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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Gale. A wind of "Force 8" or above on the Beaufort Scale of Wind Force defined as strong enough to "break twigs off trees and generally impede progress".

Gap. The distance separating the mainplanes of a biplane, triplane or quadruplane.

GAPAN. The Guild of Air Pilots and Air Navigators of the British Empire.

Gas Bag.A gas container in a rigid airship. Rigid airships are provided with a large number of gas bags (usually about 16) which hold the supporting gas and are anchored within the rigid framework.

Geared Engine. An engine in which the power developed on the crankshaft is transmitted to the propeller shaft through a reduction gear. The vast majority of large aero-engines are of this type, although in the smaller classes direct-drive engines are still widely used. A geared engine makes possible a slower-moving bigger propeller of higher efficiency.

General Inference. A general description of the meteorological situation at a given time deducted from the pressure distribution shown on a synoptic weather chart, together with the changes in progress and a statement of the kind of weather likely to be experienced over a particular area as a result of these changes.

Geodetic Structure. A structure built up of geodetic members, that is to say, of members which have the shape of the shortest (surface) distance between two points on a curved surface.

Geometric Pitch. The distance which an element of a propeller would advance in one revolution under conditions of no slip.

Gibli. A hot dry southerly wind in Tripoli, similar to the Sirocco.

Glacier Breeze. A cold breeze, blowing down the course of a glacier, owing its origin to the cooling of the air in contact with the ice.

Glazed Frost. A layer of smooth ice formed by rain falling on exposed objects when air temperature is below freezing point.

Glide. The gradual descent of an aeroplane with gravity alone providing the motive power.

Glider. A fixed-wing aerodyne intended for motorless flight. Gliders are generally divided into four classes:

(1) Primary Glider. Used for training and designed for simple glides with continuous loss of height.

(2) Intermediary Sailplanes. A more advanced form of glider capable of continuous flight without loss of height.

(3) High Performance Sailplanes. A highly developed glider of great aerodynamic efficiency designed for competition and other high performance flying.

(4) Towed Glider. This is towed behind an aeroplane singly or in a "train" carrying troops or freight.

Gliding Angle. The angle between the path of flight of an aeroplane and the horizontal while gliding.

Best Gliding Angle. That gliding angle at which an aerodyne will cover a maximum horizontal distance for a given loss of height.

Gliding Certificates. There are five classes of Gliding Certificate: "A", "B", "C", "Silver C" and "Gold C." Some 1,639 people had been awarded certificates in England up to the time when gliding was stopped by World War Two.

Gnomonic Projection. A method of projecting a map used for charts of the Polar regions and for harbour surveys. The Gnomonic projection produces a map on which direct measurements are accurate only over short distances. It assumes that each subdivision of the surface of the earth is flat. The Great Circle then appears as a straight line. Its advantages are that for small areas it is nearly accurate in every way.

Gordon Bennett Balloon Race. The first classic air race, held between 1906 and 1938. Free balloons were wafted from a joint starting place by the wind and the one to go farthest won. The record distance was 1,368 miles, set up by France in 1912 - one of the few air records standing for so long.

Gore. A fabric segment of a parachute, airship or balloon.

Gosport Tube. A system of voice pipes and headphones widely used in training aircraft for communication between instructor and pupil. The name comes from the fact that it was first used at the Special School of Flying, Gosport, under Smith-Barry in 1916. Sometimes called "de Havilland 'phones."

Göttingen. The German aeronautical research centre. A family of related aerofoils are called "Göttingen sections" after their place of origin.

Gradient Wind. A wind induced in a certain direction by flow of air balancing the pressure gradient caused by the earth's rotation and centrifugal force.

Gravity. The force exerted on a body by the mass of the earth. It is equal to the "weight" of the body in pounds. The speed of a falling body under the action of gravity depends in practice on its density; that is, on the relation between its weight and the area it presents to air resistance. In a vacuum, the rate of descent of any body increases at the rate of 32ft per second every second.

Gravity Tank. A fuel tank so situated that the contents flow to the engine by gravity alone.

Great Circle. The line over the surface of the earth in the shortest curved path. It is, in fact, the shortest line joining two points on the earth's surface.

Gregale (pronounced "gregahley"). A strong wind from the North-East usually in Winter, in the Ionian Sea and neighbouring parts of the Central Mediterranean.

Grid Deviation. The angle between a grid line and True North.

"Grids," that is to say patterns of squares, are super-imposed on some maps to enable a point to be quickly defined by co-ordinates.

Many flying - and other - maps, because of the projection employed, have slightly converging meridians. The grid lines are parallel, however, and are therefore inclined to all the meridians (except that through the origin) which converge at True North. These angles of inclination are termed "grid deviations".

Gross Weight. The maximum weight at which an aeroplane is permitted to fly. This is stated on the certificate of airworthiness for a civil aeroplane.

Ground Engineer. A person licensed to certify the safety for flight of an aeroplane or its engine or to inspect and improve repairs, replacements and overhauls to aeroplanes or aero-engines.

Groundloop. An involuntary, uncontrolled and usually sudden circular turn when moving on the ground, caused by directional instability. Groundlooping can afflict tailwheel aircraft (as distinct from tricycle-undercarriage aircraft) on take-off or landing, and if it happens at high speed the undercarriage may collapse.

Ground Speed. The speed of an aeroplane relative to the earth's surface, as distinct from its speed through the air. Thus if an aeroplane is flying at 200 m.p.h. through the air and is flying against a steady wind of 20 m.p.h. its ground speed will be 180 m.p.h.

"Ground Strafing". A term given to low-flying gun attacks on ground targets.

Group. The RAF formation into which the Commands are divided. The Group, in turn, is subdivided into Wings and the Wings into Squadrons.

Group Captain. A commissioned rank in the RAF equivalent to that of Captain in the Royal Navy and Colonel in the British Army.

A Group Captain may be distinguished by the four rings on his sleeve or the four bars on the shoulders of his greatcoat.

Guild of Air Pilots and Air Navigators of the British Empire. Founded on October 1, 1929, the Guild has as its object the promotion, consideration and discussion of all questions affecting air pilots and air navigation, and the protection by all proper legal methods of the interests of its Members and the procession of Commercial Aviation.

Gust. A sudden local fluctuation in velocity of the wind.

Gyroplane. A mechanically-driven aeroplane deriving its lift from the reaction of the air on one or more rotors freely rotating in a horizontal plane. This type of aircraft, of which the Autogiro is one example, should be clearly distinguished from the helicopter, in which the rotors are power-driven in normal flight.

Sometimes termed "Rotaplane."