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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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LAC: Leading Aircraftman - an RAF rank between Aircraftman 1st Class and Corporal, indicated by an "airscrew" badge on the arms of the tunic.

Lagging: The extent to which a rotor of a gyroplane in flight is displaced rearwards by air resistance from its normal position relative to the rotor head. The amount of lag is limited by the action of the drag pivot, a damping device which gives the rotor flexibility in the plane of rotation.

Laminar Flow: Airflow free of turbulence, that is to say, free of any eddying motion - a smooth stream.

Laminar Flow Wing: An aeroplane wing of thin section designed so that turbulence is reduced to a minimum. The Davies wing is an example.

Land and Sea Breezes: Offshore and onshore winds which occur up to about 1,000ft at night and in the daytime respectively. They are caused by the more rapid heating and cooling of the air over land than over water. Thus during the day when the land heated by the sun becomes hotter than the sea, the air above is warmed and expands. When air expands, its pressure falls and thus a region of lower pressure is created over land. The air over the sea has not heated to such an extent and has therefore a higher pressure. Because of these conditions there is a wind from sea to land - this is known as a "sea breeze." At night the above conditions are reversed, the land tending to become cooler than the sea. This creates a "land breeze."

Landing: The act of bringing an aeroplane under full control into contact with the ground.

Landing Area: An area of smooth and level land or of sheltered water providing sufficient space for the take-off and alighting of an aeroplane.

Landing Flare: A pyrotechnic flare attached to the underside of an aeroplane which, when lighted by the pilot, provides a bright illumination of the ground beneath, thus facilitating landing at night.

Landing Ground: A selected piece of ground suitable for the landing of aeroplanes, but without the facilities of an aerodrome.

Landing Light: A lamp (or lamps) usually installed in the leading edge of the mainplane of modern aeroplanes for use in illuminating the surface on which the aeroplane is alighting.

Landing Run: The distance travelled over the ground by an aerodyne after landing. Thus it is the distance between the first point of contact with the ground and the point at which the aeroplane comes to rest.

Landing Speed: That speed at which an aeroplane with a normal type two-wheel undercarriage stalls when making contact with the ground in the course of a "three-point landing." Aeroplanes with tricycle undercarriages do not have an exact landing speed, but are flown on to the ground at some speed within a specified range and are "pulled up" by wheel brakes.

Landing "T". A large symbol shaped like a letter T which is the standard indicator used on landing areas to show the direction of the wind. The cross stroke of the T is up wind. Landing T indicators may be illuminated for use at night.

Landplane: An aeroplane with a wheel undercarriage designed to operate from an aerodrome or aircraft carrier.

Laps: Defects in steel caused by splashing of the molten metal when pouring it into the ingot mould. They appear as a crack on the surface of the metal after rolling.

Lapse Rate: The rate at which temperature decreases with height. The average lapse rate in the troposphere is 3°F per 1,000ft, but the actual rate varies a good deal between the two extreme conditions of dry and saturated air.

Lateral Axis: An imaginary line passing through the centre of gravity and parallel to a line joining the wing tips. Movement about the lateral axis is termed "pitching."

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

Lateral Divergence: A combination of rolling, yawing and side-slipping of an aeroplane which precedes a spin or spiral descent with increasing rate of turn.

Lateral Stability: That quality which tends to correct any disturbance out of the plane of symmetry (that is in side-slipping or rolling). An aeroplane is laterally stable when its wings are not easily tilted sideways by external forces.

Lay Off: A term used in hull and float construction. It means to transfer the design of the hull or float lines from scale drawings to full size on the mould loft floor.

Leading: The extent to which a rotor blade of a gyroplane is displaced forward from its normal position relative to the rotor head.

Leading Edge: (1) The forward edge of a streamline body or aerofoil. (2) The structural member at the leading edge of such a body or aerofoil.

Leading-edge Radiator: A radiator built into the leading edge of a mainplane.

Leading Sweep: As applied to a propeller blade, a general curvature of the blade towards the leading edge.

Leakage: The loss of gas from the envelope or gas bags of an airship or balloon.

Leakage Drag: The drag caused by the leaks in the structure of an aeroplane. This only becomes important at high speeds. In modern fast aeroplanes leakage drag is an important component of the total drag. Experiments, chiefly carried out by Wing Commander Sidney Cotton, showed that a worthwhile increase in maximum speeds of existing aeroplanes can be achieved by carefully sealing all leaks in the structure.

Leak Detector: An instrument, used in airships, which detects the presence of hydrogen and other gases in the air. It is used to find leaks in an envelope or gas bag.

Leste: (pronounced Lestey): A hot, dry, southerly wind in Madeira and Northern Africa in front of an advancing depression.

