Light Signals
Air traffic controllers have a backup system for communicating with pilots if the aircraft's or controller's radios fail or if they are deaf. Controllers use a light gun with different colors to tell pilots what to do.
n the case of a radio failure or aircraft not equipped with a radio,or deaf pilots, air traffic control may use a light gun to direct the aircraft. The light gun 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.
Aircraft in flight Aircraft on the ground Ground vehicles or personnel
Flashing white N/A Return to starting point Return to starting point
Steady green Cleared to land Cleared for takeoff Cleared to cross/proceed
Flashing green Cleared to approach airport, or return to land Cleared to taxi N/A
Steady red Continue circling, give way to other aircraft Stop Stop
Flashing red Airport unsafe, do not land Immediately taxi clear of runway in use Clear the taxiway/runway
Alternating red and green Exercise extreme caution Exercise extreme caution Exercise extreme caution
Red-red-green sequence Visual Warning System [1] N/A N/A
Blinking runway lights Vehicles, planes, and pedestrians immediately clear landing area in use
for on big airplane never been on before 
