on our left just after the Goodwood Racecourse heliport.
A primary road runs on our starboard side that leads into the town of Midhurst about one mile on the
right.
Two disused railway tracks are shown.
And just north of Midhurst there's a river running approximately east-west.
This picture shows how difficult it is to see some of these features from the air.
You'll probably have noticed the maroon lines that denote controlled airspace.
At this stage the important thing for you to know is how to determine the type of airspace and its
lower limit because you must not under any circumstances go into controlled airspace.
Just above the present aircraft positioning you will see some maroon lettering which says R8A in a box 4500 feet
and a plus sign.
This means that the block of airspace is R8A.
It's class A airspace and its lower limit is 4500 feet above mean sea level.
The plus sign denotes an upper limit of FL245.
Therefore as you fly along this track you must be aware of the lower limits of the various areas of
controlled airspace that you will fly under.
You'll be under R8 as you pass the 814 foot spot height.
And as you pass the town of Liphoek you'll go from under R8 to under part of the London Terminal
Control Area with a lower limit of 3500 feet above mean sea level.
About three miles to your right as you pass Midhurst there is a hang and paragliding site.
The site is 80 feet above mean sea level.
And winch launching up to 2100 feet above mean sea level is indicated.
Four miles right of track is a mast or more accurately an air navigation obstacle.
The top of which is 996 feet above mean sea level.
However the smaller figure in brackets tells you that the top of the mast is 376 feet above the ground.
Your track now takes you between the towns of Liphoek and Hazelmere where you cross first a multiple track railway
and then a dual carriageway road.
As you pass Liphoek there are two areas of regulated airspace about three miles on your left.
The Oak Hanger high intensity radio transmission area which extends to 10,500 feet above mean sea level.
And the danger area Delta 130 which normally extends to 800 feet above mean sea level.
But occasionally it extends to 2,300 feet above mean sea level.
You'll see that the danger area number is preceded by three symbols.
The first symbol means that pre-flight information for the danger area is available.
The second symbol means that the danger area activity information service is available for the area.
And the third symbol means that the area is subject to by-laws that prohibit entry when the area is
active.
Not long after crossing the dual carriageway road you pass between two unnamed towns and pass over another danger area.
See if you can find out what the contract frequency is for getting information on the status of the area.
Then on your left you'll see a large blue circle indicating the Odium Military Air Traffic Zone or MATs.
There's a contact frequency printed under the name if you wish to call them.
LAARS stands for Lower Airspace Radar Service. Details of this service can be found in the Air Pilot.
In this example the LAARS is controlled by Farnborough.
And there are two visual reference points within the area. These are used when making a VFR approach to the
airfield.
Your track now takes you through the Farnborough Air Traffic Zone.
The contact frequency is printed under the airfield but can also be found in Block 2.
There is also a visual reference point which is a railway station north-east of the airfield.
After passing Farnborough the track takes you very close to the Blackbush Air Traffic Zone.
Blackbush has a non-directional beacon, an NDB, which is tied in with Distance Measuring Equipment, DME.
The frequencies for both of these aids are shown.
You will also see that there is an aerodrome light beacon which flashes white indicating that it is a civil
aerodrome.
This type of light beacon does not identify the aerodrome upon which it is placed.
Other active aerodromes will have flashing beacons, some of which may flash a Morse Identifier.
These are known as Aerodrome Identification Beacons or AIBs, Military AIBs flash red and Civil AIBs flash green.
After passing Blackbush your track takes you over an unnamed town with a multiple track railway line about a mile
on your left.
The track then passes over a significant road junction between a dual carriageway and a primary road, followed by the
M4 motorway.
The white office block towers of Bracknell are now on your right.
You then transit the White Waltham Air Traffic Zone. The aerodrome symbol indicates that it has limited facilities.
The track then crosses the River Thames before entering the Wickham Air Park Air Traffic Zone.
By now you should have an idea of what many of the symbols on the chart mean and, if you
are unsure, where to find the information.
You will now learn how to make use of this knowledge to enable you to map read.
Map reading can be defined as visualising the physical features of the ground as represented on the map by symbols
and forming a mental picture of the ground by relating the features one to another.
This is known as reading map to ground and is the basic technique used when you are confident that you
are close to your track and to your planned time.
There is a reverse process used when less certain of position called reading ground to map.
This involves seeing an arrangement of features on the ground and being able to locate those features on the map
and thus find your position.
Visual navigation using map reading is a skill which needs practice and which will develop with experience.
A number of factors can make the task easier. The most important of these are thorough flight planning and pre
-flight route study.
It's also important in flight to try to remain on track and on time and, if off track, to regain
it as soon as possible.
However, don't concentrate too much on navigation alone. Remember that it's equally important to keep a good lookout, to be
aware of any potential weather problems and to manage the aircraft systems.
In the first part of the lesson you looked at the principal features on the route from Chichester to Wickham
Air Park.
We'll now fly the route in reverse and see what features you would use for map reading.
Although map preparation will be discussed in a later lesson, there are one or two points which you need to
be aware of here.
When map reading it's good practice to have the map oriented in the direction in which you are going.
For example, for this flight the track from Wickham to Chichester is virtually due south, so you'd plan to have
the map oriented on your knee in the cockpit as shown here.
Although the map is upside down, all the ground features are oriented as you would expect to see them from
the air. This makes map reading easier.
Any annotations you might write on the map during flight planning or in flight should be written with the map
oriented in the direction of flight.
The only disadvantage is that, depending on the orientation, the printing on the map may not be so easy to
read.
Finally, you should fold your map ideally so that the whole route is visible. If this is not possible, then
you must be able to reveal each section of the route without having to unfold the map more than necessary.
During flight planning you should study the route and select visual checkpoints at about 5 to 10 minute intervals along
your track.
The ideal visual checkpoint will be recognisable from some distance away and have the following features.
Large. A large feature will be recognisable from some distance, but may be too large to use for precise navigation.
A good principle is to use part of a large feature as the checkpoint.
For example, if you're using Bracknell as a checkpoint, you'll probably see the town from some distance. But use the
road junction with a railway bridge next to it to the west of the town as your accurate checkpoint.
Unique. Be sure that the checkpoint chosen could not be mistaken for a similar checkpoint in the same area.
An airfield such as White Waltham should stand out well as there are no other airfields in the vicinity. However,
Blackbush and Farnborough could possibly be confused as they are fairly close together.
Vertical extent. Features which have a significant vertical extent such as TV masts should always be identified for safety reasons,
but they also make good visual checkpoints because they can be identified in good visibility from several miles.
But the value of features with good vertical extent alone may diminish if you fly higher. For example, at 5
,000 feet above ground level you may not easily see a 400 foot high mast.
Let's now have a look at what features we could use as checkpoints on the route from Wickham to Chichester.
The distance is 45 nautical miles and if we assume a ground speed of 90 knots it will take us
30 minutes.
We'll therefore need to select 4 or 5 checkpoints to ensure that we have one every 5 to 10 minutes.
However, remember that this is not a hard and fast rule.
If there are no suitable features for more than 10 minutes you just have to choose one as soon as
you can. Don't be tempted to select a feature which is obviously unsuitable because it will only lead to confusion.
We've already looked at the road junction just to the west of Bracknell which is about 8 minutes from Wickham.
However, you could also use White Waltham airfield. It's about 4 and a half minutes after take-off. It's large
and only a mile off track.
After the Bracknell road junction, Farnborough airfield is just over 5 minutes away and is a possible checkpoint for the
same reasons as White Waltham. However, you would make a mental note not to confuse it with…
…Blackbush, which should be on the right of your track, slightly further away and only 3 minutes from the road
junction.
After Farnborough the next suitable feature would be the railway between Hazelmere and Liphook, which is about 8 minutes away.
The railway will give you a good timing check and Liphook is less than a mile off track.
Liphook to Chichester Aerodrome is about 9 minutes flying time, so there will be no need for a further checkpoint.
This will give you time to think about your arrival procedures as you approach your destination.
To sum up, your recommended checkpoints would be White Waltham Aerodrome, the road junction west of Bracknell, Farnborough Aerodrome and
a beam Liphook as you cross the railway line.
You would therefore mark these features on your map with the estimated flight time from Wickham. Use of these marks
will be covered in Lesson 12.
These features have been chosen on the assumption that the visibility will allow you to see them clearly.
In Lesson 11 we'll discuss what other considerations you might take into account when selecting checkpoints if you expect the
weather, particularly the visibility, to deteriorate during your flight.
The purpose of this lesson is to introduce you to the basic principles of air navigation. These principles are the
same for any type of aircraft, large and small, and it's essential that you understand them before trying your hand
at navigating.
There will be some repetition of material covered in other lessons. This will be drawn to your attention at the
time.
The assumption is made that you have never done any navigation in the air. If you have, then you can
skip over those topics with which you are familiar, or you can use them to consolidate your existing knowledge.
At the heart of navigation lies the triangle of velocities shown here. You can see that there are three vectors,
and in this lesson we'll look at each of the vectors and see how they interact with each other.
Notice that the orientation of the vectors indicates direction, for example the direction of the wind, plan track, and heading,
and the lengths of the vectors are scaled in order to indicate speed, such as wind speed, true air speed,
and ground speed.
Speed and direction taken together give us velocity, hence the triangle of velocities.
For instance, putting representative values for the various speeds and directions onto the triangle of velocities shown here, we have
a wind velocity of 030 degrees at 35 knots, and a true air speed of 90 knots, requiring us to
fly a heading of 011 degrees in order to attain a desired track of 360 degrees. The resulting ground speed
is 60 knots.
In this triangle of velocities we have a wind velocity of 290 degrees at 20 knots, and a true air
speed of 100 knots, requiring us to fly a heading of 029 degrees in order to attain a desired track
of 040 degrees. The resulting ground speed is 104 knots.
Detailed instruction on how to resolve triangle of velocities problems is provided by Oxford Aviation Training's flight navigation computer CD
-ROM.
The heading and true air speed vector represents the movement of the aircraft through the air.
The true air speed, or TAS, is the speed of the aircraft through the air mass in which it's flying.
It's unaffected by wind because wind moves the air mass and the aircraft together, but it is affected by air
density.
To see why this is so, we need to examine two other speeds, namely indicated air speed and rectified air
speed, also known as calibrated air speed, particularly in Europe and America. The term rectified air speed will be used
throughout this and other lessons.
The indicated air speed, or IAS, is what you see on your air speed indicator in the cockpit. This speed
is a measure of the air pressure, or dynamic pressure, felt by the aircraft as it moves through the air.
Put very simply, it's a measurement of the number of air molecules entering the pitot tube.
