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This glossary of historic aviation terms is reproduced from 'The Spotters Glossary' which appeared in 'The Aeroplane Spotter' from January 1940 onwards. 'The Aeroplane Spotter' was the first journal devoted exclusively to the study and practice of aircraft recognition.

We are pleased to have been able to reproduce the entire glossary from A-Z, offering a unique insight into aviation terminology (and therefore aviation history and technology) prior to and during the 1940s.

NB: In the interests of clarity we have substituted current equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Cabane. The term for a tripod or similar arrangement of struts, usually at the centre-section of a biplane. Sometimes called a pylon.

Cabin. An enclosed compartment in an aeroplane or airship for housing the crew and passengers (if any).

Calm. The absence of any appreciable wind.

Camber. The curvature of the surface and/or centre line of an aerofoil section which causes lift. A heavily cambered aerofoil is usually a high-lift section. A slightly cambered aerofoil is usually a high-speed section.

Camera Obscura. A darkened enclosure which has a lens in the roof designed to throw the image of an aeroplane or cloud on a horizontal screen. Sometimes used for determining the speed of an aeroplane.

Canopy. The large envelope of a parachute which, when opened, offers resistance to motion through the air and so slows up the fall of the parachutist.

Cant. To tilt or to incline at an angle.

Cantilever. A term for a structural member held at one end by a rigid support, and free from any external bracing members. The term is often used with reference to wings or undercarriage legs which have no external bracing.

Captive Balloon. A balloon secured or towed by a cable.

Carburetter. A device which, with the aid of a draught of air, converts the fuel supply of an aero engine from a fluid into a vapour which is mixed with an appropriate amount of air and introduced into the combustion chamber as an explosive mixture.

Cartridge Starter. A means of starting an aero engine in which a cartridge is electrically fired in a cylinder and forces down one piston, thus rotating the crankshaft.

CAS. Chief of the Air Staff.

Cascades. Blades of aerofoil section set vertically, after the style of baffles, in a return-flow wind tunnel, to turn the airstream smoothly round a bend in the closed section.

Castoring Nosewheel. A nosewheel of a tricycle undercarriage which is not directly steerable but is free to swivel.

Castoring Tailwheel. A small wheel at the tail of an aeroplane which is free to swivel to assist manoeuvring on the ground.

Catapult. A mechanism for assisting the take-off of an aeroplane by giving an initial acceleration in addition to the normal thrust from the aero engine.

Ceiling. The maximum height to which an aeroplane can climb. The Absolute Ceiling is the height at which the rate of climb is zero and at which the aeroplane has only one possible flying speed. Absolute ceiling is sometimes called the theoretical ceiling. The Service Ceiling is the height at which the rate of climb of any aeroplane has dropped to 100ft. per minute.

Centre-Line (of an aerofoil). A line drawn through an aerofoil from leading to trailing edges. Each point of this line is equidistant from the upper and lower surfaces of the aerofoil. In aerofoil design the curvature of the centre line is used for designation of the type of wing section.

Centre-line Camber. The ratio between the chord of an aerofoil and the maximum height of the centre line above the chord line. A measure of the amount of curvature between the leading and trailing edges.

Centre of Buoyancy. The point through which the entire upward thrust of an aeroplane, glider, gyroplane, balloon or airship, or the hull or floats of a seaplane may be assumed to act.

Centre of Gravity. The point in a body through which the sum of the weights of the parts which make up that body may be assumed to pass whatever the attitude of the body.

Centre of Pressure. An imaginary line along the span of an aerofoil along which all the aerodynamic forces on the aerofoil affecting lift may be assumed to act. The centre of pressure of an aerofoil is normally about one-third of the way back from the leading edge, but in certain flying attitudes and with flaps extended it may vary greatly and may even move right off the aerofoil altogether. The movement in the centre of pressure is of importance structurally as well as aerodynamically.

Centrifugal Supercharger. A supercharger in which air or the ready mixed charge is compressed by centrifugal action.

Centripetal Force. The equal and opposite of centrifugal force. It is the force acting on the body and maintaining it in the curved path.

Chart Board. A table carried in most long-range aeroplanes on which the navigator can work. There is usually provision for attaching a map and parallel motion arms carrying a protractor.

Chassis. An archaic term often applied to the undercarriage of an aeroplane.

