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Simple principles are incorporated in the instruments on which pilots rely

A dismantled turn indicator

THE PENDULUM DEVICE in the centre of this picture of a dismantled turn indicator operates the pointer which indicates sideslip. The lower pointer is controlled by the gyroscopic device in the left-hand bottom corner of the picture. Air is sucked through the nozzle shown to the right of the gyroscopic device and drives the gyroscope by impinging on buckets on the periphery of the wheel.

PILOTS now depend far more upon their instruments than they did in the early days of flying, because flying in bad visibility or in thick cloud is now common. Even when the visibility downwards is fairly good, the horizon may be obscured, and instrument aid will be necessary. In addition, it has been found that bodily sensations mean little: the wind on one side of the face or the other, the pressure of the cockpit against left shoulder or right shoulder, or the pull of a safety belt do not necessarily mean that the aircraft has taken up the attitude that might be deduced from these sensations.

Instruments do not always tell the truth, but they are much more truthful in an aircraft than the sensations communicated to a pilot by changing attitudes at a time when visibility is poor and no horizon is visible.

When the pilot seats himself in the cockpit the instruments on his dashboard will already be giving certain indications. If the engine is ticking over, the revolution indicator may show 800 revolutions a minute; the oil pressure gauge may be standing at 20 or 30 lb. to the square inch; the air speed indicator will be at zero unless the aeroplane is facing into a wind of more than 40 miles an hour; and the altimeter, or height indicator, unless adjusted to zero, will be standing at some figure determined by the height of the aerodrome above sea level and the barometer reading on that particular day.

If the aeroplane is standing on level ground the bank indicator will be at zero and, as no turn is being made, the turn indicator needle (the bottom one on the scale of the two-needles turn indicator) will also be at zero. As the tail of the aircraft is on the ground and the aircraft not in flying position, however, the pitch indicator will read some figure between 10 and 15 degrees, showing that the aircraft is in the attitude of climbing.

Assuming that the pilot is now ready to take off, the first indication he will require is that of his engine speed indicator. Either with chocks under the wheels, or with the brakes hard on, he opens his throttle to the full, and the engine speed pointer moves from 800 to a figure in the region of 2,200. The pilot knows the revolutions which would be given at full throttle on the ground if the engine were in perfect tune, and can rapidly obtain definite confirmation of this point before he takes off.

Normally, the aeroplane will be facing into the wind, and if it is also facing the open aerodrome instead of the buildings, the pilot can take off without turning. At one time, during the takeoff, a pilot would always fix his eyes firmly upon some distant point such as a tree or a steeple; but in modern flying the visibility may fail to provide him with some definite mark, and in taking off he might have to rely upon his turn indicator.

The chocks are pulled away, the pilot opens up his engine again and the aeroplane begins to move across the aerodrome. The pilot looks to see that he has a clear run and then, in anything but clear weather, keeps his eye upon the bottom needle of his turn and bank indicator, which tells him the moment that the aeroplane begins to swing to one side or the other; that is, out of the direct line of the wind.

For an ordinary take-off the pilot will have the assistance of his eyes in keeping the aeroplane on a straight course, and in lifting it off the ground when the air speed indicator has reached a satisfactory take-off speed. Experience with any particular type of aircraft provides a definite knowledge of the most suitable air speed for leaving the ground, for cruising, for climbing and for gliding down to make a landing.

It is unusual for pilots to watch the air speed indicator at the moment of landing, as there might be a risk that they would run into something or fly into the ground; but the speed at which the aircraft will cease to be airborne can be determined at a safe height by gliding with gradually reduced speed, until the aeroplane “falls out of the pilot’s hand”. This sudden dip down at low air speed is the stalling speed; it is also the speed at which the aeroplane wheels should meet the surface of the aerodrome.

Let us suppose that the pilot has attained a suitable air speed to take off and that the aeroplane has left the ground. The pilot will now begin to climb to the height at which his flight is to be made. If he is to remain on a straight course, still facing directly into wind, the next instrument to claim his attention will be the altimeter.