The air speed indicator is calibrated at sea level. Therefore, at sea level, indicated air speed and true air speed
are the same. In other words, an indicated air speed of 200 knots gives a true air speed of 200
knots.
Strictly speaking, the two are only identical when sea level atmospheric pressure conforms to the international standard atmosphere pressure of
1013.2 mbar at a temperature of plus 15 degrees Celsius. However, for the purpose of this explanation, we'll assume
that indicated air speed and true air speed are the same at sea level.
If we now climb, maintaining an indicated air speed of 200 knots, the aircraft will have to move faster through
the air mass in order to keep the same number of air molecules entering the pitot tube. This means that
the true air speed will increase as the air gets less dense, even though the indicated air speed remains constant.
By the time you reach 40,000 feet, the true air speed will be approximately twice the indicated air speed.
While your cruising indicated air speed for any particular aircraft will always be the same, the equivalent true air speed
will vary from day to day. You must, therefore, calculate as accurate a true air speed as possible for your
planned altitude.
No machine is perfect, so the air speed indicator suffers from two errors, called instrument error and position error. Instrument
error is caused by imperfections in the manufacture of the instrument, as well as friction in the mechanism. Position error
is the result of the positioning of the pitot tube and static vents on the aircraft.
Details of these errors can be found in the pilot's operating handbook, and when applied to indicated air speed, show
what the air speed indicator should read if it was perfect.
Indicated air speed, then corrected for position error and instrument error, gives us rectified air speed.
The errors are normally quite small. Nevertheless, if you want to navigate accurately, you should use rectified air speed rather
than indicated air speed, as the basis for calculating true air speed.
The heading true air speed vector therefore represents the path of the aircraft through the air mass. Although you fly
the aircraft using the indicated air speed, it's the true air speed which determines your speed through the air. True
air speed therefore plays a central role in navigation.
This vector is referred to as the wind velocity vector, and by definition it shows speed as well as direction.
By convention it's shown with three arrows, and the wind blows in the direction of the arrows.
If we assume that due north is the top of the screen, the wind vector shown here indicates a wind
blowing from the northeast.
Forecast wind direction is always given in degrees true, so we can say that the direction of this wind is
030 degrees true.
Although the length of the vector represents the speed, we have ignored this for sake of clarity. However, if we
say that the wind is 10 knots and the true air speed is 100 knots,
then the wind vector should be one tenth of the length of the heading true air speed vector. Such a
wind would be shown as 030 degrees 10 knots. Notice how, for a given heading and true air speed, any
change in the wind vector affects our track and ground speed.
As the aircraft moves through the air, represented by the heading and true air speed vector, it's affected by the
wind velocity, represented by the wind vector. The resultant is the track and ground speed vector. This represents the actual
path and speed of the aircraft over the earth's surface.
The angle between the heading true air speed vector and the track ground speed vector is the drift angle. This
shows the effect of the wind on the aircraft as it moves through the air. You always drift from your
heading to your track, so this particular example shows port drift.
The track element of this vector, therefore, is referred to as the Track Made Good, or TMG.
The TMG is different from the planned track. As its name implies, the planned track is the track you wish
to fly along. In other words, it's the line joining your departure airfield to your destination airfield.
So the planned track is what you want to fly, and the TMG is what you actually fly.
Clearly, you want to fly so that your TMG is the same as your planned track. As we shall see
in a later lesson, when flight planning, you calculate the heading required using the forecast wind, so that you can
make good your planned track.
In the triangle of velocities shown here, the length of the planned track vector now gives you your ground speed.
Of course, as with all your navigation calculations, you'll calculate your heading and ground speed using your navigation computer.
The difference between your planned track and your required heading is known as the wind correction angle.
In our treatment of the triangle of velocities, we have referred to the direction of wind and track with respect
to true north. Remember that in order to find the course to steer on your aircraft's compass or direction indicator,
you'll need your heading with respect to magnetic north.
Your calculated true heading, therefore, must be converted to magnetic heading by applying the magnetic variation as taught in the
lesson on direction.
The speed of the aircraft over the ground is what will determine the time taken to fly between two points
on the ground.
Knowing your ground speed is therefore an essential part of navigation, since you will then be able to calculate how
long it will take you to fly between any two points.
Having calculated your heading to achieve your planned track, the length of the planned track vector will give you your
ground speed.
Similarly, it will enable you to work out your estimated time of arrival, or ETA, at your destination.
During the flight planning process, you calculate what your ground speed is likely to be based on the forecast wind.
Once in the air, you can determine what your actual ground speed is by measuring the time taken to fly
between two points.
If it's different from your planned ground speed, you can then alter your estimated time of arrival.
All this will be covered fully in a later lesson, but it's mentioned here to make the point that ground
speed plays a vital role in successful navigation.
Finally, there are some practical points you need to remember about navigation.
When flying on a cross-country flight, you act as pilot, navigator and radio operator.
In other words, you aviate, navigate and communicate, and the order of the words indicates the priority you must place
on these activities.
First and foremost, you must fly the aircraft accurately and, of course, safely.
You'll soon learn that navigation is much easier if you fly accurately the headings, speeds and heights that you've planned.
Efficient navigation requires you to coordinate your navigation activities with the safe operation of the aircraft.
If you plan your flight accurately and thoroughly and then fly the aircraft accurately, you'll find not only is navigation
easier, but also that you have plenty of time to attend to the operation of the aircraft.
By the same token, you will also have plenty of time to think about your RT calls so that you
can make them clear and concise.
What's the time?
This question is asked millions of times a day, but as we look at our watches many of us probably
don't realise the complexity of the question.
Normally, of course, the person asking the question expects the answer in local time. However, we could respond to the
question with another question.
What time do you want the answer in?
Possible answers are shown here. As you can see, there is a bewildering array of different times.
You don't need to know what they all mean, so we'll look at those times which are important to you
when flying.
They are shown here.
The Earth rotates daily around its geographic axis, anticlockwise if observed from above the North Pole.
However, it's convenient to consider the Earth as stationary and the Sun travelling around the Earth once a day in
a clockwise direction as shown here.
This alternative fits in with our perception that the Sun rises in the East and sets in the West.
When the Sun transits, or crosses, a particular meridian, the local mean time at all places on that meridian is
1200 hours, in other words midday or noon.
Similarly, when the Sun transits the anti-meridian of a point, the local mean time at that point is 2400
hours or midnight.
By convention, midnight of a particular night, say the night of the 15th-16th, is regarded as 2400 hours local
mean time on the 15th or 0000 hours on the 16th.
It takes the Sun one hour to travel 15 degrees of longitude.
Therefore, if the local mean time at 45 degrees East, approximately Madagascar, is 1200 hours on the 16th of May,
at 90 degrees East, approximately India, the local mean time is 1500 hours on the 16th of May.
At 135 degrees East, approximately Japan, the local mean time is 1800 hours on the 16th of May.
At 180 degrees East, approximately mid-Pacific, the local mean time is 2100 hours on the 16th of May.
Now, if we move to the West from the 45 degree East meridian, we come to the Greenwich meridian at
0 degrees East or West, where the local mean time is 0900 hours on the 16th of May.
Local mean time at the Greenwich meridian is known as Greenwich mean time or GMT.
The Earth is divided into time zones of 15 degrees or 1 hour as shown here.
The Greenwich meridian is the datum, and as you can see, it lies in the Zulu zone.
Zulu time used to be the Earth's standard time. However, the present standard time is called coordinated universal time.
or UTC.
UTC changes at a constant rate and is regulated against International Atomic Time, derived from atomic clocks.
For all practical navigation purposes, UTC is the same as GMT.
In the UK, the standard time is UTC at all times of the year.
In the summer, we adjust our clocks forward 1 hour and refer to the time as British summertime, or BST,
a time which is 1 hour added to our standard time.
Many countries may carry summertime, or daylight saving time as it's called in America, in the summer months.
As in the United Kingdom, their summertime is normally 1 hour in advance of their standard time.
Thus, if the standard time in the United Kingdom in June is 1200 hours UTC, the British summertime is 1300
hours BST.
If the daylight saving time in New York, USA, in June is 0900 DST, the standard time is 0800 ST.
Flight Plan
Whenever you fly, you flight plan.
Even for a short local area flight, you will, at the very minimum, look at the weather situation to see
if it's suitable for your intended flight, and decide on the format of the flight.
That is, where you're going to, and what you're going to do.
You'll also make sure the aircraft has sufficient fuel, and has been properly serviced and prepared for the flight.
All these activities constitute planning for your flight.
However, the expression of flight planning normally refers to the activities associated with planning for a cross-country flight, either
VFR or IFR.
In this lesson, we'll examine the following flight planning activities.
Pre-flight briefing, map preparation, completing the flight log, fuel and communications plans, weight and balance, take-off and landing
performance, and completing an air traffic flight plan.
Before starting any planning for a cross-country flight, you should make sure that you have proper personal equipment.
The following list is a guide. With experience, you'll decide what you really need, and what you can do without.
Relevant charts covering your intended route, navigation computer, ruler and protractor, reliable watch or stopwatch, pencils and pens, clipboard or
knee pad, sunglasses.
Pre-flight briefing covers all of your activities prior to getting airborne.
The starting point is obviously the route you wish to fly.
For this lesson, we'll assume that you wish to fly from Oxford to Howarden, about 10 miles south of Liverpool.
You then have to decide if the weather is suitable for the flight.
Then you must find out if there are any NOTAMs, airspace restrictions, royal flights, or other hazards which may affect
your flight.
You need to know if your destination aerodrome will be open when you arrive, whether it's prior permission only, and
what its serviceability state is.
You need to do the same for your alternate aerodrome.
Remember to check the time of sunset, particularly in winter, and plan to arrive at your destination at least 30
minutes prior to this.
A great deal of this information can be obtained from the Met Office and National Air Traffic Services websites.
This is a quick and easy way of getting the relevant information for a flight, and allows you to do
some pre-flight preparation at home.
However, remember that nearly all the information should be available in paper form at your aerodrome operations facility.
We'll now have a look at these points in more detail.
There are several factors which may influence your choice of route.
The most obvious is the weather.
For example, if you intend to plan across country just for training purposes, you'll want to plan to go where
the weather is best.
On the other hand, if you have to go to a particular destination, in our case we want to go
to Hardin, then you'll have to accept the weather as it is, although you may be able to pick a
route which avoids the worst of it.
Either way, you need to include the weather forecast in your route decision.
If you want to get from A to B as quickly as possible, then the direct route is usually the
best, but not always.
Take the flight to Hardin. You could go direct from Oxford.
However, this route would take you under the Birmingham Control Area, or CTA.
