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§ 91.125
ATC light signals.
ATC light signals have the meaning shown in the following table:
Color and type of signal
Meaning with respect to aircraft on the surface
Meaning with respect to aircraft in flight
Steady green
Cleared for takeoff
Cleared to land.
Flashing green
Cleared to taxi
Return for landing (to be followed by steady green at proper time).
Steady red
Stop
Give way to other aircraft and continue circling.
Flashing red
Taxi clear of runway in use
Airport unsafe—do not land.
Flashing white
Return to starting point on airport
Not applicable.
Alternating red and green
Exercise extreme caution
Exercise extreme caution.