The mechanism of the altimeter is not much different from that of a barometer and so it responds to weather changes. Before taking off, the pilot will have adjusted it to a figure depending upon the nature of his flight. If he is going to land again on the same aerodrome, he will set the altimeter to zero; but if he intends to land upon a distant aerodrome the height of which he knows, it is safer to set the altimeter to that height, when it will read zero as soon as he again reaches ground level.

As the pilot climbs, the pointer will begin to move round the circle; when a certain height has been reached the needle begins to go round for the second time and it will be necessary to read on a second scale inside the first. While the aircraft is climbing, the engine revolutions will be at their climbing maximum. The air speed indicator will show a figure much lower than the maximum speed of the aircraft, and lower also even than its cruising speed, as the aeroplane is in effect going uphill. The altimeter will be showing a continuously increasing reading as greater height is attained, and the pitch indicator will show an angle of about 15 degrees or more, that is, the angle of climb.

When a pilot has reached a height with which he is content, movement of the control column brings the aeroplane back to a level keel: its nose is no longer above the line of the horizon. This indication is confirmed by the reading of the pitch indicator, which falls back to the zero mark. The air speed will be higher when the aircraft is flying level than when it was climbing. The altimeter needle will remain constant at the height of the aircraft above the aerodrome or above sea level, according to the figure to which it was set before the aeroplane left the ground.

Let us suppose again that the pilot does not make his first turn until he has reached the selected altitude. He has been flying directly into the wind in continuation of his take-off and climb, and it will now be necessary for him to turn on to the course which will bring him to his intended destination.

He therefore banks the aeroplane, puts on an appropriate amount of rudder and begins to turn. In making his turn, especially in conditions of bad visibility, he will be greatly assisted by the turn and bank indicator.

The bottom needle moves over a scale reading from 1 to 4 (see illustration at the top of this page). If the needle stands at 1, a slow turn is being made; if the needle were to remain there until a complete circle had been made, it would take as long as two minutes. Rate 2 means a much faster turn. At the most rapid rate on the scale, Rate 4, a complete circle would be turned in less than forty seconds. The top needle of the turn and bank indicator does not really show bank; either end of the scale is marked SIDE SLIP.

THE CONTROLLING HEAD of the air speed indicator

THE CONTROLLING HEAD of the air speed indicator is mounted on the aircraft clear of the propeller slipstream. It is visible under the nose of the aeroplane in the picture below. The illustration in the top left shows the insides of three air speed indicators. The instrument in front is of the capsule pattern; the other two are diaphragm types. A typical dial of an A.S.I. is shown also.

The scale is marked in degrees; if the aircraft were banked at 20 degrees, without any turn, the needle would stand at 20 degrees. Because of that bank, the aeroplane would be sideslipping downhill fairly rapidly, though the pilot could tell this by comparing the angle of his wings with the horizon line, if he could see it, or by the feel of the side wind on his face if no horizon were visible. The sideslip pointer is connected to a small pendulum; when there is no turn, the pendulum will hang pointing to the Earth and the needle will indicate the angle of bank.

Supposing now that the aeroplane begins to turn at a gradually increasing rate with a fixed degree of bank; as the speed increases, the little pendulum will tend to be flung outwards and upwards by the centrifugal force due to the speed of the turn. The greater the speed of the turn, the nearer to zero will the pointer stand; a speed is finally reached at which the pointer is standing at zero, although the aeroplane wings may be banked at 60 degrees. At that point, a perfect turn is being made; the downward pull of gravity on the little pendulum is exactly balanced by the upward and outward pull of centrifugal force; and there is no sideslip in either direction.

The Air Log

If the speed were now still further increased, centrifugal force would send the pendulum upwards until it stood above zero, and now the aeroplane would be sideslipping again, but this time upwards and outwards instead of downwards. If therefore the sideslip

needle stands to one side or the other of zero, the aeroplane is slipping in that direction. The attitude of the needle is approximately that of the control column.

On the dashboard of any light aeroplane there is a small clock; sometimes it may carry either an additional pair of hands or a small dial, which when set to zero at the beginning of a flight will record the exact duration of the flight.

