lightning (atmospheric condition). A bright flash of light in the atmosphere caused by a discharge of electricity from one cloud to another or between a cloud and the earth. Lightning is actually an extremely large spark.
lightplane (type of airplane). A simple, single-engine airplane, used for training or noncommercial flying.
light sensitive diode (semiconductor device). A semiconductor diode whose ability to conduct in a forward direction is determined by the amount of light falling on the PN junction.

light year. The distance traveled by light in one year. The light year is a unit of measurement commonly
used in discussing astronomy and is equal to 5.879 trillion (5.879 x 1012 miles).
lime grease. A semisolid lubricant made by mixing oil with calcium carbonate. Lime grease is more suitable for use around water than grease having a sodium carbonate base.
limit switch (electrical switch). A precision electrical switch, used to turn a mechanism on or off when some part of the mechanism reaches a specific position or location. Limit switches are actuated by a very short movement of the actuating control, and they actuate at exactly the same position of the control each time.
Lindberg fire detection system. A type of aircraft fire detection system that detects either a fire or a general overheat condition. The detector element consists of a sealed, stainless steel tube filled with a material that absorbs gas when it is cool and releases it when it is heated. This tube is fitted with a pressure-actuated switch that initiates a fire warning signal when the tube is heated and releases enough gas to close the switch.