A quote from the Australian Civil Aviation Regulations...
Distress signals
(1) The distress signal shall be transmitted only when the aircraft is
threatened with grave and immediate danger and requires immediate
assistance.
(2) In radio telegraphy, the distress signal shall take the form of SOS
(... ...), sent 3 times, followed by the group DE, sent once, and
the call sign of the aircraft, sent 3 times.
(3) The signal specified in subregulation (2) may be followed by the
automatic alarm signal which consists of a series of 12 dashes, sent in
one minute, the duration of each dash being 4 seconds, and the
duration of the interval between consecutive dashes being one second.
(4) In radiotelephony, the distress signal shall take the form of the word
MAYDAY, pronounced 3 times, followed by the words THIS IS,
followed by the call sign of the aircraft 3 times.
(5) By other means the distress signal shall take one or more of the
following forms:
(a) the Morse signal ... ... with visual apparatus or with sound
apparatus;
(b) a succession of pyrotechnical lights, fired at short intervals, each
showing a single red light;
(c) the two-flag signal corresponding to the letters NC of the
International Code of Signals;
(d) the distant signal, consisting of a square flag having, either above
or below, a ball or anything resembling a ball;
(e) a parachute flare showing a red light;
(f) a gun or other explosive signal fired at intervals of approximately
one minute.
Urgency signals
(1) The following signals, used either together or separately, shall be used
by an aircraft for the purpose of giving 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
a manner as to be distinctive from the flashing lights described in
subregulation 196 (2);
(c) a succession of white pyrotechnical lights.
(2) The following signals, used either together or separately, shall be used
by an aircraft for the purpose of giving notice that the aircraft has a
very urgent message to transmit concerning the safety of a ship,
aircraft or vehicle, or of some person on board or within sight:
(a) in radiotelegraphy, 3 repetitions of the group XXX
( .. .. ..), sent with the letters of each group, and the
successive groups clearly separated from each other, and sent
before the transmission of the message;
(b) in radiotelephony, 3 repetitions of the words PAN, PAN, sent
before the transmission of the message;
(c) a succession of green pyrotechnical lights;
(d) a succession of green flashes with signal apparatus.
Safety signal
(1) The safety signal shall be transmitted when an aircraft wishes to
transmit a message concerning the safety of navigation or to give
important meteorological warnings.
(2) The safety signal shall be sent before the call and:
(a) in the case of radiotelegraphy shall consist of 3 repetitions of the
group TTT ( ), sent with the letters of each group and the
successive groups clearly separated from each other; and
(b) in the case of radiotelephony shall consist of the word
SECURITY, repeated 3 times.