ATC Light-Gun Signals

If your radio fails at a towered airport, the controller directs you with a light gun. The same colour means different things airborne versus on the ground.

Signal	On the ground	In flight
Steady green	Cleared for takeoff	Cleared to land
Flashing green	Cleared to taxi	Return for landing (followed by steady green)
Steady red	Stop	Give way to other aircraft and continue circling
Flashing red	Taxi clear of the runway in use	Airport unsafe — do not land
Flashing white	Return to starting point on the airport	(not used)
Alternating red & green	Exercise extreme caution	Exercise extreme caution

Acknowledge by day: rock your wings; by night: flash your landing/nav lights.

Reference only — verify against the current FAA Aeronautical Information Manual (AIM) and regulations before flight. More quick reference.