It is rare to find such an instrument as the air log on the dashboard of light aircraft. The air log is in effect a clock which works only while the aircraft is moving at a speed above stalling speed. It therefore keeps a permanent visible record of the times of each trip, provided that the pilot remembers to set it to zero at the beginning of the flight.

When the pilot has sighted the aerodrome for which he is bound, he normally begins to reduce height by throttling back his engine and slightly depressing the nose of the aircraft. This descent under power may sometimes be many miles long; glides have been made, with the engine throttled back, as long as twenty or thirty miles.


INSTRUMENT LAY OUT OF THE PERCIVAL Q.6. At the top of the centre panel are the two engine starters, with air-intake controls below. Under these are the two throttles, with aileron trimmer on their left and elevator trimmer on their right. The oval-topped panel behind the control wheel carries a Sperry-Horizon and a directional gyro. Other instruments behind the control wheel are altimeter and A.S.I., with two engine revolution counters below them and a turn indicator at the bottom. The two levers to the right of the centre panel control the pitch of the propellers. Below these levers are the flap switch and flap-position indicator. At the top on the right are two air temperature thermometers for the engines, and below them a group of four petrol gauges. The clock is at the bottom right with two boost gauges on its left.

During this glide the air speed indicator will reach a slightly greater figure than cruising, as the aeroplane is in effect running downhill. The height will be constantly falling and the pitch indicator will be reading an angle below zero, showing the gliding angle of the aircraft.

On arrival over the aerodrome, the pilot turns the aircraft into the wind as shown by smoke flare or wind stocking, and glides down until he is sufficiently close to the surface of the aerodrome to land.

In that flight, every instrument on the dashboard has provided the pilot with useful and sometimes essential indications. It remains only to describe the construction of these instruments and to point out the occasions on which they may not tell the exact truth.

The liquid type of fore-and-aft level is extremely simple. There is a glass tube provided with a reservoir, the whole built in the form of a triangle. The glass tube is mounted within two “Bakelite” mouldings, which are screwed together, and a scale of translucent plastic material is mounted inside the glass of the front cover.

When the aeroplane is gliding, the liquid runs along the lower tube into the reservoir and so stands at a lower figure than zero in the front vertical tube; the greatest angle of climb or glide which can be recorded by this type of pitch indicator is 20 degrees.

When the aeroplane is climbing, fluid runs from the reservoir along the bottom tube and increases the reading in the front vertical tube.

Gravity Balanced by Centrifugal Force

A well-known type of cross-level or liquid sideslip indicator behaves exactly as the pendulum of the two-needles turn indicator, described above. With a perfectly banked turn the liquid will be horizontal in relation to two triangular marks shown on the dial. Its tendency to flow downhill under the influence of gravity is exactly balanced by the effect of centrifugal force tending to throw it uphill. As the liquid level does not therefore indicate in a turn the fact that the aeroplane is banking, it is not telling a truthful story of the bank indication.

A similar minor untruth occurs in the use of the liquid pitch indicator. We may assume that the aeroplane is gliding downhill at a fixed speed and that the pilot, by opening up his engine, suddenly increases the speed of glide, without altering the angle. The aircraft will be suddenly pulled forward and the liquid will tend to run back into the reservoir. The indication given on the scale makes it appear that the angle of glide has momentarily changed, whereas it has remained constant. Again, if the pilot pulls his aeroplane out of the glide and so reduces its speed, the liquid will run forward from the reservoir into the vertical tube and make it appear that his change of angle has been much more steep than it really is.

The little pendulum, which remains in the centre of its scale even when the aeroplane is banked at 80 degrees, is shown in the centre of the illustration at the top of this page of a dismantled turn indicator. On the left of the illustration is the gyroscope, consisting of a small wheel run at 5,000 revolutions a minute and driven by an air jet. The jet from which the air issues is a conical component shown in the foreground. Air is continuously sucked out of the case of the instrument by means of a suction pump or venturi, and continuously enters through the pointed jet direct against the buckets on the periphery of the wheel.

