Glossary

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equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Absolute ceiling. The greatest height which can be reached by an aerodyne or aerostat in Standard Atmosphere. The theoretical height at which the rate of climb is zero.

Accelerometer. An instrument for measuring acceleration in a definite direction.

Accessory Gearbox. A gearbox, usually attached to the back of an aero engine, to transmit the drive of the engine to accessories such as instruments, electric generators, de-icing apparatus, undercarriage retracting mechanism, etc.

Acorn. A streamlined fitting placed at the intersection of bracing wires to prevent abrasion.

Acrobatics. A term used erroneously to describe voluntary evolutions in the air other than those requied for normal flying. The proper description is 'Aerobatics'.

Action, Radius of. Half the range of an aeroplane in still air.

Adjustable Pitch Propeller (airscrew). A propeller the angle of which can be adjusted on the ground.

Aerial. The wire, fixed or trailing, used for radio reception and transmission from aircraft. Sometimes erroneously used as an adjective such as 'Aerial Reconnaissance' which should be Air Reconnaissance'.

Aerobatics. Voluntary evolutions performed in the air other than those required for normal flying.

Aerodrome. A prepared area of land or water, including all building and fixtures, intended to be used for the operation of aircraft.

Aerodynamics. The science of the laws relating to forces acting on bodies moving in the air.

Aerodyne. A generic term for heavier-than-air aircraft which derives its lift when flying chiefly from aerodynamic forces - aeroplanes, gyroplanes, helicopters, ornithopters, kites or gliders.

Aero-engine. The power-plant for an aerodyne or aerostat.

Aerofoil. A surface designed to produce an aeropdynamic force at approximately right angles to its direction of motion. Usually a wind, tailplane or fin.

Aeronautics. The study of all branches and activities of flying.

Aeroplane. A mechanically driven heavier-than-air flying machine which has fixed wings. An aerodyne with fixed wings) the terms includes landplanes, seaplanes and amphibians. The word should be used in preference to the generic term 'aircraft' when an aeroplane is specifically meant. The abbreviation 'plane', commonly used is not correct and should be avoided. 'Plane properly means a wing.

Aerostat. A generic terms for aircraft which derive their lift chiefly from buoyancy in the air: lighter than air craft, balloons or airships.

Aileron. Movable aerofoil fitted near the wing-tip of an aeroplane and designed to make possible a rolling movement about the longitudinal axis. Ailerons are invariably connected differentially to the control column so that when one is raised to depress a wing the other is lowered to raise its wing.

Air Brake. A device, usually in the form of a flap, designed to increase the drag of an aeroplane at will. Used to steepen the glide, shorten the approach and reduce the landing run.

Aircraft. A generic term for all types of flying machines, both heavier-than-air and lighter-than-air. The term 'aircraft' includes aeroplanes, gyroplanes, helicopters, ornithopters, kites, gliders, airships an balloons.

Aircraft Carrier. A ship specially designed for the housing and servicing of aircraft and providing means for them to take off and alight (land).

Airframe. The structure of an aeroplane without the engine(s).

Air Intake. The scoop through which air is fed to the induction system of an aero-engine. Forward facing air-intakes give a degree of supercharge from the forward speed of the aeroplane.

Airline. A concern operating transport aircraft over an air route.

Airliner. A slang term for a transport aeroplane.

Airplane. Americans derivative of 'aeroplane'.

Air Pocket. An erroneous term describing the effect of vertical currents which cause 'bumps' to a flying machine passing through them.

Airport. An aerodrome provided with Customs amenities and used as a station on a commercial airway.

Air Route. A definite route between two or more aerodromes, towns or countries.

Air Scoop. A scoop through which air is taken into the ballonnets of a balloon or an airship by wind pressure.

Airscrew. All. types of screws with helical blades designed to rotate in air and more particularly power-driven screws designed to produce thrust by rotation. Synonymous with the term 'propeller'. Airscrew slip is the ratio of the actual advance per revolution of an airscrew to the theoretical r advance per revolution. Airscrew efficiency is an expression of this ratio, normally about 82 per cent.