While this would be perfectly legal, if you strayed slightly right of track, you would be under a part of
the CTA with a lower limit of 2,500 feet.
You would therefore be unwise to plan on flying higher than an altitude of 2,000 feet along that part
of the route.
This would restrict your ability to climb if you wished to get out of turbulence or to get better visibility.
Furthermore, although you would not actually be in regulated airspace, good airmanship would dictate that you contact Birmingham Air Traffic
to let them know you're there, thereby increasing your communications workload.
Your direct route would also take you through the Cosford Air Traffic Zone, and through the combine military air traffic
zones at Turnhill and Shawbury.
Again, this would be both feasible and legal. However, it would not only add significantly to your communications workload, but
also there would be a strong possibility that you would have to deviate from your planned track as you pass
through the military zones.
And finally, you should consider the fact that the route goes through areas where the traffic density is likely to
be fairly high, so the risk of collision would be commensurately higher.
You can avoid all these potential difficulties by routing via Ludlow. By doing so, you increase the distance by only
9 miles.
However, the route does take you over some high ground north of Ludlow, so you'd need to check the weather
in that area carefully.
To summarise, when choosing a route always check the weather before rather than after you have decided which way to
go. And, even if the weather is good, do not assume that the direct route is necessarily the best.
A very comprehensive weather briefing can be obtained from the Met Office website. There are four forecasts to choose from.
You can therefore select a forecast which is closest to the time of your flight.
For your flight to Hardin, you'll need information on the weather and the winds you can expect en route, all
of which can be obtained from this page on the website.
This is a UK low level forecast chart.
The left chart shows the forecast for a specific date and time, and the right chart shows the outlook for
the date and time shown.
The weather depicted on the left chart is divided into zones. The weather in each zone is summarised underneath. Note,
however, that these charts do not show wind.
The forecast shows that your route will be within zone 2, and that the weather will be good, with 20km
visibility and the lowest cloud base 3,000ft.
An occluded front is approaching the area, but will not affect your flight, which we will assume is planned to
depart at 1400 hours.
With a cloud base of 3,000ft, and good visibility, you decide that a cruising altitude of 2,000ft would
allow you to maintain good VMC.
For information on the wind you can expect along your route, you need the current UK low level spot wind
chart shown here.
Information is given for the latitude and longitude shown in each box.
Altitudes are in thousands of feet above mean sea level, and temperatures are in degrees Celsius.
To select a wind you look for the box nearest your intended route, and select an appropriate wind for your
flight log.
You may need to interpolate if your cruising altitude lies between the published altitudes.
For example, the 5230 North 0230 West box lies about midway along our track from Oxford to Hardin.
As you have decided to cruise at 2,000ft you don't have to interpolate, so you can use the 2
,000ft wind of 180°10' and the temperature of plus 6°C.
Now that you have your en route weather, you need to find out what the weather will be at your
departure and destination airfields.
Terminal aerodrome forecasts, or TAFs, and met actual reports, METARs, can be obtained from many UK aerodromes via the TAF
and METAR bulleting guide.
TAFs are updated every 3 hours, and METARs every 30 minutes.
However, you should bear in mind that many aerodromes are not included, and this applies to both Oxford and Hardin.
To get a general indication of the weather at these locations, you should first select the nearest aerodromes with TAFs
and METARs shown in the guide.
For example, Bryce Norton, IKO designator Echo Golf Victor November, is nearest to Oxford, and Liverpool Echo Golf Golf Papa
is nearest to Hardin.
To get a more detailed forecast for your arrival time at Hardin, you would use the telephone.
The UK Aeronautical Information Publication, AIP, contains information on long-standing and permanent operational matters.
More recent changes and amendments are issued as AIP supplements.
In addition, Aeronautical Information Circulars, AICs, contain information of a more administrative nature, although they are also used to give
advanced warning of impending operational changes.
Most of this information, including pre-flight route and aerodrome information bulletins, should be available at your flying school or
club.
The information is also available on the National Air Traffic Services website.
In addition, updated information on Royal Flights is available by telephoning 0500 354 802.
Among the services on offer are a series of briefings shown here.
To obtain information on NOTAMs and all other warnings or restrictions pertaining to a particular route, you click on UK
Route Briefing, and this form will appear.
It saves you a lot of time because you avoid the need to look through many NOTAMs to find the
few which apply to your route.
When completed, you'll get a pre-flight information bulletin, an example of which is shown here.
Aerodrome information is available from several sources, such as Pooley's Flight Guide and the AIP.
Information from the AIP obtained from the website will be used in this lesson.
Information on all UK aerodromes can be obtained from the AD Aerodrome section of the UK AIP.
From here you can get a detailed map of the aerodrome layout.
And find out such things as hours of operation, radio and navigation aid frequencies, local flying restrictions, warnings and obstacles,
and a variety of administrative details.
In general, this is a very good place to start, particularly if you're planning to go to an unfamiliar aerodrome.
From time to time this information will be amended by NOTAM, and by clicking on UK Aerodrome Briefing, you can
select the aerodromes you're interested in and get up-to-date information.
You are now armed with sufficient information to start preparing your map and flight log.
Map preparation is largely done in conjunction with preparing your flight log, so we'll look at both of these activities
together.
You must always carry the appropriate chart or charts for your proposed flight.
Make sure the chart includes your alternate aerodrome and that it covers at least 50 nautical miles either side of
your intended track. This should ensure that you don't fly off the edge of the map.
We'll assume that you'll use the IKO half million chart for this flight.
Flight logs come in a wide variety of layouts and you will, no doubt, use many of them in the
future.
The log shown here is therefore a typical example which we will use for this and the subsequent lesson.
The first thing you must do is draw the tracks for your route on the chart. Make sure you use
a pen with permanent ink.
Grease pencil or chinagraph and water-soluble inks tend to rub off too easily.
The colour of the ink is a personal choice, but it must stand out clearly on the map.
It's a good idea to draw small circles around your destination and departure aerodromes and any intermediate turning points as
shown here.
This will ensure that these features are not obscured by the tracks.
In a similar vein, don't make the tracks too thick as they may hide vital information on the chart.
To complete your map preparation you need to draw in track guides as shown here.
They are normally drawn at the end of each track 10 degrees either side and in a different colour from
the track so that they stand out.
We'll see how these are used in the next lesson.
Next you can enter the route information in your flight log, together with your planned cruising altitude.
Then you measure the tracks and distances and enter them in the log.
You've already decided what wind you're going to use so you can put this in the log.
With your navigation computer you can now calculate your true and magnetic headings, your ground speed and your leg times.
We'll assume a cruising indicated airspeed of 100 knots.
The spot wind forecast chart shows a temperature of plus 6 degrees Celsius at 2000 feet, which gives a true
airspeed of 103 knots.
If you're unsure how to do these calculations, refer to Oxford Aviation Training's Flight Navigation Computer CD-ROM.
Finally, you would complete a line in the log to your designated alternate aerodrome.
Liverpool is the nearest suitable aerodrome, so the entry in your flight log is shown here.
An essential part of your planning is to calculate safety altitudes for each leg.
The reason is that, if you lose sight of the ground, flying at your safety altitude will keep you clear
of any obstacles within a given distance of your track.
Of course, if you are navigating VFR you must remain in sight of the ground in order to map read.
Nevertheless, it's common sense and sound airmanship to calculate your safety altitude and enter it in your flight log.
There are a number of ways of calculating safety altitude which are listed here.
We'll take the highest maximum elevation figure, or MEF, and add 1000 feet.
On the first leg, the highest MEF is 2100 feet, so our safety altitude is 3100 feet.
Enter this in your flight log.
On the second leg, the MEF is still 2100 feet, so the safety altitude remains the same.
So enter this in your flight log.
Now that you've worked out your ground speed for each leg, you can put timing marks on your map.
There are several ways of doing this.
The simplest is to put marks at a quarter, half and three quarters of the way along the track, and
then work out your time to each point.
This system works satisfactorily, provided there is some prominent ground feature at or near each point from which you can
take your time.
If there is no such feature, then the timing mark is of no use. The system is therefore not very
precise.
Nevertheless, you may well use it when you start cross-country flying until you're ready to cope with the more
sophisticated system.
In the lesson on map reading, you were advised to select visual checkpoints along track at between 5 and 10
minute intervals.
By using these points for timing as well as map reading, you will increase the accuracy of your timing.
Note that we normally measure time to the nearest half minute.
On the first leg, with a ground speed of 108 knots, checkpoints need to be between 9 and 18 miles
apart.
Therefore, with the length of 60 miles, no more than 4 checkpoints will be required.
Suggested ones are shown here.
The town of Morton in Marsh has a railway line running through it and a disused airfield to the north
east.
It's 16 nautical miles along track and will take 9 minutes to get there, so it falls nicely within the
5 to 10 minute bracket.
You should mark your map as shown, so that the timing mark and the time are clearly legible.
The next large feature is the town of Evesham, which has a river and a railway running through it.
It's 11 miles beyond Morton in Marsh and will take a further 6 minutes to get there.
Our total elapsed time is now 15 minutes.
An alternative might be the town of Pershaw, some 5 miles further along track.
However, the only large feature between Pershaw and the turning points at Ludlow is Worcester,
which, depending on what part of the city you choose, is between 5 and 7 miles from Pershaw.
For this reason it would be better to use Worcester rather than Pershaw as a check and timing point.
From the map there are two features at Worcester which look as though they may stand out from the air.
The first is the motorway junction to the south east of the city.
The other is the racetrack beside the river.
The motorway junction will almost certainly be easier to see, as the moving traffic on motorways normally stands out pretty
well,
so giving you warning of its approach.
The distance from Evesham to the junction is 10 miles, which will take 5 and a half minutes,
giving a total elapsed time from take-off of 20 and a half minutes.
After you have passed Worcester there are no major features to use as checkpoints before reaching Ludlow,
with the possible exception of the town of Tenbury Wells.
However, as it's only 5 miles from Ludlow it would not be much use as a timing point,
but you could certainly use it as a checkpoint.
If you've drawn the route on your map, before moving to the next screen select four checkpoints on the leg
from Ludlow to Howarden.
Then click on the continue button to compare them with the ones selected for this lesson.
Don't worry if they aren't the same. There are a variety of features that could be used on this leg.
Like the first leg, three to four checkpoints will be sufficient, and if you choose ones that are reasonably spaced,
you can use them for timing as well.
In the next lesson we'll discuss, with the aid of photographs, how we use all the checkpoints to aid our
navigation.
For obvious reasons, the amount of fuel you carry is very important.
You want to carry enough fuel to get you to your destination, and maybe to your alternate if you can't
land at your destination.