Chili. A dry southerly wind which blows in Tunis. Similar to the Sirocco.

Chine. The line which runs along the side of the hull of a flying-boat or a float, parallel to the keel and marks the change in angle between the side plating and the planing bottom.

Chinook. A warm dry wind which blows from the West on the eastern side of the Rocky Mountains. Similar to the 'Foehn' in Europe.

Chock. A wedge placed in front of the wheels of an aeroplane on the ground to prevent it from moving forward when the engine is run up.

Chord. The width of an aerofoil section, usually the wing, measured in a straight line from leading to trailing edge.

Cirro-cumulus (mackerel sky). High clouds, usually found above 20,000 ft. and composed of ice crystals. Appears like ripples in the sky resembling sand on the sea shore or in groups and lines. Cirro-cumulus is invariably white and flaky.

Cirro-stratus. High clouds usually seen in a thin whitish veil which does not blur the outline of the sun or moon but sets a halo around them. Like Cirro-cumulus, is composed of ice crystals and found around 20,000 ft.

Cirrus (mares' tails). High cloud around 20,000 ft. which appears in detached and fibrous wisps. Usually dead white and silky looking.

"C" Licence (Ground Engineer). A licence issued to Ground Engineers for the inspection of aero engines before flight.

Clinometer. An instrument for indicating the lateral attitude of an aeroplane.

Closed-jet Wind Tunnel. A wind tunnel in which the air stream passing over the body under test is enclosed by rigid walls.

Cloud Amount. The proportion of sky obscured by cloud. Usually expressed in tenths of the sky covered. Thus 10/10ths indicates complete overcast. Also expressed in eighths or 'oktas'.

Cloud Base. The height at the bottom of a towering cloud.

Cloud Height. The height of the cloud base above the ground at the point indicated.

Clouds. Clouds, like fog, are caused by condensation of water vapour in the air when it is cooled.

Cloud formation is usually caused by the expansion of rising air. There are three main conditions in which clouds can arise:-

(i) GEOGRAPHICALLY. Wind blowing against mountains and thus causing the air to rise over them, with consequent fall in temperature. A long bank of cloud results.

(ii) CONVECTION. By the sun heating the ground and causing, large vertical movements of air wit a mushroom of cumulus at the top of each rising column. Clouds of this type are formed by day and disappear at night and are usually bumpy to fly through. Clouds also form in this way behind' a depression when a current of cold air meets a mass of warm air and undercuts it, forming a cold front.

(iii) BY THE RISING OF AIR SLOWLY OVER A BIG AREA. In front of a depression clouds are formed by the gradual ascent of air over a large area. A warm front is thus formed by the advance of warm air over a wedge of colder air. Warm air rises in a gentle slope over the colder and denser air and gives rise to clouds usually at great heights. The warm front is distinguished from the cold by the fact that it is the body of warm air which moves forward and by the relative slowness of its descent. Clouds are also formed by eddy motion when a current of cold air blows over a warmer surface and by the mixing of two air currents of different temperatures.

Flying in Cumulo-nimbus, Cumulus, Nimbo-stratus and Strato-cumulus clouds is liable to be very bumpy.

Alto-stratus clouds are usually smooth and smooth flying is encountered above Strato-cumulus.

Clouds can be divided into four general categories, according to height:

(a) HEAP CLOUDS. Clouds with a vertical structure: Cumulus, Cumulo-nimbus and Fracto-cumulus. Height from 1,500ft. to 20,000ft.

(b) HIGH CLOUDS. Clouds which have a base normally higher than 20,000ft. Usually composed of ice crystals: Cirro-cumulus, Cirro-stratus, Cirrus.

(c) LOW CLOUDS. Clouds which have a base normally lower than 8,000ft: Nimbo-stratus, Strato-cumulus, Stratus, Fracto-stratus.

(d) MEDIUM CLOUDS. Clouds which have a base normally between 8,000ft. and 20,000ft: Alto-cumulus, Alto-stratus.

Cockpit. The portion of a fuselage, hull or nacelle designed to accommodate the pilot and/or crew.

C of A. Certificate of Airworthiness.

Col. With reference to a weather map, a region of weather between two areas of relatively high pressure and two areas of relatively low pressure. A col is usually associated with light winds, with thunderstorms in Summer and fog in Winter.

Cold Front. The boundary line between a mass of advancing cold air and a mass of warmer air under which it pushes.