Airship. A mechanically driven aerostat (lighter-than-air craft) .Includes rigid airships, semi-rigid airships and non-rigid airships. A dirigible (i.e. steerable) balloon.

Airspeed. The speed of a flying machine or airship relative to the air as distinct from its speed relative to the ground. Thus an aeroplane flying at 200 m.p.h. airspeed against a 50 m.p.h. headwind will have a ground speed of only 150 m.p.h.

Airspeed Indicator. An instrument which registers the speed of aircraft through the air, as distinct from relative groundspeed .

Airway. An air route with ground organisation.

Airworthy. A term used to denote that an aeroplane has been examined and passed as safe for flying.

Alighting. The act of making contact with water.

Altimeter. An instrument, usually a converted type of aneroid barometer, graduated to indicate height above a given datum, usually sea level.

Altitude. Height of aircraft above sea level. The word should be used with discretion. Thus 'high altitude' means merely 'high height' and should be avoided.

'Maximum Power Altitude' is the lowest height at which full throttle should be used at maximum permissible r .p.m. of an aero engine. For a supercharged engines it is the greatest height at which the rated boost pressure can be maintained at max. permissible r .p.m.

'Rated Altitude' - preferably 'rated height' - is the lowest height at which full throttle should be used at normal r.p.m. For a supercharged engine it is the greatest height at which 'the rated boost pressure can be maintained at normal r.p.m.

Alto-cumulus. Clouds of average height (between 7,000 and 20,000 ft.), usually in the form of a layer made up of small masses arranged regularly in groups, lines or rolls.

Alto-stratus. Clouds of average height! (between 7,000 and 20,000ft.), usually in the form of a bluish-grey fibrous veil.

Amphibian. An aeroplane designed for taking off from and alighting on both land and water. Examples: Supermarine Walrus, Grumman Goose, Consolidated Canso.

Anemometer. An instrument for measuring the speed of the wind.

Angle of Attack. The angle between the chord line of an aerofoil and the relative airstream, no matter what the angle of the aeroplane. This is also the True Angle of Downwash - the angle through which the airstream relative to the main axis of the aeroplane is deflected by the aerofoil.

Angle of Incidence. The angle between the chord line of a wing and the centre-line of the aeroplane.

Angle of Incidence (Rigging). The angle between the chord line of the mainplane of an aeroplane and the horizontal when the aeroplane is in the specified 'rigging position' on the ground. It should not be confused with the 'True Angle of Incidence' See Angle of Attack.

Angle of Pitch. The angle between the relative airstream and the longitudinal axis of the areoplane.

Angle of Roll. The angle between a line joining the wing-tips and the horizontal.

Angle of Sideslip. The angle between the longitudinal axis of the areoplane and its direction of motion.

Anodic Treatment. A treatment for metal parts of aeroplanes whereby a protective coating of oxide is imposed by electrolytic action.

Antenna. Aerial wire for radio communication.

Anticer. A device for preventing the formation of ice on aircraft, usually in the form of liquid paste. Should not be confused with 'de-icers', which are devices for removing ice when it has formed.

Anticyclone. Refers to weather. A region of relatively high barometric pressure in which the pressure is highest at the middle. The winds circulate clockwise around the centre in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. Anticyclone is the opposite of Depression.

Anti-drag Wires. Wires or cables, usually in the inside of non-stressed-skin wings, which resists forces set up by the drag of the aeroplane as it flies.

Anti-lift wires. Wires or cables, usually running from the top of the top centre-section plane to the outer wing struts of a biplane, designed to resist forces opposite to the normal direction of lift. They are sometimes called 'landing wires' because they help support the weight of the wings on the ground. They are units of the bracing system of a biplane.

Aperiodic. Non-oscillating. Applied for example, to the need of an instrument, usually a compass, which is so designed that, after a slight lag, it indicates the correct reading with no over-swing.