On the other hand, you may want to carry extra weight in the form of passengers or freight, which may
limit the amount of fuel you can carry.
It's therefore common sense and good airmanship to know the minimum fuel you need to carry out the flight safely,
and to make sure that you have at least this on board.
For this flight, and to keep it simple, we'll assume that you have a fuel consumption of 10 US gallons
per hour.
Your total flight time to Howarden is 59 minutes, which equates to 10 gallons of fuel.
Your flight time from Howarden to Liverpool is 6 minutes, which equates to 1 gallon.
It's a good idea to assume that your consumption will be slightly higher than expected, and to allow for this
by adding a 10% contingency.
This equates to 1.1 gallons.
You don't want to arrive at your destination or alternate with almost empty tanks, so you must have fuel in
reserve.
Sometimes called a fixed reserve or a holding reserve, where a light aircraft 45 minutes is adequate, this equates to
7.5 gallons.
Adding all this up to the nearest gallon, your minimum fuel requirement is 20 gallons.
You now have the choice of taking just 20 gallons, or of taking extra fuel up to full tanks, which
we will assume is 50 gallons.
Whatever fuel load you take, it's sound airmanship to record what this represents in terms of endurance, so that you
know just how long you can stay in the air.
For a VFR cross-country flight, your radio communications are going to be fairly simple.
Nevertheless, to minimise your workload in the air, it's a good idea to make a note on your flight log
of the frequencies you're likely to use, so that you have the information readily available.
Radio frequencies for aerodromes can be found in the AIP AD section, under the appropriate aerodrome name.
The information for Howarden is shown here.
En route, you are unlikely to need more than the London Flight Information Service frequency, which is displayed on your
chart.
You should put all the frequencies you think you'll need on your flight log as shown here.
Note that your alternate aerodrome, Liverpool, is included.
It's good airmanship always to calculate your mass and balance to ensure that you're not exceeding weight limitations, and that
the centre of gravity is within prescribed limits.
If you're unsure how to do this, refer to the lesson on mass and balance.
With regard to aircraft performance, you should always confirm that the aircraft will be able to operate from the planned
runways.
If you're unsure how to do this, refer to the lesson on performance.
The flight plan is an air traffic message in a set format, which is transmitted to the appropriate air traffic
organisation concerned with the flight.
It's compiled by, or on behalf of, the pilot in command, and is the basis for air traffic clearance to
be given for the flight to proceed.
As a PPL pilot, you may file a flight plan for any flight. However, if you do not, you must
contact the air traffic control or operations agency at the departure aerodrome before leaving.
This is known as booking out.
You are advised to file a flight plan if you intend to fly more than 10 miles from the coast,
or over sparsely populated or mountainous terrain.
Flight plans must be filed for all flights.
Within Class A airspace, IFR only.
Within controlled airspace in IMC or at night, excluding special VFR.
Within controlled airspace in VMC if the flight is to be conducted under IFR, within the Scottish and London upper
flight information regions.
Where the destination is more than 40 miles from departure, and the total weight authorised exceeds 5,700 kilograms.
To or from the United Kingdom, which will cross the United Kingdom FIR boundary, during which it is intended to
use the air traffic advisory service.
A flight plan must be filed at least 30 minutes before taxi or start-up clearance.
And, if you file a flight plan and then land at an aerodrome other than the one specified as your
destination, you must inform the air traffic unit at your specified destination within 30 minutes of your ETA there. Otherwise,
overdue action will be initiated.
An example of a completed flight plan is shown here. It's not intended to detail how to fill in a
flight plan in this lesson.
Full instructions are contained in an aeronautical information circular.
We are now going to examine how you would navigate round the route that you have just flight planned. However,
in view of the many aspects of VFR navigation that need to be covered,
we will stop the flight at times or even backtrack in order to illustrate particular points.
When you have completed your flight planning and before going out to your aircraft, you should check that you have
the following.
Map, flight log, pen and pencil, watch, preferably a stopwatch, and also make sure that you have got an accurate
time check, checklist, headset, any other personal items you may require.
When in the aircraft, remember that you should check your compass deviation card to see if there is any correction
to be applied to your planned magnetic headings.
There are two commonly used departure techniques for VFR navigation and we will have a look at both in relation
to our route.
You should decide which of these techniques you are going to use when flight planning.
The first is the overhead departure which you will probably use during your early cross country flights.
The second is the en route departure.
The overhead departure involves climbing to your planned altitude close to the airfield and then setting heading from directly overhead
as shown here.
Make sure that when you arrive overhead you are at your planned altitude on the correct heading and at the
correct speed.
You would start your stopwatch when overhead.
The en route departure is slightly more complicated but it gets you on track and clear of the airfield more
quickly.
However, local airfield traffic procedures may prevent you from carrying out the ideal departure for your planned first track.
For example, at Oxford specific clearance is required for a right turn after take-off because it is against the
circuit direction.
This clearance may be denied if the circuit is very busy.
The angle at which you intercept the outbound track is your choice but anywhere between 30 degrees and 45 degrees
is normally adequate.
You intercept the outbound track visually by looking back at your departure airfield and then turn onto your first heading
when you estimate that you are on track.
Whichever departure you use, when you are on track it is vitally important that you fly your pre-planned heading,
height and speed as accurately as possible.
If you do and you have planned carefully you will stand a very good chance of arriving at your next
turning point, or destination as the case may be, on time and with a minimum number of heading changes.
If you do not, no matter how carefully you have planned, you will soon wander off track. Your timing will
become more and more inaccurate and you may well end up getting lost.
Always remember, aviate, navigate, communicate and aviate means fly accurately.
Whichever departure you fly, when you have settled down on track and on your first planned heading it is important
to check four things.
Firstly your heading. Is it correct? And if you have a direction indicator, is this synchronised?
On this flight you would make sure that your heading was 301 magnetic and not 310 magnetic which is a
very easy mistake to make.
Secondly your altitude. Are you at your planned altitude and is the altimeter set correctly?
If you are flying at below 3000 feet, when clear of the aerodrome circuit you would normally set the appropriate
regional pressure setting on your altimeter subscale.
Thirdly the time. Is your stopwatch running? You can then work out what your ETA will be at Ludlow and
enter it in the flight log.
If we assume that you started the watch at 1352 then your ETA at Ludlow will be 1425.
Remember that flying an accurate indicated airspeed of 100 knots is a vital part of good timekeeping.
Finally you should carry out a visual check that you are in fact flying in the right direction.
You should use readily identifiable features on the ground such as this railway line which should be on your left
as you set heading.
And the aerodrome at Enstone should be visible on your right after two to three minutes depending on the visibility.
Now that you have satisfied yourself that you are on track on the correct heading you must concentrate for the
next few minutes on aviating.
In other words flying accurately and keeping a good lookout.
Do not attempt to map read continuously. You know which checkpoints you are going to look out for and the
time when you should reach them.
So do not confuse yourself by trying to map read using less suitable features.
It is going to take you nine minutes to get to your first checkpoint at Moreton in Marsh so you
will expect to arrive there at 1401 which is your start time plus nine minutes.
Therefore at around 1359 you should start to look for Moreton in the Marsh.
This is the sort of view you might see about two minutes away.
You can see the disused aerodrome with the town to the left just beyond.
Note the way cloud shadows obscure features on the ground.
You may be able to see the railway converging with your track from the left and then the town and
disused aerodrome slightly to the right of the nose of the aircraft.
Do not panic if you cannot see them immediately.
Keep flying accurately and they will appear.
If not exactly where you expect then somewhere pretty close.
For the purpose of this lesson we will assume that when you get to Moreton in Marsh you find that
your track made good takes you just to the north of the aerodrome.
You arrive on time, in other words at 1401, so there is no need to revise your ETA at Ludlow.
However you can see that you are not on track.
You may have noticed that you were going right of track as you approached Moreton but quite correctly you resisted
the temptation to alter heading until you had a reliable fix and had made a sensible calculation to determine a
new heading.
In this situation there are two methods of determining a new heading, shown here.
You will normally be taught the double track error method first as it is slightly easier.
When confident with this method you would then move on to the closing angle method which has a more commercial
application.
However both of these require you to determine the size of the angle you are off track which we call
the track error and there are two methods of doing this.
The first is by using the 1 in 60 rule. This provides you with a simple method of estimating angles
or distances during a revision of heading.
As the diagram shows, with a distance of 60 miles each mile off track subtends an angle of 1 degree.
For example if you are 1 mile off track after travelling 15 miles this is the same as being 4
miles off track at 60 miles so the angle of error is 4 degrees.
Let us assume that you estimate your position as 1.5 miles right of track and you have gone 17
miles. This shows you are about 5 degrees off track.
The other method is to use the 10 degree drift lines which you drew on your map during planning to
help you estimate your track error.
This has the advantage of not requiring any mental arithmetic as you merely find the angle by observation.
In this situation 5 degrees looks about right.
Once you have decided your track error you then have to determine by how much you are going to alter
heading.
We will first look at the double track error method.
Having decided your track error is 5 degrees you alter heading by doubling the track error towards the planned track.
You would therefore alter heading left from 301 degrees to 291 degrees.
Note the time at which you make the alteration, say 14.02, which represents an elapsed time since you started
the watch of 10 minutes.
You then maintain a heading of 291 degrees for a further 10 minutes and at 14.12 you would alter
heading right by the track error of 5 degrees to 296 degrees. This should put you on or very close
to your planned track.
Notice that your heading is now 296 degrees rather than 301 degrees.
Assuming that you were off track originally because the actual wind was either stronger or from a different direction from
the wind you used for planning, using the double track error method takes account of this by changing your original
heading.
This way of regaining track is suitable up to the halfway point. Beyond that point you estimate your track error
in the way we have just seen and you estimate your closing angle using the drift lines at the other
end of your track.
The required alteration of heading for your turning point is then the sum of your track error and closing angle.
We will have a look at an example of this after we have passed the halfway point.
If the actual wind is different from the planned wind then it is reasonable to assume that your ground speed
will be different from what you have planned. Therefore you should pay particular attention to your time at the next
checkpoint to see if this is so.
To look at this problem of timing we will jump back to your position north of Malton Aerodrome.
Your correctional heading of 291 degrees means that your track made good will take you slightly to the right of
your next planned checkpoint at Evesham.
Nevertheless you should pass over the centre of the town which will allow you to take an accurate time fix.
You expect to arrive there after 15 minutes elapsed time. In other words your planned time of arrival is 14
.07. However let us assume that you arrive 1 minute late at 14.08.
Evesham is 28 miles along a track of 60 miles which is pretty close to halfway. Therefore if you are
1 minute late at the halfway point you will be 2 minutes late at your turning point so your revised
ETA for Ludlow will be 14.27.