Levanter: An easterly wind experienced in the Straits of Gibraltar. This wind is stronger in winter than in summer. When of moderate force it causes a cloud to appear over the Rock. This phenomenon is known as the "Levant."

Leveche (pronounced Levetchey): A hot, dry south-west wind in Spain which occurs in front of an advancing depression.

Life Line: A feature of certain designs of parachute. The flexible connection between the rigging lines and the harness when these are not directly connected.

Lift: (1) The component in a vertical, upward direction in straight and level flight of the resultant force created by the relative wind acting on the lifting surfaces of an aeroplane. (2) The total buoyancy of an airship.

Lift Axis: An arbitrary line through the centre of gravity of an aeroplane, which is vertical in straight and level flight.

Lift Wires: Wires or cables the principal function of which is to transfer the lift of non-cantilever wings to the main structure.

Light Aeroplane: An aeroplane having a total all-up weight of less than 1,200lb.

Limit of Proportionality: In effect this is the elastic limit of a material. The load applied is proportional to the extension it produces up to this limit. Beyond this point the material stretches far more rapidly for a given increase of load, and when the load is removed the material remains partly stretched.

Line Squall: A squall advancing on a wide front caused by the replacement of a warmer by a cooler body of air. The passage of a well-developed line squall is usually marked by a sudden or very rapid change of wind direction; heavy rain, hail or snow, thunder and lightning; rapid rise in barometric pressure; a sudden fall of temperature; and violent vertical disturbances. A frequent characteristic is a long arch or line of low black cloud.

Link Trainer: A device which was widely used for ground instruction in instrument flying. It resembles a small aeroplane built round an exact replica of an aeroplane's cockpit. The cockpit is provided with a hood so that "blind" conditions can be reproduced and is fully equipped with all the normal controls and instruments of an aeroplane. These are ingeniously linked so that actual flying conditions are reproduced when the student pilot sits inside the trainer and "flies" it under the hood.

Load Factor: The ratio of the load at which a member or structure fails to the assumed working load under specified conditions of flight. The load factor of an aeroplane, when no conditions are specified, is that existing in flight with the centre of pressure in its most forward position. Load factor should not be confused with Factor of Safety, which is the ratio of the failing load to the assumed maximum load.

Load Ring: A ring on which the basket suspensions and the net of a free balloon are secured.

Load Water Line: A horizontal line on a seaplane hull or float, marking the level of the water when in the fully loaded condition.

Loadings. (1) Power loading. The loaded weight of an aeroplane divided by the total power of the engine(s). (2) Span loading. The loaded weight of an aeroplane or glider divided by the square of the wing span. (3) Surface loading. The mean force on unit area carried by an aerofoil under specified aerodynamic conditions. (4) Wing loading. The loaded weight of an aeroplane or glider divided by the net wing area.

Longeron: A main longitudinal strength member of a fuselage or nacelle.

Longitudinal Axis: A line fore and aft through the centre of gravity of an aeroplane and parallel to the line of flight.

Longitudinal Divergence: The pitching motion of an aeroplane which precedes a dive or a stall.

Longitudinal Stability: That quality which tends to correct any disturbance in the plane of symmetry (that is, in pitching). An aeroplane is longitudinally stable when it resists any external force tending to raise or lower the nose.

Loop: A manoeuvre consisting of a complete revolution in flight in a vertical plane. The upper surface of the aeroplane or glider performing the manoeuvre is on the inside of the circle.

Loop Aerial: A radio aerial consisting of one or more turns of a conductor, either self-supporting or in a protective covering. Loop aerials are mounted on the fuselage and may be of three types. (1) The fixed-loop aerial. A loop aerial permanently fixed with respect to the centre line of an aeroplane and used with a homing receiver. (2) Rotatable-loop aerial. A loop aerial used for direction finding. It can be rotated in a horizontal plane. (3) Streamline-loop aerial. A rotatable loop aerial enclosed in a streamline fairing of insulating material.

Louvre: A movable vane for controlling an opening for the flow of cooling air.

Low: A meteorological term used to denote a region of low pressure, or depression.

Low Clouds: Clouds which have an average height below 7,000ft. They are:
(a) Nimbo-stratus. A low uniform layer of dark grey cloud.
(b) Fracto-nimbus. Low, dark, ragged clouds associated with bad weather.
(c) Strato-cumulus. A layer or bank of cloud composed of large masses showing some vertical structure and following one or two general directions.
(d) Stratus. A uniform layer of cloud resembling fog, but not resting on the ground.
(e) Fracto-stratus. A layer of cloud broken up into irregular, ragged fragments.

Lubber Line: A line fixed to the forward end of a compass bowl. It corresponds to the nose of an aeroplane and enables the pilot to determine the direction in which the aeroplane is heading at any time.