The tendency of the gyroscope is to remain fixed in the plane in which it is running, irrespective of any turns. As, however, this gyroscope is mounted in a frame or gimbal which is fixed at either end, it translates this tendency into a sideways movement, and the needle of the turn indicator travels across the lower scale of the instrument accordingly.


A SOLO TAKE-OFF BY INSTRUMENTS in an R.A.F. Avro trainer. To perform such a take-off the pilot under the hood must have absolute confidence in his instruments, and must follow their guidance implicitly, ignoring any impressions he may receive from his senses of the attitude of the aeroplane. The pilot allows the aircraft to attain an ample margin of flying speed before taking off.

The venturi tubes on aircraft exhaust the air from the case of this and other gyroscopic instruments; or this can be done by a suction pump coupled to the engine. For small aircraft a venturi tube will continue to be used for years, being light, inexpensive and efficient.

Venturi tubes are generally made of aluminium spinnings, fixed to a bracket which is a die casting. They are mounted on some part of the aircraft in the slipstream, with the smaller end forward. The stream of air, passing into the small trumpet and then expanding down the large trumpet following upon it, causes a considerable reduction of air pressure at the junction of these two parts. This reduction of air pressure creates a suction at that point, and pipes are connected, from a small hole or circular slot there, to the suction outlet of the instrument.

The revolution indicator is little different from that used on motor cars. It is coupled to the engine by a flexible shaft which drives a small gearwheel inside the instrument; this gearwheel drives another gearwheel rotating a vertical shaft on which a spring governor is mounted. The faster the vertical shaft revolves the more the four weights of the governor tend to fly outwards.

This tendency causes a scissors-like movement of the frame connecting them, which is transmitted to the pointer of the revolution indicator.

The altimeter consists in principle of a box from which most of the air has been removed. When this box or capsule is taken to a greater altitude, where there is a reduced air pressure upon its surface, the box is expanded by the pull of a strong spring. The expansion of the walls of the capsule is magnified by gearing and transmitted to a pointer.

The air speed indicator also uses a capsule or exhausted box, though at one time a mere disk of oiled silk was used instead. The aeroplane is provided with what is known as a pitot-static head, consisting of two tubes, generally mounted on the front of a wing, pointing in the direction of flight. One is open-ended, so that the wind can blow freely into it, creating a pressure at the far end. The other has a closed end, except that just behind the end there are a few small holes to provide indication of the external atmosphere ; this is known as the static tube. Formerly one tube was brought up to either side of a silk diaphragm, causing a pressure on one side and static air or even a slight suction on the other. The movement of the diaphragm towards the low-pressure side was transmitted by magnified gearing to the pointer.

The replacement of this silk diaphragm by a metal capsule has made the construction much more robust and efficient. One tube is connected to the inside of the box, the other to the inside of the case of the instrument, which is airtight. Comparison of these two pressures causes movement or breathing of the walls of the box magnified by gearing to produce a full-scale reading of the pointer.

The various types of pressure gauges on an aeroplane dashboard are for measuring the pressure of liquid, or for measuring air pressure. The oil pressure gauge, for example, is coupled to some part of the lubrication system and measures the pressure of the liquid at that point. Air pressure gauges include the boost gauge, and those which may be connected to the petrol tank to show the air pressure on the fuel. Pressure gauges on aircraft are generally for measuring boost, or induction pipe pressure; fuel pressure, in the supply system; and oil pressure at various engine speeds and temperatures.

With the increasing efficiency of aero engines, a boost gauge has become necessary to indicate whether or not the induction pipe pressure is above or below the one figure at which the engine would give its maximum permissible output.

GYROSCOPIC INSTRUMENTS are generally driven by venturi tubes

GYROSCOPIC INSTRUMENTS are generally driven by venturi tubes. Types of these tubes are shown in the small picture, and one may be seen in place just below the name of the aircraft in the other picture. The air blowing through the tube produces suction across a small orifice which is connected by tubing to the instrument in the cockpit. The suction causes a jet of air to blow on to the gyroscope wheel, and thus drives it round.

Not only does the gauge show the most suitable pressure for level flight, but it also provides a warning of too much boost pressure, which might cause damage to the engine at low altitudes. For this reason, the gauge is generally provided with a red warning label showing the maximum permissible boost pressure in level flight.