You should enter the revised ETA on your flight log.
We will now examine how you calculate your heading alteration if you find yourself off track after the halfway point.
Let us assume that as you approach Worcester you find that you will pass over the motorway junction to the
north east of the city rather than the junction to the south east which is the one on your track.
They look pretty similar from the air so you take particular care to confirm that you are in fact approaching
the one to the north east mainly because of its position relative to the city.
You then need to either measure or estimate the following. Your flying instructor will teach you the techniques for doing
both of these at the same time as you fly the aircraft.
Your distance off track which you estimate as 2.5 miles, your track error which you estimate as 5 degrees
and your closing angle which you estimate as 7 degrees.
You would use the drift lines to estimate the angles but you could equally well use the 1 in 60
rule.
If you did the calculation would look like this. The fact that your track error calculation is 1 degree different
from your estimate underlines the fact that the calculation method is inherently more accurate.
Assuming you were still on your planned heading of 301 degrees you would add the track error and the closing
angle figures and alter heading to the left towards Ludlow onto 289 degrees.
In other words 301 minus 12 or 290 degrees if you calculated both angles using the 1 in 60 rule.
As you approach your turning point you must be absolutely sure that it is Ludlow.
There are several features which you could use to positively identify the town.
You might use the town of Tembury Wells to reassure yourself that you are approaching Ludlow.
The distinctive road and river configuration and the high ground in the distance to the right of track would help
you identify the town.
The chart shows a ring road to the east of Ludlow which looks fairly distinctive.
Also there is a spot height of 1750 feet to the north east and a 585 foot mass to the
south.
However, masks are notoriously difficult to see even in good visibility.
As you approach the town the ring road immediately becomes apparent as does the spot height.
When overhead the town the curve of the ring road, the path of the river and the general layout of
the town confirm that it is Ludlow.
Having positively identified the turning point you would then carry out very much the same procedure as you did when
starting the first leg as follows.
Note your time of arrival. We will assume you arrived 14.27 as you anticipated when you passed Evesham.
Turn over the town onto your new heading of 352 degrees magnetic.
Start your stopwatch.
Note the ETA for Harden in your flight log. 14.27 plus 26 that is 14.53.
We will now take a short break from navigating to cover a few additional points.
As was pointed out in the lesson on flight planning, your radio communications on a VFR cross country are normally
fairly simple.
For this particular route you would stay on the Oxford approach frequency until you were say 10 miles away then
change to the London information frequency of 124.75 for a flight information service.
You could stay on this frequency until you were established on the second leg when it might be prudent to
call Shawbury on 129.775 for a flight information service.
Finally when you were about 15 miles from Harden you would change to 123.35 to warn them of your
arrival and to get the relevant airfield information.
So far whenever you have wanted to find out the effect of the wind on the aircraft, in other words
drift, you have used either in flight planning your navigation computer or in the air the 1 in 60 rule
or drift lines.
However it is useful to be able to estimate drift without these aids when taking off and landing and if
you have to carry out an unplanned diversion to another airfield because of say bad weather.
The maximum amount of drift can easily be calculated if the wind speed and the TAS are known, the equation
is shown here.
So for example on our cross country flight we have a forecast wind of 180 10 knots and a TAS
of 103 knots.
With this wind on our beam our maximum drift would be 6 degrees.
To work out your actual drift you need to know the angle between the wind and your track. An easy
way of calculating the amount of drift is to use a clock face.
Drift is taken to be at its maximum if this angle is between 60 degrees and 90 degrees.
For other angles you use the proportions shown, for example if the wind is 15 degrees off track then you
would call this 15 minutes or a quarter of an hour so the drift would be a quarter of maximum
drift.
Press P to pause the lesson while you examine this example.
The more accurate drift table is shown here.
For those of you who are mathematically minded the figures in black represent the sine of the angle between track
and the wind direction. You will see that they are very close to the figures given using the clock face.
All light aircraft have a maximum crosswind limit for takeoff and landing. When the wind is not down the runway
we can adapt the drift calculations to allow us to work out the crosswind so that we do not exceed
the limitation.
For example if you are taking off on runway 27 and the wind is given as 220 degrees 16 knots
the crosswind would be 12 knots.
The flight log used in this lesson is similar to what you can expect to be given for your PPL
navigation exam. However there are many variations on this format in use.
Many of which have space for observations made during the flight. The purpose of these observations is to provide a
brief summary of your flight. An example is shown here.
The idea on this particular version is that whenever you alter altitude, heading or ETA you make an entry recording
the time, the observation and any change to ETA.
The observation would be the new heading or altitude and any other pertinent information.
We will now return to your cross country and assume that you are on your second leg and that you
are approaching your first checkpoint, a beam the road rail bridge, south west of Condova, a disused aerodrome.
You start looking for the checkpoint after about 6 minutes have elapsed and after 8 minutes there is no sign
of the bridge.
You feel sure it is there somewhere but you just cannot see it as it is underneath you. So quite
correctly you decide to ignore that checkpoint and continue to fly the aircraft accurately towards the next checkpoint.
For the time being you must assume that you are on track.
When you have mastered the double track error method of calculating a heading alteration you will progress to the closing
angle method. The basic concept of this is that when a track error has occurred, corrections to heading are made
relative to your destination rather than your departure point.
Before looking at a practical example let us examine the principles involved.
An assessment of the amount of correction to be applied is based on the distance travelled along track and the
closing angle to the destination as follows.
During a flight from A to B a pinpoint is reached at X. The closing angle between the track AB
and the track XB is say 5 degrees and the proportion of the distance covered is one quarter of the
total.
In order to obtain the change in heading required you invert the fraction of the distance covered, in other words
a quarter becomes 4 over 1 and multiply by the closing angle of 5 degrees. The result is a change
of heading of 20 degrees. The aircraft should now maintain a new track from X to B.
Let us assume that you then reach another point at Y. You estimate the closing angle between track XB and
track YB at 3 degrees. The proportion of the distance covered from X to B is a half so you
invert the fraction and multiply by the closing angle to get a heading change of 6 degrees.
You should remember that for the second and subsequent corrections the closing angle required is between the new track, in
this example YB, and the previous track being used, in this example XB and not the original track.
We now return to your route and assume that you are within sight of Shrewsbury. You estimate your position, as
shown here.
You now need to assess your closing angle with your destination. As we have already seen, you can do this
by either estimating the angle using drift lines or by using the 1 in 60 rule.
You estimate your position to be 1 nautical mile right of track and your distance from Ludlow 17 miles. This
leaves you 32 miles to run to reach Hardin.
Using the 1 in 60 rule, your closing angle with Hardin is 1 over 32 times 60, which is 2
degrees for navigational purposes.
To calculate the required heading alteration, we will use the closing angle method that you have just learnt.
You have flown 17 miles along a track of 49 miles. That is, you have flown just over one third
of the leg distance.
You therefore take the reciprocal of one third, giving us 3, and multiply this by your closing angle of 2
degrees.
Therefore your change in heading to reach Hardin is 6 degrees, in this case to the left. Your new heading
then is 346 degrees, that is 352 minus 6.
Your next checkpoint is the disused airfield at Monford Bridge. This is 12 minutes along track, so you expect to
be there at 14.39.
Also, you now know that you are slightly right of track, so you will expect to see the airfield on
your left side.
Sure enough, the airfield appears on your left, but you get there at 14.38.
As the airfield lies within a mile or so of the halfway point, you can quickly revise your ETA by
2 minutes, from 14.53 to 14.51.
When you arrive at your next checkpoint at Ellesmere, you confirm that you are slightly to the right of the
lakes, which is what you expected with your new track, but you also note that instead of arriving 2 minutes
early at 14.42, you actually arrive at 14.41, 3 minutes early.
You estimate that Ellesmere is about two-thirds of the way along track. In order to find out how early
you will be at your destination, you invert the fraction to 3 over 2 and multiply by the number of
minutes early. This gives you 4.5 minutes.
Your revised ETA is therefore your original ETA of 14.53 minus 4.5 minutes, making 14.48 30 seconds.
Clearly, your ground speed is higher than you had planned, so you decide to call Hardin shortly after passing Ellesmere
to ensure they have plenty of warning of your impending arrival.
When you get your joining instructions, you are told that the active runway is runway 23 and that the surface
wind is 190 degrees, 15 knots. From this, you calculate that the crosswind for landing will be 11 knots, which
is within your limits, so you join the circuit and land. After checking in with air traffic, you go for
a well-earned cup of coffee.
Most ground stations in the aeronautical mobile service are equipped to take automatic bearings of an aircraft's VHF transmissions in
the frequency range 118 to 137 MHz. The ground installation comprises a suitable aerial and a cathode ray tube display
as shown here.
Information on stations providing VHF direction finding can be found in the United Kingdom Air Information Publication and documents such
as Pooley's Flight Guide.
The aerial is vertically polarised and has an array of vertical elements arranged in a circle. This is known as
an adcock aerial.
The equipment resolves the bearing from the transmissions received at each aerial in the array. The bearing is then displayed
on a cathode ray tube. Hence, the system is sometimes known as Cathode Ray DF, CRDF.
The latest high resolution equipment uses Doppler principles to determine the bearing and the bearings may be displayed as digital
readouts with an accuracy of plus or minus 0.5 of a degree, UHF or VHF.
The bearing information displayed on the cathode ray tube can be displayed in several ways depending on the switch selection
on the front of the display.
There are four principle types of display. The two which are most commonly used are
QDM. This is the aircraft's magnetic heading to steer to reach the ground station. Note that it does not take
wind into account. It's used mainly for homing to an aerodrome and for letdowns in poor weather.
QTE. This is the aircraft's true bearing from the station and is frequently used in en route navigation to obtain
position lines which can be plotted on a chart. A minimum of two position lines is required for a fix.
QUJ. This is the aircraft's true track to the station.
QDR. This is the aircraft's magnetic bearing from the station and it's used for en route navigation.
A QTE or a QDR can be used to check the true or magnetic tracking from a VDF station or
obtain en route position lines. A QDM can be used to home to a VDF station or
to let down in cloud using published procedures.
As VDF utilises the VHF band or UHF as required, the range will obey the line of sight formula.
Intervening high ground will limit range, especially for low flying aircraft in hilly terrain.
The power of airborne and ground transmitters will limit ranges. Gradients of temperature and humidity can give greater than line
of sight ranges.
Synchronous transmissions by two or more aircraft will cause momentary errors in the bearings.
VDF bearings are classified as follows.
Class A accurate to within plus or minus two degrees.
Class B accurate to within plus or minus five degrees.
Class C accurate to within plus or minus ten degrees.
