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How a pilot plots his course and uses a compass to maintain it while in flight


SWINGING THE COMPASS OF AN AVRO ANSON. The metal in an aeroplane produces inaccuracies in the readings shown by its compass. These inaccuracies vary with the compass bearing in which the aircraft is pointing. They are corrected as far as possible by small magnets placed in the compass. The residual differences between compass bearings and correct magnetic bearings are termed deviation. Deviation readings are taken at bearings 45 degrees apart. This process is known as swinging the compass, and is carried out on a circular concrete base on which are marked true magnetic bearings.

THE first aeroplane pilots followed tracks upon the ground - roads, railways, canals or rivers. Claude Grahame-White even had a motor car running along the roads in the twilight with its headlamps switched on during his race towards Manchester against Paulhan in 1910.

In those days pilots considered it unwise to fly across country at more than about 500 feet. To fly higher than that, they said, interfered with their ability to follow the ground tracks.

To follow ground routes is still the simplest form of aerial navigation. Over some countries (such as the Argentine), where the roads or railway tracks run straight for perhaps hundreds of miles, this method of flying from place to place works well enough. One woman pilot, however, who used this method of flying across country in Great Britain could always find her way when flying north, but when flying south she was frequently lost. She said this was because the names of all the towns and villages were

then printed upside down on her map. Ability to fly across country - even by the simplest of all methods outlined above - requires an elementary knowledge of map reading. This is not difficult to acquire. Many people, having learnt geography at school, can read maps with some facility.

The aeroplane pilot needs to know how to choose his map. It should be between the limits of 4 and 14 miles to the inch and reasonably up to date. Good maps bear the last correction date.

For ordinary flights there are three essentials. Firstly, the map should incorporate the (north and south) lines of longitude as straight lines, and the (east and west) lines of latitude as straight or nearly straight lines. On maps of this kind a straight line drawn between two places not too far apart will show the desired track or route line (or something very near it), so that the pilot can fly on a compass course. Examples of such maps are Mercator, British Ordnance Survey, International 1/1,000,000 scale and aeronautical maps.

Secondly, the difference between true (or geographic) north and magnetic (or compass) north should be indicated.

Thirdly, the scale to which the map is drawn should be marked. The scale shows how many miles there are to the inch.

With this information the pilot can measure beforehand the course he has to fly and how long it will take him to cover the distance between two places on the map.

For simple navigation the details of map construction (as, for example, the kind of projection) may be ignored. Projection means the method by which the curved surface of the Earth is depicted on a flat sheet. Something has to be warped or distorted. Each projection has some fault. But if the map conforms to the three rules given above, that is good enough.


FIG. 1. SYMBOLS USED ON AVIATION MAPS. Some of these symbols are printed on the map in red and some in black. The spaces in between the parallel lines which represent roads are coloured yellow. Roads are shown less prominently and railways more prominently, than on road maps. Features which are most easily visible from the air are given the greatest prominence on an air map.

It is useful - and sometimes essential - to know the various symbols used on a map, for as the pilot flies over the ground he may see a church below, and he ought to be able to look at the map and identify it. The special aeronautical maps contain additional information required by the air pilot and navigator - such as aerodromes and prohibited areas. Fig. 1 shows a number of typical symbols.

For simple navigation the essential working instruments required for laying out the route are a rule, protractor, pair of dividers, pencil and eraser.

It is assumed that the pilot is going to fly from A to B - a distance of 500 miles. Between A and B he draws a straight line on the map. He observes whether the line runs over any prohibited or danger areas. (If he is not using an up-to-date aeronautical map, this information must be sought elsewhere, for prohibited and danger areas must be avoided.) If the line crosses either prohibited or danger areas the route must be altered to pass round them. In this event the pilot draws a straight line on one side of any such area, and then connects either end of this line with A or B respectively. Then he erases the original line.

If there are any radio masts or hills on the route, he makes a pencilled cross upon the line and enters a note upon the border of the map opposite the cross as a quick reminder of what the cross represents.

