Almost all modern aircraft are equipped with landing lights if they are intended and approved for nighttime operations. Landing lights are usually of very high intensity because of the considerable distance that may separate an aircraft from terrain or obstacles; the landing lights of large aircraft can easily be seen by other aircraft over 100 miles away.
In the design of landing lights, key considerations are intensity, reliability, weight, and power consumption. Ideal landing lights are extremely intense, require little electrical power, are lightweight, and have long and predictable service lives. Technologies used in past and present have included ordinary incandescent lamps, halogen lamps, various forms arc lamps and discharge lamps, and LED lamps.
Landing lights are typically only useful as visibility aids to the pilots when the aircraft is very low and close to terrain, as during take-off and landing. Landing lights are usually extinguished in cruise flight, especially if atmospheric conditions are likely to cause reflection or glare from the lights back into the eyes of the pilots. However, the brightness of the landing lights makes them useful for increasing the visibility of an aircraft to other pilots, and so pilots are often encouraged to keep their landing lights on while in flight below certain altitudes or in crowded airspaces. Some aircraft (especially business jets) have lights that can be operated in a flashing mode when not needed to directly illuminate the ground to enhance the visibility of the aircraft further. One convention is for commercial aircraft to turn on their landing lights when changing flight levels.
Landing lights are sometimes used in emergencies to communicate with ground personnel or other aircraft, especially if other means of communication are not available (radio failures and the like).
In many jurisdictions the landing light fixtures in the aircraft and the lamps they use must both be certified for use in a given aircraft by a government authority.
The actual use of landing lights may or may not be required or forbidden by local regulations, depending on time of day or night, weather, airport conditions, aircraft conditions, the type of operation being carried out (take-off, landing, etc.), and other factors.
In the United States, for example, landing lights are not required to be present or used for many types of aircraft, but their use is strongly encouraged, both for take-off and landing and during any operations below 6000 ft AGL or within ten nautical miles of an airport. For transport category aircraft and some operations with other types of aircraft, landing lights are required to be present and used. Landing lights must be certified safe and adequate for their purpose before installation.
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