Combustion Starter. A device to start an aero engine by the firing of a charge within a cylinder.

Commercial Load. That part of the disposable load of, an aeroplane from which revenue is derived. (Passengers, mails or freight.) Usually termed 'Payload'.

Compass. An instrument which consists essentially of a magnetic needle free to swing and which, subject to correction for conflicting magnetic fields, always points to the magnetic North. It thus indicates the angle in the horizontal plane between the magnetic North and the longitudinal axis of the aeroplane.

Compass Base. A circular area, usually on the edge of an aerodrome, marked with magnetic bearings upon which aircraft may be orientated for the compensation of errors in their compasses.

Compass Course. The angle between the: longitudinal axis of an aeroplane and the compass needle, measured clockwise from the compass needle between 0 degrees and 360 degrees.

Composite Cooling. (Usually termed Evaporative or Steam Cooling.) A cooling system for aero engines which makes use of the latent heat of evaporation by allowing the cooling fluid to boil, then passing the vapour to a condenser and returning the resultant liquid to the cylinder jackets. An example of the steam-cooled engine was the Rolls-Royce Goshawk of 1932.

Compressed Air Starter. A device for starting an aero engine by making use of the expansive energy of compressed air to push down a piston in a cylinder.

Compressed Air Wind Tunnel. (Sometimes called the Variable Density wind tunnel.) A wind tunnel in which the pressure of the circulated air can be increased so that the small scale of the models tested can be offset by the compensating factor of higher pressure.

Compressibility Drag. The great increase in drag, which arises when a body is moving at speeds around the speed of sound (770 m.p.h. at sea level decreasing steadily to 660 m.p.h. at 36,000ft., after which it is constant).

Compressibility Stall. The sudden loss of lift of a wing which is moved through the air at such a speed that the air flow over its upper surface reaches the speed of sound.

Compression Ignition Engine. An engine in which the ignition of the explosive charge is produced by the heat of compression alone without any electric spark. CI engines usually run on oil fuel and work on the Diesel cycle.

Compression Ratio. The ratio of compression, in the cylinder of an aero engine. Determined by the formula (r + R)/r, where 'r' is the compression space in the cylinder when the piston is at Top Dead Centre and 'R' is the volume swept by the piston in the cylinder.

Condensation. The conversion of a fluid from vapour into a liquid state. This happens when air is cooled below its dew point. Near the ground or sea this produces fog. When rising air is cooled, cloud forms.

Coning. The upward angular displacement of the rotor blades of a gyroplane or helicopter under the influence of lift. The Coning Angle is the angle between the rotor blades and the plane normal to the axis of the hub.

Connecting-rod Assembly. The complete assembly of two or more connecting rods of an aero engine working on one crankpin.

Constant-speed Propeller. Propeller in which the pitch of the blades is varied automatically by a governor so that the aero engine preserves a set, constant speed of rotation, whatever the load.

Consumption. The quantity of fuel or oil consumed by an aero engine or engines, defined in gallons per hour.

"Contact". A colloquial term used to announce that the switch of an aero engine is on and the pilot is ready for the engine to be started. Largely a relic now from the days when propellers swung by hand.

Contours. In maps, the imaginary line joining points of equal height above sea level.

Contra-rotating Propeller. Two propellers mounted one in front of the other on concentric shafts and designed to rotate in opposite directions. By this method twice the horsepower is absorbed by a given diameter propeller and propeller torque is cancelled out. This method was used by the Italians on the 2,500hp Fiat A.S.6 motor of the Macchi Castoldi 72 racing seaplane of 1931, which set a world's Speed Record of 440.67 m.p.h. in 1934.

Control Column. The lever, sometimes surmounted by a wheel, by which the elevators and ailerons are operated. Sometimes called the Control Stick or Joystick, the control column is pulled back to raise the elevators and depress the tail to put an aeroplane into a climb. It is pressed forward to lower the elevators, raise the tail and so promote a dive. Movement of the column to one side raises the ailerons on that side and lowers the ailerons on the other side, so that the aeroplane is tilted towards the side on which the ailerons are raised. When a wheel is set on top of the column this wheel is used to operate the ailerons instead of moving the stick sideways.


This diagram illustrates the means by which a conventional aeroplane is controlled. Elevators control pitching movement, ailerons control rolling movement. Directional movement is effected by the rudder. The lower inset diagrams show how control surfaces are operated. The small upper diagram shows the forces operating on an aeroplane in straight and level flight.