Approach Beacon. Radio equipment on the ground which, in conjunction with a radio receiver in an aeroplane, indicates its displacement to one side or another of a predetermined radio beam. The approach beacon is used for the final approach to landing, usually in conjunction with two marker beacons, when an aeroplane is coming in to land in bad visibility or in darkness.

Apron. A prepared area near the hangars on an aerodrome, usually made with a hard surface, designed to facilitate the manoeuvering of aeroplanes on the ground.

Arrester Gear. Cables laid across the deck of and aircraft carrier to engage with the deck arrester hook of a deck-landing aeroplane to return it's run on touch-down.

Artificial Horizon. An instrument, operated by a gyroscope, devised to keep an indicator permanently parallel with the true horizon and thus show the pilot the attitude of his aeroplane when flying in cloud or bad visibility.

ASI. Common abbreviation for for an 'Air Speed Indicator'.

Aspect Ratio. The ration of the span to the mean chord of an aerofoil; the ration of the square of the span to the total area of an aerofoil. Determined by the formula: 'span multiplied by span divided by total wing area' for all aerofoils - tapered, elliptical, circular, rectangular or square. Aspect Ration is also important to the structural engineer in building the wing and to the performance estimator in calculating the rate of climb and ceiling of an aeroplane. Aspect Ratio has an important bearing on the drag of a wing at low speeds high up. Aspect ratio vries between 12:1 (high) and about 4:1 (low)

Atmosphere. The mass of air which surrounds the earth and rotates with it. International Standard Atmosphere is an imaginary condition of the atmosphere to which the performance of all aeroplane is referred for exact comparison. It assumes, at mean sea level, Temperature = 15º C; Pressure = 1013.2 millibars. The temperature is calculated to fall by 6.5º C. for every additional 1,000m (3,281ft) of height up to 11,000m (36,089ft) above sea level. A that height it is assumed to be constant at minus 56.5º C.

Attitude (Flying). The inclination of the axes of an aeroplane to the relative airstream.

Attitude (Ground). The inclination of the axes of an aeroplane relative to the ground.

Autogyro. A trade name copyrighted by the Cierva Autogiro Company Limited for the gyroplanes made by that Company.

Automatic Boost Control. A device which automatically regulates the boos pressure from the supercharger of an aero engine so that a predetermined value is not exceeded.

Automatic Mixture Control. A device which automatically adjust the fuel mixture fed to an aero engine so that the quantities of air and fuel are varied by a predetermined amount with increase in height.

Automatic Parachute. A parachute which is opened by a static line attached to and aerostat or aerodyne. Also a parachute the pack of which is attached to an aerostat or aerodyne and opened by the freely falling body.

Automatic Pilot. An apparatus, operated by gyroscopes, which automatically moves the flying controls of an aeroplane to keep it on an even keel and on a fixed course.

Auxiliary Fuel Tank. A tank in which a reserve a reserve supply of fuel is carried.

Aviation. The operation of all types of aircraft.

Axes. Imaginary lines about which an aeroplane may change its attitude of flight. An aeroplane has three axes. They are:

(i) Lateral Axis. The straight line through the Centre of Gravity parallel to a line which would join the wing tips.

(ii) Londitudinal Axis. The straight line through the Centre of Gravity parallel to a line which would join nose to tail.

(iii) Normal Axis.The straight line through the Centre of Gravity at right angles to the lateral axis and the longitudinal axis. When an aeroplane is flying so that the longitudinal axis and the lateral axis are horizontal the normal axis is vertical.

There are also the Lift Axis, the straight line through the Centre of Gravity perpendicular to the relative air-stream, and the Drag Axis, the straight line though the Centre of Gravity parallel to the airstream.

Azimuth. Used in aeronautics usually with respect to instruments to denote angular displacement in a horizontal plane. In astronomical navigation, azimuth is the true bearing of the star under observation from the North point on the horizon as seen by the observer.