If the flight is over more than one country there may be some special frontier corridors over which the aeroplane must pass. This may require a modification of the route line. If such a requirement does exist, the pilot notes upon the border of the map the height at which the frontier must be crossed. Different countries impose different rulings - even on either side of a common frontier.

The pilot now knows that his route from A to B avoids all prohibited and danger areas, and complies with all frontier regulations. In addition he has observed any special places along the route where care is needed.

The instrument by which he will follow this route is the compass. The air compass reads from 0° to 360°. The course is the direction in which the pilot flies. It is an angle, not a distance, and for air navigation is measured to the right, or clockwise, from the true north line; the complete circle of 360° is used.

THE BEARING of a route to be followed is measured clockwise from a meridian of longitude

FIG. 2. THE BEARING of a route to be followed is measured clockwise from a meridian of longitude. It is the angle this meridian makes with a line running in the direction in which the route is to be flown.

FIG. 3 shows how a side wind would blow a machine off its course if it were headed straight along the desired route.

To average out any distortion on the map the track or route line angle should be measured in the following way (see Fig. 2). On the pencilled line between A and B - or between each straight section of the line if a constant course cannot be flown - find the north and south longitude line (meridian) that cuts it nearest to the halfway point. With the protractor centred where this meridian cuts the route line, measure the angle between the two. This angle is the true (or geographical) course. It would be also the compass course to be flown if it were not for a number of inconstant factors that have to be taken into consideration.

The inconstant factors are three in number: variation - the angle between true north and magnetic north; deviation - the angle of error in the compass itself; and drift, or wind allowance. The first two are known (1) from the map and (2) from the compass deviation card mounted in the aeroplane near the compass.

As the magnetic and true poles do not coincide but lie about 1,400 miles apart, the angle which is formed between them varies according to geographical position. Because the variation changes, it is necessary to examine the maps to find out what the variation is, not only at the start, but also along the whole line from A to B. If it is not the same throughout, the pilot must average out the variation or make allowance during the flight when the point is reached where a change in variation occurs. On short flights to average out is generally the better way. Variation may be nil, or either east or west, according to the pilot’s situation on the Earth’s surface.


DRIFT CORRECTION is illustrated in Fig. 4. By steering a course to windward of the desired track the aeroplane can be made to follow the desired track. Fig. 5, to the right, explains how the correct course to steer to allow for drift can be worked out on paper when the speed and direction of the wind at a desired height are known. The aeroplane must be flown the whole time at the height chosen, because wind strength and direction vary with height.

To find the magnetic course from the true course westerly variation is added and easterly variation is subtracted.

Deviation is the name given to inaccuracies in the action of the compass. These inaccuracies are produced by local magnetism in the aeroplane. The extent of the inaccuracy changes as the aeroplane’s head is turned round a circle. Readings taken at, every eighth part of the circle are expressed in degrees east or west of magnetic north, and are recorded on the deviation card.

To find the compass course from the magnetic course, it is necessary to make allowance for the deviation. The following doggerel rule serves as a mnemonic:

Deviation west, Compass best.

Deviation east, Compass least.


Magnetic Course 315°, Deviation 4° W, Compass Course 319°

Magnetic Course 315°, Deviation 4° E, Compass Course 311°

Variation is constant for every aeroplane; deviation is not. Both affect the course to be steered compared with the true course, but neither affects speed.

The third variant is produced by the direction and force of the wind. This may affect both the course to be steered and the speed (Figs. 3 and 4).


AN AIRCRAFT COMPASS. The ring carrying the degree markings and the four parallel lines can be turned to any position and then locked. It should be set so that the compass needle is parallel to the four lines when the aircraft is heading on the correct course. The deviation for the eight different compass points will be marked on the card on the front of the instrument.

If the wind is dead ahead or astern there will be no change in the angle to be steered. But if the wind blows from the left side of the aeroplane the steering angle must be decreased; if from the right side the angle must be increased. Wind always affects the speed.

If the wind strength and direction are known (from meteorological advice before the flight) the pilot can readily calculate the required change in angle and the alteration in speed.

