thumb|right|A [[United States Air Force airman with a signal light gun that can be used to control aircraft with radio failure.]]thumb|right|Air traffic control signal light gun in use at base flight tower
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 (called a "signal light gun" or "light gun" by the FAA) 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 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 rocking their wings, moving the ailerons if on the ground, or by flashing their landing or navigation lights during hours of darkness. and are visible for roughly in clear daylight conditions. The table below describes the meaning of the signals.
{| class="wikitable"
|-
! Signal !! Aircraft in flight !! Aircraft on the ground !! Ground vehicles or personnel
|-
! Flashing white
| ICAO – Land at this airport and proceed to apron (this is not a clearance to either land or taxi. Clearances to land and taxi will follow.)<br />
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
| colspan=2 align=center | STOP
|-
! Flashing red
| Airport unsafe, do not land || Taxi clear of the runway in use || Clear the taxiway/runway
|-
! Alternating red and green
| colspan=3 align=center | Exercise extreme caution
|-
|}
See also
- Formation light
- Landing light
- Navigation light