ICAO describes the following distress signals in Annex 2 (Rules of the Air):
The following signals, used either together or separately, mean that grave and imminent danger threatens, and immediate assistance is requested:
a) a signal made by radiotelegraphy or by any other signalling method consisting of the group SOS (. . . — — — . . . in the Morse Code);
b) a radiotelephony distress signal consisting of the spoken word MAYDAY;
c) a distress message sent via data link which transmits the intent of the word MAYDAY;
d) rockets or shells throwing red lights, fired one at a time at short intervals;
e) a parachute flare showing a red light.
Note that a) mentions "any other signalling method", which could, for instance, be flashing aircraft lights. d) and e) require no radio or transponder.
In addition, the following urgency signals exist:
The following signals, used either together or separately, mean that an aircraft wishes to give notice of difficulties which compel it to land without requiring immediate assistance:
a) the repeated switching on and off of the landing lights;
b) the repeated switching on and off of the navigation lights in such manner as to be distinct from flashing navigation lights