Class D accurate to greater than ten degrees.
VDF bearing information will only be given when conditions are satisfactory.
Normally, no better than Class B bearing will be available.
The operator assesses the accuracy of the bearings and passes a classification to the pilot.
For example, your true bearing is 060 Class Bravo.
Propagation error and sight error are caused by the aircraft's transmissions being reflected from the ground as they travel to
the VDF site or being reflected from buildings at the site.
As the VDF system and VHF communications are vertically polarised, the attitude of the aircraft can affect accuracy.
The best reception and results will be obtained if the aircraft is flying straight and level.
Poor accuracy will result if the aircraft is in the overhead of a VDF receiver, particularly with the later stopper
systems.
The reception of both direct wave and ground reflected wave can cause signal fading or loss, although this phenomenon is
usually short lived.
Together with other multipath signals, this can give rise to bearing errors.
This summarises the main features of ground direction finding.
Automatic direction finding, ADF equipment in the aircraft, is used in conjunction with the Simple Low and Medium Frequency Non
-Directional Beacon, NDB, on the ground to provide an aid for navigation and for non-precision approaches to airfields.
The NDB transmits vertically polarised radio signals in all directions in the low frequency and medium frequency bands.
When an aircraft's ADF is tuned to an NDB's frequency and the NDB's call sign is identified, the direction of
the NDB will be indicated.
A cone of silence exists overhead the NDB transmitter, during which the aircraft does not receive any signals.
The diameter of the cone increases with aircraft height.
The ADF measures the bearing of an NDB relative to the fore and aft axis of the aircraft.
If a loop aerial is placed in the plane of the transmitted radio frequency, a voltage will be generated in
the vertical elements of the loop, because of the phase difference of the wave in each of the vertical elements.
As the loop is rotated, the voltage induced will decrease until it becomes zero, when the loop is perpendicular to
the radio wave.
As the loop continues to rotate, a voltage will be induced in the opposite sense, etc.
The polar diagram formed is a figure of eight, as shown here.
You can see that there are two null positions, and that by rotating the loop until a null is reached,
the direction of the beacon can be determined.
This is fine if the approximate direction of the beacon is known, but if it is not, then there are
two possible null positions.
However, if the equipment is to determine the direction of the NDB automatically, then with only the single loop, it
would have an insoluble problem.
To resolve this ambiguity, a simple dipole aerial, called a sense aerial, is added.
The polar diagram of the sense aerial is circular.
The currents generated are combined electronically, as if the sense aerial was in the middle of the loop aerial.
The relative signal strengths of the two signals are shown.
It is so arranged that the field from the sense aerial is in phase with one element, in this case
the left-hand element of the loop aerial shown here.
The resultant polar diagram is known as a cardioid.
The cardioid has a single null, which as can be seen is ill-defined, and would not in itself provide
an accurate bearing.
However, the correct null in the loop aerial can be defined by introducing a logic circuit, which defines the correct
null as being that null in the loop aerial, which, when the loop aerial is rotated clockwise, produces an increasing
signal strength in the cardioid.
Nevertheless, the resultant null with a single cardioid is not precise enough to meet the IKO accuracy requirement of plus
or minus 5 degrees.
To meet the accuracy requirements, the polarity of the sense aerial is reversed to produce a right-hand cardioid.
Then, by rapidly switching, about 120 Hz, between the two cardioids, the null is more precisely defined, and hence the
accuracy improved.
In reality, it is not feasible to have a rotating loop outside the aircraft, so the loop is fixed and
has four elements, two aligned with the fore and aft axis of the aircraft, with the other two perpendicular to
the fore-aft axis as shown in this diagram.
The electrical fields are transmitted to a similar four elements in a goniometer, reproducing the electromagnetic field detected by the
aerial.
The signal from the sense aerial is also fed to the goniometer, where a search coil detects the unambiguous direction.
The resultant is fed to an instrument in the cockpit, which indicates the bearing of the NDB aerial relative to
the fore and aft axis of the aircraft.
The allocated frequencies for NDBs are 190 to 1750 kHz in the low frequency and medium frequency bands.
Since the mode of propagation used is surface wave, most NDBs will be found between about 250 and 450 kHz.
There are two types of NDB in current use.
Locator – L
These are low-powered NDBs, used for airfield or runway approach procedures, or are co-located with, and supplement, the
outer and middle markers of an ILS system.
They normally have ranges of 10 to 25 nautical miles, and may only be available during an aerodrome's published hours
of operation.
Enroute NDBs
These have a range of 50 nautical miles or more, and, where serving oceanic areas, may have ranges of several
hundred miles.
They are used for homing, holding, enroute, and airways navigation.
The aircraft equipment comprises a loop aerial, a sense aerial, a control unit, a receiver, a display, which is either
a relative bearing indicator, RBI, or a radiomagnetic indicator, RMI.
The NDBs have a two- or three-letter identification, and there are two types of emission.
N0N-A1A
N0N-A2A
The N0N part of the emission is the transmission of an unmodulated carrier wave, which would not be detectable on
a normal receiver, so a beat frequency oscillator is provided on ADF equipment.
The beat frequency oscillator, when selected, produces an offset frequency within the receiver, which, when combined with the received frequency,
produces a tone of, say, 400 or 1,020 Hz.
The A1A part is the emission of an interrupted, unmodulated carrier wave, which requires the beat frequency oscillator to be
on for aural reception.
The A2A is the emission of an amplitude modulated signal, which can be heard on a normal receiver. Hence, when
using N0N-A1A beacons, the beat frequency oscillator should be selected on for manual tuning, identification, and monitoring.
N0N-A2A beacons require the beat frequency oscillator on for manual tuning, but off for identification and monitoring. The beat
frequency oscillator may be labelled tone, or tone voice on some equipment.
The information may be presented on a relative bearing indicator, RBI, or a radiomagnetic indicator, RMI. In both cases, the
information being presented is relative bearing.
Note that on the RMI, the ADF indicator is the double yellow needle. The single yellow needle is the VOR
indicator, which is covered in the next lesson.
The RBI has a standard compass rose, where 360° is aligned with the fore and aft axis of the aircraft.
On some RBIs, the compass rose can be manually set to the aircraft's heading.
When this is done, the needle shows the magnetic bearing of the NDB. On both the RBI and RMI the
head of the needle always points to the beacon, the QDM, and the tail of the needle gives the QDR.
In the case of the RMI, the information is still relative. However, the RMI compass card shows the magnetic heading
of the aircraft, and the needle shows the magnetic bearing of the NDB.
NDBs are used in a number of ways. En route navigational bearings. Bearings from two NDBs are one way of
fixing your position, homing to or flying from the NDB when maintaining airway centre lines. The use of NDBs in
airways is becoming less common as they are replaced by more advanced and accurate equipment.
Holding overhead at an assigned level in a racetrack pattern. NDBs are still widely used by aircraft waiting to approach
a busy airport.
Runway instrument approach procedures. In this context, NDBs are used to position an aircraft for a runway instrument approach.
The designated operational coverage, or DOC, defines the maximum range from an NDB at which you will receive the required
level of bearing accuracy, which is plus or minus 5 degrees by day only.
DOCs are typically anywhere between 15 and 50 nautical miles. Beyond the DOC you will experience a gradual increase in
bearing errors.
The following are brief descriptions of the errors to which an ADF is prone.
Precipitation static is generated by the collision of water droplets and ice crystals with the aircraft. This causes a wandering
needle and a background hiss in the audio.
The powerful discharges of static electricity in thunderstorms will cause significant bearing errors in the ADF. For this reason caution
must be exercised when using the ADF in the vicinity of thunderstorms.
At night the characteristics of the atmosphere change with respect to the transmission of radio waves. The effect is most
marked around dawn and dusk and is characterised by needle hunting and audio fade.
Due to the number of stations in the medium frequency and low frequency bands there is the possibility of interference
between stations which are on or near the same frequency.
By day the DOC will afford protection from interference. By night the interference can be experienced even within the DOC.
Identification of an NDB should therefore be carried out with particular care at night.
Mountainous areas can cause reflections and diffraction of the transmitted radio waves to produce errors in ADF systems. These errors
will increase at low altitude and can be minimised by flying higher.
Radio waves speed up over water compared to over land. This leads to a bending of the waves as they
cross the coast which leads to errors in the indicated bearing.
The theoretical polar diagram of the loop aerial is distorted by the airframe which produces a strong electrical field along
the aligned fore and aft axis of the aircraft.
Incoming NDB signals are thus refracted towards the fore and aft airframe axis. This is known as quadrantal error. The
angle of bank during a turn causes a current to be induced in the horizontal elements of the loop thereby
leading to a bearing error which is referred to as dip error. This error is only present when the aircraft
is not in level flight.
Finally, it's important to realise that false indications due to a failure in the system are not readily detectable because
of the absence of failure warning on most ADF instruments. Particular care should therefore be exercised in identifying and monitoring
the NDB and independent cross checks made with other navigational aids where possible.
The major factors which affect the range of NDB and ADF equipment are listed here. The range is proportional to
the square of the power output. That is, to double the NDB range you must quadruple the power output of
the transmitter. All precipitation, including falling snow, reduces the effective range and accuracy of ADF bearings.
N0NA1A NDBs have greater ranges than N0NA2A. Quality of the ADF receiver
The main features of the ADF and NDB are summarised here.
The VHF omnidirectional range, or VOR, was adopted as the standard short-range navigation aid in 1960 by IKO.
It produces 360 radials at 1 degree spacing, which are aligned in relation to magnetic north at the VOR location.
It is practically free from static interference, and is not affected by sky waves, which enables it to be used
day and night.
When the VOR frequency is paired with a co-located distance measuring equipment, or DME, an instantaneous range and bearing
fix is obtained.
The equipment operates within the frequency range of 108 to 117.95 MHz.
A VOR has the following uses. Marking the beginning, the end and centreline of airways or sections of airways.
As a let-down aid at airfields using published procedures.
As a holding point for aircraft.
As a source of en route navigational position lines.
An aircraft's VOR receiver measures the phase difference, the angular difference, between two signals from the VOR transmitter.
The first is a 30 Hz frequency modulated omnidirectional reference signal, which produces constant phase regardless of a receiver's bearing
from the VOR, and...
The second is a 30 Hz amplitude modulated variable phase directional signal created by the rotating transmission pattern. This is
called a Lemacon.
The 30 Hz FM reference signal is synchronised with the 30 rpm rotational directional AM signal, the Lemacon. This diagram
shows how.
The two 30 Hz modulations are in phase to an aircraft's VOR receiver when the aircraft is due north, magnetic,
of the VOR beacon.