Reference has been made above to the expansion of exhausted capsules; a boost gauge generally contains two such capsules, in an air-tight case connected to the induction pipe. Changes of pressure in the induction pipe are communicated to the inside of the case, and cause expansion or contraction of the capsules, these “breathing” movements being transmitted to a pointer by gearing. Oil pressure gauges normally incorporate a Bourdon tube, that is, a tube sealed at one end, and bent into a complete circle or an arc of a circle. Increase of pressure in the inside of the tube will cause a tendency for the arc to straighten out; this tendency is transmitted by suitable gearing to a pointer.

Some types of oil pressure gauges are connected directly to the lubrication piping of the engine; but then, if the pipes should break, the oil pump would continue to pump oil through the orifice until the tank ran dry. This danger is overcome by the use of a transmitting type of pressure gauge, in which a metal capsule is submerged in the liquid in which the pressure is to be measured; variations of pressure within the capsule are transmitted to the dial through fine piping.

Fuel pressure gauges are generally of the transmitting type, but they differ from the oil pressure gauges in that the dial reading for oil pressure may be as much as 100 lb. to the square inch, whereas the maximum pressure on the scale of a fuel gauge would be no more than 5 lb. to the square inch.

Whether for oil or fuel pressure, instruments may be either of the circular dial type, or provided with an edgewise indication. Pilots prefer the circular dial because of the clearness of indication of the long needle; but the edgewise type lends itself better to incorporation in a crowded dashboard, especially where oil and fuel pressures for four engines or tanks have to be indicated.

Aircraft thermometers do not consist of a slender glass tube with a bulb filled with mercury or alcohol; they are required to give distant indications and to be proof against shocks which would shatter glass bulbs.

Distant reading thermometers are therefore used to transmit a temperature reading from the radiator, the oil pipes or the outer air to the dials in the cockpit.

These indications are up to the present always provided on the circular dial, so that, without reading the figures, the pilot can see by a casual glance the attitude at which the indicating needle stands.

Thermometer as Pressure Gauge

Each thermometer consists of a bulb located at the point where temperature is to be measured, the dial, and a length of capillary tubing connecting the two. Increase of temperature expands the contents of the bulb, and causes an increase in pressure to be transmitted to a Bourdon tube in the dial. A transmitting thermometer is therefore a delicate type of pressure gauge with its dial marked in degrees.

Bourdon tubes used in thermometers may be flattened and wound upon themselves in as many as six or twelve coils; this multiplies the effort of the tube to straighten out, and less multiplication is therefore necessary in the gearing.

Fuel gauges in aircraft may comprise a float in the tank connected by a wire to a gauge in the cockpit, a tube in the tank in which the pressure of the liquid remaining there is transmitted to a diaphragm gauge by piping, or a system of electrical transmission in which there is no current flow whatever within the tank. The electrical system is specially suitable for large aeroplanes with long transmission distances; several tanks can be indicated on the same dial.

Each tank attachment consists of a float operating a spiral cylinder which moves a little slider outside the tank over a balanced electrical circuit, the out-of-balance voltage being indicated on a sensitive voltmeter calibrated in gallons of petrol.

In the cockpit there is a switch connecting the tank unit of each tank to the dial as required; to avoid the necessity for continuous current flow, the switch is fitted with a push button which is pressed when a reading is required.

Petrol gauges of this type frequently have two scales, one above the other; the upper scale is calibrated to read correctly when the aeroplane is in flying position; the lower scale shows the amount of petrol when the tail of the aeroplane is on the ground.


A LIQUID FORE-AND-AFT LEVEL consists of a triangular glass tube containing coloured liquid. Such a level is shown, dismantled, to the left of this picture. The reservoir on the tube is at the back of the instrument when the instrument is in place on the aeroplane. On the right is shown a dismantled cross level, also using liquid as an indicating medium. In front of this liquid cross level is another type of cross level, a ball in a glass tube.

You can read more on “Blind Flying”, “The Principles of Navigation” and “Progress to Solo Flying” on this website.

How Aircraft Instruments Work