Control Surface. A surface, movable in flight, to control the motion of an aeroplane about its axes. The three principal control surfaces are the elevators, the rudder and the ailerons.

Controllable Pitch Propeller. (Abb. "c.p. prop"). Propeller, the pitch of the blades of which can be changed, within previously determined limits during rotation.

Convection. In meteorology the transference of heat by the movement of vertical currents in the atmosphere.

Coolant. The liquid which is made to flow around the walls and head of the cylinders of a liquid-cooled engine and then passed through a radiator to disperse the excess heat from the engine. Coolant is usually water or glycol or a mixture of both.

Cooling Drag. The part of the drag of an aeroplane which is caused by the cooling system of the engine. For a liquid-cooled engine this is the drag of the radiator. For an air-cooled engine it is the drag caused by the air passing over the finned cylinders of the engine. The cooling drag needs on an average about 3 per cent of the total horsepower to overcome it.

Corrosion. The destruction of the surface of a metal by the chemical action of the fluid in which it is immersed. This takes the form of rust in steels and pitting in light alloys. Corrosion is caused particularly through the action of sea water. Various chemical treatments such as anodising are used to combat corrosion.

Course. In air navigation, the direction of the horizontal longitudinal axis of an aeroplane with reference to the angle it makes with a specified datum. The course is set on the verge ring of the compass and includes both the bearing of one's destination and an allowance for the effect of wind and the consequent drift.

Course and Distance Calculator. A navigational device used for the solution of the triangle of velocities. Sometimes called the Course and Speed Computor. With this instrument the air navigator, knowing wind speed and direction, the bearing of his destination and his own cruising speed, can read off the true compass course which must be flown and the distance in "air miles." Any of these five quantities can be read off, provided four of them are known.

Course and Drift Indicator. A device for measuring the angle between the longitudinal axis of an aeroplane and any other object and between that axis and the line of travel. Sometimes called a 'Bearing Plate'.

Cowling. A metal cover enclosing the whole or part of the power unit of an aero engine.

Cracked Spirit. The fuel obtained by high temperature distillation of crude oil. Petrol of high octane value is prepared by this process.

Crew. Persons carried in an aeroplane who are actively engaged in its pilotage, navigation, maintenance and in other duties connected with its commercial or military purposes. The crew varies from one in a fighter up to about ten in a large bomber or flying-boat.

Cross-bracing. Wires, cables or girders used in certain forms of construction to keep the structure of wings or fuselage rigid.

Cross-level. An instrument used to indicate the direction of the resultant force, or apparent direction of gravity, in a transverse plane. Sometimes called the Lateral Climometer.

Cross-section. A section of the fuselage, hull, float or ring of an aeroplane perpendicular to the longitudinal axis.

Crosswind Axis. The straight line through the centre of gravity of an aeroplane perpendicular to the axes of lift, drag and thrust.

Crosswind Force. The component of the resultant force on an aeroplane along the cross-wind axis, caused by the relative air stream. This force is zero except when the aeroplane is yawed or side-slipped.

Cruising Speed. The normal operating speed of an aeroplane. This may vary widely according to circumstances. An aeroplane normally cruises at about 85 percent of its top speed at any particular height.

Cumulo-nimbus. Heap clouds formed of heavy masses, which may rise to a great height. The tops of these clouds pile up like mountains and appear to have a fibrous texture.

Cumulus. Thick clouds of a woolly type, usually with flat base low down and rising up to cauliflower tops.

Cyclogyro. A flying machine which is supported in the air by power-driven rotors, which rotate about a horizontal axis, like the paddle-wheels of a steamboat. These paddle-wheels replace the normal wings. Each paddle-plane consists of long thin aerofoils, which are feathered at the turn. Machines of this type have been designed by Rohrbach in Germany and Platt in the U.S.A., but there is no record of any successful flights.

Cyclone. A tropical revolving storm with winds of hurricane force, circulating anti-clockwise in the northern Hemisphere and clockwise in the Southern hemisphere. In temperate latitudes a cyclone is properly a Depression. In the West Indies a cyclone is called a hurricane. In the China Seas it is known as a typhoon. In Australia it is called a 'Willy-Willy'. The name cylone originated in the Indian Ocean.