Aviation light signals
In the case of a radio failure or aircraft not equipped with a radio, or in the case of a deaf pilot, air traffic control may use a signal lamp to direct the aircraft. ICAO regulations require air traffic control towers to possess such signal lamps. The signal lamp has a focused bright beam and is capable of emitting three different colours: red, white and green. These colors may be flashed or steady, and have different meanings to aircraft in flight or on the ground. Planes can acknowledge the instruction by rocking their wings, moving the ailerons if on the ground, or by flashing their landing or navigation lights during hours of darkness. Air Traffic Control signal light guns are typically specified with a center beam brightness of > 180,000 - 200,000 candela, and are visible for roughly 4 miles in clear daylight conditions. The table below describes the meaning of the signals. The use of handheld combination red/green/white signal lamps for air traffic control dates back to at least the 1930s.
| Signal | Aircraft in flight | Aircraft on the ground | Ground vehicles or personnel |
| Flashing white | ICAO – Land at this airport and proceed to apron FAA – Not applicable | Return to starting point on airport | Return to starting point on airport |
| Steady green | Cleared to land | Cleared for takeoff | Cleared to cross, proceed or go |
| Flashing green | Return for landing | Cleared to taxi | Not applicable |
| Steady red | Give way to other aircraft and continue circling | STOP | STOP |
| Flashing red | Airport unsafe, do not land | Taxi clear of the runway in use | Clear the taxiway/runway |
| Alternating red and green | Exercise extreme caution | Exercise extreme caution | Exercise extreme caution |