In case of a radio failure, or an aircraft not equipped with a radio, or in the case of a deaf pilot, air traffic control at controlled airports may use a signal lamp to direct aircraft. This signal has a focused bright beam and is capable of emitting three different colors of light: red, white, and green.
The colors are changed by rotating the handle that is on the front of the device. These colors may be either flashing (intermittent) or steady having different meanings to the aircraft in flight or on the ground. The duration of the signal is controlled by the trigger like switch on the unit. Aircraft can acknowledge the instruction by rocking their wings, moving their ailerons if on the ground, or by flashing their landing or navigation lights during hours of darkness.
If an aircraft is in flight, signals from the signal light have the following meanings: Steady green = Cleared to land. Flashing green = Cleared to approach airport, or return to land. Steady red = Continue circling, give way to other aircraft. Flashing red = Airport unsafe, do not land. Alternating red and green = Exercise extreme caution. If an aircraft is on the ground, signals from the signal light have the following meanings: Flashing white = Return to starting point. Steady green = Cleared for takeoff. Flashing green = Cleared to taxi. Steady red = Stop. Flashing red = Immediately taxi clear of runway in use. Alternating red and green = Exercise extreme caution.
The light signals may also be used to control the movement of airport service vehicles that may be moving about the airport surface.