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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Backing. A change of wind in an anticlockwise direction. This is the opposite of Veering, which is a change of wind in a clockwise direction.

Baffle. A metal plate used between the cylinders of an air-cooled aero engine to guide the cooling air. Properly: "inter-cylinder baffle".

Balanced Surfaces. Control surfaces of an aeroplane (ailerons, elevators, rudder and flaps) in which the aerodynamic movements about the hinge are wholly or partly self-balanced.

Horn Balance. A small balanced area at the tip of a control surface, usually projecting forward of the main surface.

Mass Balance. A weight set forward of a control surface designed especially to prevent flutter.

Balance Tab. A hinged area at the rear portion of a control surface designed to move in the opposite direction to the control surface and so balance it.

Bale Out. An expression which has become common parlance for the abandoning of an aeroplane and descending by parachute. It was derived from the act of baling out a boat.

Ballast. A weight carried in an aeroplane, balloon or airship to trim it with respect to its centre of gravity or to alter its buoyancy. In aeroplanes, ballast usually takes the form of lead disks. In balloons and airships sand or water is usually carried as ballast, as they can be easily discharged.

Ballonet. A compartment within the envelope of a balloon or airship into which air can be blown to counteract changes of volume of the gas in the envelope. This is necessary to maintain pressure and alter trim, and in non-rigid types to preserve envelope shape.

Balloon. An aerostat which consists of an envelope containing a gas which is lighter than air and which is not equipped for mechanical propulsion. Types of balloons are: Barrage Balloons, Captive Balloons, Free Balloons and Kite Balloons.

Bank. The angle between the lateral axis of an aeroplane and the horizontal plane. Banking is the angular motion about the longitudinal axis of an aeroplane when turning.

Barograph. A recording barometer used in aeronautics to record the varying heights reached by any aircraft during flight. The barograph is carried in the aeroplane and traces out a record on a revolving drum.

Bay. The lateral space between the interplane struts on one side of a biplane. Thus a Gauntlet is a two-bay biplane, a Gladiator is a single-bay biplane.

Beacon. An apparatus, usually on an airway, which emits light signals to indicate a particular geographical position to aircrews.

Beaufort Scale. A numerical system, ranging from 0 (Calm) to 12 (Hurricane) to classify the speed of the wind and covering a range of 0 to 75 m.p.h. or over. The system was originated by Rear-Admiral Sir Francis Beaufort and is based on observing the effect of winds on common objects.

Biased Fabric. Multi-ply fabric used for covering aircraft with one or more plies cut diagonally so that the warp is at an angle to the length.

Big End. In an aero engine, the crankpin end of the connecting rod.

Biplane. An aeroplane or glider which has two mainplanes (main wings) one above the other.

Bise. A cold, dry wind which blows in the winter from a northerly direction in Southern France.

Blade Angle. Of an propeller, the acute angle between the flat undersurface of one blade and the right angle from the propeller shaft. - the plane of rotation. The blade angle is in effect the angle at which a propeller blade bites into the air. It is variable in most propellers by turning the blade in its socket to alter the 'pitch'.

Blade Loading. Used with reference to a gyroplane or helicopter. It is the loaded weight of the machine divided by the total area of the rotor blades, not to be confused with disc loading, which, which is the equivalent of wing loading.

Bleeding. A term used with reference to the hydraulic system of an aircraft which concerns the expulsion of air from a system.

Bonding. The joining together of all metal parts of an aeroplane to form a continuous electrical conductor so that no sparks are caused between separate parts of the structure when they are charged with static electricity.

Bonding Noise. Interference in radio reception caused by the relative movement of the metal parts of an aeroplane in contact with each other.

Boom Well (or Trough). A depression in the plating of a float of a seaplane to take the end fittings of struts or booms.

Booms. Usually Tail Booms, a longitudinal structure which is used on some aeroplanes in place of the fuselage to carry the tail unit.

Boost. The pressure, which may be above that of atmospheric pressure, in the induction manifold of an aero engine. It is measured in lb/in2 above or below standard sea-level atmospheric pressure or in inches of mercury absolute. In the USA, boost pressure is usually termed Manifold Pressure.

Bora. A cold, squally wind which blows in the winter from the North-East on the northern shore of the Adriatic and Aegean Seas.

Boss. Used only with a fixed-pitch propeller, is a collar with flanges which fits on to the propeller shaft and on to which the hub of the propeller fits. Should not be confused with a propeller hub which, in a fixed-pitch and in a controllable pitch propeller, is the central portion on to which the roots of the blades are attached.

Boundary Layer. The thin layer of air immediately surrounding the surfaces of an aeroplane or airship. This layer is slowed down in flight by the friction exerted on the surface as it moves through the air and represents an important element of drag.

Boundary Light. A light, of which there are usually a number, arranged round the limits of an aerodrome. It marks the extent of the landing area.

Bracing. A system, usually consisting of struts and/or wires, by which a compound structure is made rigid.

Brake Horsepower. (Abbreviated b.h.p.) The Horsepower available at the propeller shaft of an aero engine. It differs from the Indicated Horsepower by the power needed to overcome the internal friction of the engine. The term is derived from the convenient method of measuring the horsepower developed at the propeller shaft by running the engine against a calibrated brake.

Breeze. A wind of moderate strength. In the Beaufort Scale graduated from 'Light Breeze' to 'Fresh Breeze' for a wind speed from between 4 to 24 m.p.h.

Bubble Sextant. A sextant used in air navigation in which a bubble level is provided to afford an artificial horizon for determining the altitude of a celestial body.

Buffeting. An irregular oscillation of any portion of an aeroplane distinct from 'flutter', because it is produced and maintained by the eddying wake behind some other part of the aeroplane, whereas 'flutter' is an unstable oscillation in a normal airflow. Buffeting can occur in the tailplane of a low-wing monoplane when it is literally buffeted by the wake of the mainplanes at high angles of incidence or when the flaps are down.

Bulkhead. A solid partition which is used to isolate one part of a structure from another. Used particularly with regard to the Fireproof Bulkhead behind an aero engine.

Bump. A colloquial term used to denote and sudden vertical or lateral movement of an aeroplane caused by thermal or eddy currents. Often erroneously referred to as 'air pockets'.

Bunt. The first half of an inverted or outside loop in which an aeroplane dives past the vertical into an inverted position and then returns to a normal attitude by a half roll or through half a normal loop. In a bunt the forces tend to throw the pilot from his seat and to break the wings downwards. Because of the abnormal stresses put upon an aeroplane in a bunt it is not encouraged as a manoeuvre.

Buran. (pronounced Boorarn). A strong wind from the north-east in Russia and Central Asia.