And the phase difference measured at any other point will equate to the aircraft's magnetic bearing from the VOR.
The two 30 Hz signals are modulated differently to prevent interaction and merging at the aircraft's receiver.
The rotating Lemacon polar diagram, which provides the directional information, is created by electronically combining the polar diagrams of the
rotating loop and reference signal.
There are three main components of the VOR equipment in the aircraft.
For slower aircraft, the aerial is a whip type fitted on the fuselage, and for high-speed aircraft it's a
blade type or is flush-mounted on either side of the vertical fin.
The receiver is a box fitted in the avionics bay.
Information derived from the VOR signal received at the aircraft may be fed to a flight director system or to
the more simple displays, such as the RMI, the radio magnetic indicator, or the course deviation indicator.
A radial is a magnetic bearing from a VOR as shown here. In this diagram, the tail of the needle
on the RMI indicates the radial that the aircraft is on, and the head of the needle indicates the magnetic
track to the VOR.
In this diagram, the instrument is a course deviation indicator, or CDI. It displays VOR information and is widely used
in light aircraft.
The instrument indicates the displacement of the aircraft with respect to a bearing, to or from the VOR station, which
the pilot has selected on his omni-bearing selector, the OBS.
This displacement, or deviation, is presented by left-right movement of the course deviation indicator.
This diagram shows that the displacement of the indicator depends on the angular position of the beacon relative to the
selected bearing, and is independent of the way the aircraft is pointing.
In other words, for a given position and bearing selection, the heading of the aircraft does not affect the display
on the deviation indicator.
The various types of VOR are shown here.
A broadcast VOR gives weather and airfield information between beacon identification. A Doppler VOR has improved bearing accuracy.
A terminal VOR has low power, and is used at major airfields.
A VHF omnidirectional test, VOT, is found at certain airfields and broadcasts a fixed omnidirectional signal for a 360 degree
test radial.
This is not for navigation use, but is used to test an aircraft's equipment accuracy before IFR flight.
Co-located VOR and TACAN beacons. TACAN stands for Tactical Air Navigation, and is military equipment fulfilling a similar function
to DME.
The higher the transmitter power, the greater the range. Thus en route VORs with a 200 watt transmitter will have
about a 200 nautical mile range, and a terminal VOR will normally transmit at 50 watts to give a range
of about 50 nautical miles.
The transmitter and receiver height will also have an effect on the operational range of VOR, as the transmissions give
line of sight ranges, plus a slight increase due to atmospheric refraction.
Range can be assessed by using the formula shown here.
Uneven terrain, intervening high ground, mountains, man-made structures etc. cause VOR bearings to be stopped, screened, reflected or bent,
scalloping, all of which give rise to bearing errors.
To guarantee no interference between beacons with the same frequencies, known as co-frequencies, among the 160 frequencies available worldwide,
it would be necessary to separate co-frequency beacons by at least twice their anticipated line of sight range.
For example, an aircraft at a height of 25,000 feet, with a VOR situated at mean sea level, the
reception range would be 396 nautical miles.
Transmitter power, propagation paths and the degree of co-frequency interference protection required, necessitate co-frequency beacons to be separated
for planning purposes by an extra 100 nautical miles to about 500 nautical miles.
In practice, a beacon is protected as far as is deemed necessary, and this is not always the anticipated line
of sight reception range.
In the United Kingdom, this protection is denoted by a DOC, specified as a range and altitude.
For example, a DOC of 50 over 25,000, published in AIPs, means that an aircraft should not experience co
-frequency interference within 50 nautical miles of a VOR beacon, up to a height of 25,000 feet.
The DOC may also vary by sectors, and it's valid day and night.
Use of a VOR outside its DOC can lead to navigation errors, and details are published periodically in Air Information
Circulars.
Sight error is caused by uneven terrain, such as hills and man-made structures, trees and even long grass in
the vicinity of the transmitter.
The error to radiated bearings is termed VOR coarse displacement error.
Ground VOR beacon sight error is monitored to plus or minus one degree accuracy.
Propagation error is caused by the fact that having left the VOR sight with plus or minus one degree accuracy,
the transmissions are further affected by terrain and distance.
At considerable range from the VOR, bends or scalloping can occur.
VOR scalloping is defined as an imperfection or deviation in the received VOR signal.
It causes radials to deviate from their standard track, and is the result of reflections from buildings or terrain.
It causes the coarse deviation indicator to slowly or rapidly shift from side to side.
Airborne equipment errors are caused by aircraft equipment assessing and converting the phase differences to one degree of bearing.
Maximum aircraft equipment error should be plus or minus three degrees.
The above errors are aggregate errors to give a total error of plus or minus five degrees.
In addition, there is pilot error, which is due to the fact that as an aircraft approaches the VOR, the
one degree radials get closer together, and greater flying accuracy is required to maintain a given radial.
For more information, visit www.fema.gov.
Distance measuring equipment, or DME, enables a pilot to establish an accurate distance measuring equipment, or DME.
The complete DME system comprises a ground station, often co-located with a VOR beacon, and an onboard receiver and
data display.
As well as giving the distance from the beacon, the DME display may also show an aircraft's ground speed, and
the time required to arrive overhead the beacon.
The ground speed shown is meaningful only when the aircraft is tracking directly to or from a station.
The time required readout is meaningful only if the aircraft is tracking directly to the station.
By obtaining magnetic bearings from a VOR beacon, and range readings from a co-located DME, the pilot receives an
accurate fix, giving him his exact position.
The picture on the screen shows an aircraft approaching the Daventry VOR DME ground station on the 180 degree radial.
The pilot can read from his DME indicator that the distance of his aircraft from the ground station is 30
nautical miles,
that the aircraft's ground speed is 120 knots, and that the time required to reach the station is 15 minutes.
If the onboard DME equipment has no ground speed readout, an aircraft's ground speed can nevertheless be easily calculated when
flying directly to or from a VOR DME ground station.
The pilot simply times the interval between two DME range readings and carries out a mental calculation.
For instance, if in a time interval of one minute the aircraft has travelled 1.5 nautical miles directly towards
a beacon,
the ground speed of the aircraft will be 60 times 1.5, giving 90 nautical miles per hour, that is,
90 knots.
As signals from a DME ground station are propagated in all directions, the DME position line is circular as shown
here.
When en route, therefore, a pilot can obtain an accurate position fix by ascertaining the distance of his aircraft relative
to two separate DME stations.
DME operating frequencies fall within the UHF range of 962 to 1213 MHz.
These UHF frequencies, however, are not directly selectable. Your DME control unit will have an associated VHF frequency selector.
DME frequencies are most commonly paired with VOR frequencies, so when you select a VOR frequency on your VHF nav
box,
you automatically select the frequency of any co-located DME station.
DME is also paired with localizer frequencies of instrument landing systems.
DME measures the slant distance of an aircraft from the ground station. It is therefore a line of sight facility.
Though the slant distance from a beacon is slightly greater than the horizontal distance, in practice,
the two distances are near enough to each other for the difference to be negligible for the purposes of aircraft
navigation.
For instance, an aircraft at 3,000 feet at a distance of 15 nautical miles from a DME ground station
would be at a height making an angle of only 2 degrees with the ground at the DME beacon.
At this angle, the difference between the horizontal distance and the slant distance of the aircraft from the beacon
would be only one hundredth of a nautical mile. Obviously, such a small difference can be ignored.
When the aircraft is overhead the ground station, the horizontal distance will of course be zero,
while the DME indicator in the cockpit will still indicate 3,000 feet, or about half a nautical mile.
This is the position of greatest error, but as you will appreciate, half a nautical mile is also a negligible
value.
If the DME station is at the airfield of destination, visual contact will have been made long before half a
nautical mile to run.
If you are using the station as an en route overhead fix, knowing your altitude will be enough for you
to calculate the overhead reading.
DME works on the secondary radar principle. This is explained in a separate lesson.
Here, it is sufficient to say that the DME ground station is fitted with a type of transponder that responds
to interrogation signals from aircraft tuned to the correct frequency.
An aircraft sends out an interrogating pulse and the ground station transponder responds by retransmitting a strong answering signal which
is received in the aircraft.
The DME equipment onboard the aircraft converts this signal using simple distance, speed and time formulae into a digital readout
of distance in nautical miles.
DME transmissions obey the line of sight rule. Thus, the higher the aircraft and the ground beacon, the greater the
theoretical reception range.
The formula for calculating the maximum line of sight range is shown here.
Intervening high ground means that in order to receive DME readings, the aircraft will have to be at a higher
altitude than that suggested by the maximum range formula.
Range errors may also be caused by co-channel interference from two or more ground beacons having the same frequency.
A designated operating area is published for all DME stations in which co-channel interference is unlikely to be experienced.
Further information on DME ground stations in the UK can be found in the appropriate section of the UK AIP.
DME is very accurate. The total system error for DME should be no more than plus or minus 0.2
of a nautical mile for recently manufactured aircraft.
For older aircraft registered before the 1st of January 1989, the error is 0.25 of a nautical mile plus
1.25% of the range.
Slant error is only of any significance when the aircraft range is less than three times its height above the
beacon.
The development of space-based navigation systems commenced in the 1950s with the establishment of the USA Transit system.
The current generation began development in the 1970s, and the next generation is already under development.
It's intended that Global Navigation Satellite Systems, or GNSS, will eventually replace all terrestrial radio navigation facilities.
However, despite the United States' assertions that this is imminent, it's unlikely to be achieved in the foreseeable future.
The current systems have brought a new dimension of accuracy to navigation systems, with the precision measured in meters,
and where special differential techniques are used, the potential is for accuracy substantially less than one meter.
At present, there are two operational GNS systems. In addition, enhancements of the existing systems are under development, as is
a European system.
These systems are the Navstar Global Positioning System, which is operated by the USA.
The Global Orbiting Navigation Satellite System, operated by Russia.
There are some doubts about the long-term viability of this system because of the economic situation in Russia.
Local Area Differential Global Navigation Satellite System, which provides improved accuracy and integrity to aircraft-making airfield approaches.
Wide Area Differential Global Navigation Satellite System, of which the European Geostationary Navigation Overlay System, EGNOS,
currently under development, is a European contribution to a global augmentation system providing integrity and improved accuracy.
The European Galileo Project, which is at present a feasibility study, but is intended to start becoming operational in 2008.
The principal reason the Europeans are developing their own system is one of internal security.
Since access to the full GPS or GLONASS facilities is outside European control.
It can be anticipated that other countries or regions may develop their own systems for the same reason.
Johannes Kepler's three laws quantify the mathematics of planetary orbits which apply equally to the orbits of satellites.
His first law states that a satellite's orbit describes an ellipse with the Earth at one of the foci.
