From the tower
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When radio contact is lost, the tower can still talk to you with a signal lamp (light gun). The same colour means something different in flight and on the ground.
Memory aid: green = go ahead (land, take off, taxi), red = stop or do not land, white = go back.
Signal table
Click a row to show the signal in the picture.
Quiz
Radio failure at a controlled aerodrome
Check volume, squelch, frequency, headset plugs and the other radio. Then squawk 7600, keep a sharp lookout, follow the radio failure procedure of the aerodrome in the AIP and watch the tower for light signals.
Acknowledging a signal
In flight, by day: rock the wings (not on base or final). By night: flash the landing lights twice, or switch the navigation lights off and on twice.
On the ground, by day: move the ailerons or the rudder. By night: flash the landing lights twice, or the navigation lights.
EASA and FAA differences
SERA (Europe) also uses flashing white in flight (land here and proceed to the apron) and a red pyrotechnic (do not land for the time being).
The FAA has alternating red and green (exercise extreme caution) for aircraft in flight and on the ground. Flashing white in flight is not used by the FAA.
"In due course"
Flashing green in flight (return for landing) and flashing white in flight (land and go to the apron) are not a landing clearance yet. The clearance to land (steady green) and to taxi (flashing green on the ground) will follow.
Where to find it
EASA: SERA.3215 and Appendix 1 of SERA, based on ICAO Annex 2 Appendix 1. FAA: AIM 4-3-13 and 14 CFR 91.125. The light signal table is also printed in many VFR guides and on kneeboards.