Aviation light signals [EN]

An air traffic controller using a light gun that can be used to control aircraft with radio failure. In the case of a radio failure or aircraft not equipped with a radio, air traffic control may use a signal lamp to direct the aircraft. The signal lamp has a focused bright beam and is capable of emitting three different colors: 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 wiggling their wings, moving the ailerons if on the ground, or by flashing their landing or navigation lights during hours of darkness.

Signal	Aircraft in flight	Aircraft on the ground
Flashing white	N/A	Return to starting point
Steady green	Cleared to land	Cleared for takeoff
Flashing green	Cleared to approach airport, or return to land	Cleared to taxi
Steady red	Continue circling, give way to other aircraft	Stop
Flashing red	Airport unsafe, do not land	Immediately taxi clear of runway in use
Alternating red and green	Exercise extreme caution	Exercise extreme caution
Blinking runway lights	Vehicles, planes, and pedestrians clear landing area in use