His second law states that a satellite sweeps out equal areas in equal time.
And his third law states that the square of the satellite's orbital period is proportional to the cube of its
average distance.
Using these laws, and given a starting point, the satellites, or space vehicles as they are normally called, calculate their
positions at all points in their orbits.
A space vehicle's orbital position is known as ephemeris.
When you switch on your GPS in your aircraft the receiver knows from its internal clock and memory which space
vehicles it can expect to see.
It then constructs a three-dimensional fix using information from four of these vehicles.
The following diagrams will give you an idea of what is involved in this process.
Each range corresponds to a position somewhere on the surface of a sphere with a radius in excess of 10
,900 nautical miles as shown here.
However, we could be anywhere on the circumference of the sphere.
We can slightly improve the situation by using two range spheres to give a circular position line.
The introduction of a third range sphere will produce two positions several thousand miles apart.
One position will be on or close to the surface of the Earth.
The other position will be out in space, so it would be possible to use just three spheres to produce
a position by rejecting the space position.
However, a fourth sphere is needed because of the way the receiver compensates for time errors.
The details of this are outside the scope of this lesson.
Suffice to say that the use of four space vehicles provides an accurate three-dimensional fix,
which can be readily transposed into latitude and longitude or any other Earth reference system.
The GPS comprises three segments.
The space segment, the control segment, and the user segment.
GPS time is measured in weeks and seconds from 0000 UTC on 22 August 1999.
The time restarts at zero after an epoch, which is 19.7 years, or 1024 weeks.
GPS time is referenced to UTC, but does not run in direct synchronisation,
so time correlation information is included in the space vehicle broadcast.
The accuracy of GPS is of the order of five metres.
However, it has a variety of errors, most of which are compensated for within the system.
Ephemeris errors are errors in the space vehicle's calculation of position,
caused by the gravitational effects of the sun, moon and planets, and solar radiation.
The vehicle's position is checked every 12 hours, and where necessary, updated.
As with ephemeris, the space vehicle clock is checked at least every 12 hours,
and any error passed to the vehicle.
The ionosphere affects radio waves of all frequencies,
the actual effect being dependent on the frequency.
The space vehicle broadcast contains a model of the ionosphere,
which is used to make time corrections to the measured time interval.
The state of the ionosphere is continuously checked at the monitoring stations,
and the model is updated to the space vehicle at least every 12 hours.
This is the most significant of the errors in space vehicle navigation systems.
If two different frequencies aren't available,
then the space vehicle can deduce the ionospheric effect on the transit of the signals
and make appropriate adjustments to the observed times, thereby minimising the error.
Because of the inherent accuracy of space vehicle navigation systems,
the effect of variations in tropospheric conditions on the passage of radio waves has become significant.
Variations in pressure, temperature, density and humidity affect the speed of propagation.
For example, a change in transit time of one nanosecond gives an error of 0.3 of a metre.
As with ionospheric propagation error, this is minimised with the use of two frequencies.
All radio receivers generate internal noise, which can cause errors in the measurement of the time difference.
Reflections from the ground and parts of the aircraft result in multipath reception.
This can be minimised by careful sighting of the aerial and my internal processing techniques.
Geometric dilution of precision is the term for a poor cut between position lines.
GDOP occurs when the space vehicles are relatively close to each other.
Errors caused by GDOP are minimised by the geometry of the positioning of the space vehicles in their orbits
and by the receivers selecting the four best space vehicles to determine position.
The space vehicle geometry that will provide the most accurate fixing information
is one space vehicle directly overhead the receiver
and the other three space vehicles close to the horizon and spaced 120 degrees apart.
Aircraft manoeuvres may result in part of the aircraft shadowing one or more of the in-use space vehicles.
There are two possible outcomes of this.
Firstly, whilst the space vehicle is shadowed the signal may be lost resulting in degradation of accuracy
or the receiver may lock on to reflections from other parts of the aircraft, again with a reduction in accuracy.
The effect of manoeuvres can be minimised by careful positioning of the aerial on the aircraft.
Air traffic control services use ground radars extensively to serve a large number of requirements and users.
The most common radar systems used by air traffic control are listed here.
These are long-range radars, typically 200 to 300 nautical miles,
which are used for airway surveillance to provide air traffic control with the range and bearing
of all aircraft flying in airways within their area of responsibility.
Additional information is provided by secondary surveillance radar which is covered in the next lesson.
This diagram shows the locations and coverage of the London Area Control Centre
and Scottish Area Control Centre radars superimposed on a map of the United Kingdom Airways structure.
These radars have a range of up to 75 nautical miles and are used for controlling traffic in terminal areas.
Again, additional information is provided by secondary surveillance radar.
In the United Kingdom, horizontal radar separation minima may be reduced to 3 nautical miles within 40 nautical miles,
or in certain circumstances 60 nautical miles, of the radar head
and below FL245 where the procedure has been officially approved.
These are short-range radars providing positional information up to 25 nautical miles.
They provide positional information and control of aircraft approaching an aerodrome,
radar vectoring to the ILS final approach, surveillance radar approach, or SRA.
The PAR is a runway approach aid.
The system has two antennae which provide the controller with a very accurate electronic glide path.
Glide path and azimuth information is displayed on two screens positioned one on top of the other.
The pilot is talked down by the controller to his decision height.
This gives you a plan view of the PAR procedure.
When the pilot has been handed over to the PAR controller and established radio contact,
the controller then maintains a continuous flow of instructions to the pilot.
For example,
You are 3.5 nautical miles slightly above the glide slope. Increase your rate of descent.
Turn left 3 degrees heading 272.
Ranges are usually given every mile until 4 nautical miles from the runway threshold,
and thereafter at every half nautical mile.
The talk down terminates at half nautical mile from the threshold.
The PAR procedure has been replaced by the ILS at most United Kingdom civil aerodromes
and is now only available at certain military airfields.
Airports Surveillance Detection Equipment, also known as Airfield Surface Movement Indicator,
is installed at major airfields.
It provides a very accurate radar display in all weathers and visibilities
of the aerodrome infrastructure, taxiways, runways, aprons, etc.
It shows vehicular traffic and aircraft that are stationary, taxiing, landing,
or taking off.
The radar is designed to provide a detailed bright and flicker-free display
of all aircraft and vehicles on runways and taxiways,
so that air traffic control officers can be certain that runways are clear of traffic
before landings or take-offs.
They can also ensure the safe and orderly movement of traffic on taxiways.
Electronic processing can remove selected fixed features,
leaving targets on runways and taxiways and other movement areas
clearly visible and free of ground clutter.
Primary radar relies on the reception of a reflected pulse,
in other words, the echo of the transmitted pulse.
Secondary radar, on the other hand, receives pulses transmitted by the target
in response to interrogation pulses.
Secondary surveillance radar is one of several types of secondary radar system.
Both primary and secondary surveillance radars are used to track the progress of an aircraft.
Primary radar provides better bearing and range information of an aircraft than SSR,
but its biggest disadvantage is the lack of positive individual aircraft identification.
This is required for safe operations by air traffic control, particularly in crowded airspace.
Primary radars also require higher transmitter power outputs
for the two-way journey of the single pulses.
Secondary surveillance radar requires an aircraft to be fitted with a transmitter receiver,
called a transponder.
The pilot sets a four-figure code allocated by air traffic control,
and the transponder transmits information automatically in pulse-coded form
when it's interrogated by the ground station.
The transmissions are therefore only one way, from transmitter to receiver.
The ground station transmits and interrogates on 1,030 MHz and receives on 1,090 MHz.
The aircraft receives on 1,030 MHz and transmits and transponds on 1,090 MHz.
The SSR ground antenna transmits in a narrow beam,
while the aircraft transmits omnidirectionally in a circular pattern around the aircraft.
A typical aircraft transponder control unit is shown here.
The aircraft is interrogated from the ground station by a predetermined series of pulses
on the carrier frequency of 1,030 MHz.
The aircraft transponder then transmits a coded reply on a carrier frequency of 1,090 MHz.
There are two main modes of operation.
The ON position selects mode alpha, which is an interrogation to identify an aircraft.
The ALT position selects mode Charlie,
which is an interrogation to obtain an automatic height readout of an aircraft.
To achieve this, the aircraft must not only have a transponder with mode Charlie,
but also a special altimeter which can pass information to the transponder.
The IDENT button is used, normally at the request of air traffic control,
to facilitate quick identification of the aircraft.
When pressed, it produces a distinctive display
so that the controller can pick out the aircraft easily from a crowded radar screen.
The STANDBY button turns the transponder off,
but keeps it in STANDBY mode so that it's available instantly when required.
The TEST button is for ground test use.
The controller will tell the aircraft both the mode and code to set.
In this example, the aircraft has set a code of 4635 and mode alpha.
And in this example, the aircraft has kept the same code, but changed to mode Charlie.
By international agreement, some codes are reserved for special purposes,
and should be selected as follows.
7700 indicates an emergency condition.
This code should be selected as soon as is practicable after declaring an emergency situation.
However, if the aircraft is already transmitting a discrete code,
and also receiving an air traffic service,
that code may be retained at the discretion of either the pilot or controller.
7600 indicates a radio failure.
7500 indicates unlawful interference with the planned operation of the flight.
In other words, a hijack.
2000 provides recognition of an aircraft which has not received any instructions
from air traffic control units to operate the transponder.
For example, when entering the United Kingdom airspace,
from an adjacent region where the operation of transponders has not been required.
Mode Charlie should be operated with all of the above codes.
This shows a typical radar display of aircraft in the London terminal area.
SSR has the following advantages over primary radar.
It requires much less transmitting power to provide coverage up to 200 to 250 nautical miles.
It's not dependent on an aircraft's echoing area or aspect relative to the radar transmitter.
It gives clutter-free responses as it doesn't rely on returning reflected pulses.
It positively identifies an aircraft's primary response by displaying its code and callsign alongside.
It indicates an aircraft's track history, speed, altitude and destination.
It can indicate on a controller's screen that an aircraft has an emergency,
has lost radio communications or is being hijacked.
Thus, when SSR is used in conjunction with primary radar,
the advantages of both systems are realised.
SSR has the following disadvantages.
Garbling is caused by overlapping replies from two or more transponders
on nearly the same bearing from the ground station
and within a distance of 1.7 nautical miles from each other,
measured on a line from the antenna.
Fruiting is interference at one interrogator caused by replies from a transponder
responding to interrogations from another.
There are only 4,096 identification codes available in Mode Alpha.
Ghost targets which are due to reflections from obstacles or high terrain.
Shielding occurs when the aircraft attitude shields the antenna, resulting in loss of signals.