It is assumed that the pilot knows that his desired track (or true course) from A to B is 315 degrees. The air speed at which he will fly is 150 miles an hour. He intends to fly at 6,000 feet. The meteorological office reports air conditions clear at that height with the wind blowing at 35 miles an hour from 180 degrees. From the pilot’s wind diagram (Fig. 5, above), he finds that the wind will increase his speed over the ground by 22 miles an hour. Now the pilot can complete all his calculations on the ground before the start, as follows:—

Flight from A to B

True course: 315°

Variation 12° west: 315 + 12 = 327 degrees

Deviation 3° west: 327 + 3 = 330 degrees

Wind - 9°: 330 - 9 = 321 degrees

Distance: 500 miles

Air speed 150 + windage 22 = 172 m.p.h. ground speed

Time for flight = 2 hours 54 minutes.

The pilot must therefore fly on a compass reading of 321 degrees. He should reach B two hours fifty-four minutes after having left A, provided that he flies at 6,000 feet and that no change in the force and direction of the wind occurs during the flight.

The pilot heads away from A on his calculated steering course of 321 degrees. Below and in front he sees the landmarks that lie along the trackline drawn upon his map. From them be can check that his steering course is taking him along the desired track (or true course) of 315 degrees.

As he travels he can check his speed. Pencil lines drawn across his track line on the map at regular intervals of, say, 20 miles (Fig. 6, below) enable him to place any conspicuous landmark on his route into a time relationship. For instance he knows (from his estimated speed of 172 miles an hour) that in twenty-eight minutes he should be able to see the railway junction that lies just to the right of his track, eighty miles from A. Two hundred and forty miles from the start he ought to see a small triangular lake on his left. That should be visible in less than 1 hour 24 minutes.

DISTANCE MARKS at regular intervals

DISTANCE MARKS at regular intervals (Fig. 6, far left) along the route on his map enable the pilot to place any conspicuous landmarks on his route into a time relationship, because he knows his ground speed frpm previous calculations. Fig. 7, to the right, illustrates how a course may be checked by two sights taken on a landmark near the route. The two sights are taken at points which are a known distance apart.

The lake gives him an excellent opportunity to check that his steering course is still correct. Distance is about halfway between A and B. If conditions are changing a position fix on the lake will disclose the change (Fig. 7, above). Sight 1 is taken when the line of sight is an extension of the level edge of the lake farthest from A.

Sight 2 is taken when the line of sight passes through the apex of the lake and continues through its centre and cuts the middle of the side farthest from the track of the pilot should make sights 1 and 2 at 230 and 240 miles from A respectively, this gives the pilot a time check for speed over the ground. Landmarks (if any) below him when the sight comes into line should correspond with the map. The time taken to fly from Sight 1 to Sight 2 should correspond with the time taken to fly 10 miles at 172 miles an hour, that is, 10/172 x 60 mins. = 3 mins. 29 secs.

If the landmarks below correspond with the map or (if there are no suitable landmarks) the time interval between the sights is correct, then true course and steering course are both correct and the wind strength and direction are unaltered. Checks of this kind enable the pilot to continue confidently over stretches of country where there are no prominent landmarks.

Sometimes the wind conditions may be known in advance to be different between A and B. In such conditions the pilot must keep a wary lookout for the place where the change begins to take effect.

If he can see behind comfortably, excellent rear bearings can be taken (even visually) on landmarks that lie on the track. While heading for a known landmark ahead, the angle between the tail of the aeroplane and the trackline can be observed. Any serious change caused by an alteration in the wind can be seen quickly in this way.

SoWORKING OUT A COURSE for a cross-country flightme people doubtless make better air navigators than others. Probably the reason is that the art of navigation calls for the application of a number of different human qualities - keenness of observation, quickness of thought, reasoned deduction and patience. Not every one has all these qualities in equal proportion.

WORKING OUT A COURSE for a cross-country flight. The pilot, on the left, is holding an automatic calculator which works out the course to be followed and also the air speed when the strength and direction of the wind are known. It avoids the necessity of working these out on paper in the manner illustrated in Fig. 5 above.

[From Part 14, published 7 June 1938]

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The Principles of Navigation