In the case of a radio failure

NORDO

NORDO, short for "No Radio", is a North American aviation term for aircraft flying without a radio. The term originates from the 5-character uppercase abbreviated notation "NORDO" displayed on controllers' radar scopes when an aircraft transmits the "radio failure" code on its transponder...

 or aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 not equipped with a radio, air traffic control

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

 may use a signal lamp

Signal lamp

A signal lamp is a visual signaling device for optical communication . Modern signal lamps are a focused lamp which can produce a pulse of light...

 to direct the aircraft. 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 wiggling their wings, moving the aileron

Aileron

Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...

s if on the ground, or by flashing their landing or navigation lights during hours of darkness.

Signal	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
Blinking runway lights	Vehicles, planes, and pedestrians immediately clear landing area in use