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42      AIR REGULATIONS, 1938

II. Distress signals.
When an aircraft is threatened by grave and imminent danger and requests immediate assistance, the following signals shall be used or displayed, either together or separately, before the sending of a message:
(a) By radiotelegraphy:
The signal - - - -- -- -- - - - (See Note (1)).
(b) By radio telephony:
The spoken expression "MAYDAY" (corresponding to the French pronunciation of the expression "m'aider").
(c) By visual signalling:
(1) The signal - - - -- -- -- - - - with signalling apparatus.
(2) A succession of red pyrotechnical lights fired at short intervals.
(3) The two-flag signal corresponding to the letters NC of the International Code of Signals (See Note (2)).
(4) The distant signal, consisting of a square flag having, either above or below it, a ball or anything resembling a ball (See Note (2)).
(d) By sound signalling:
(1) The signal - - - -- -- -- - - - with any sound apparatus.
(2) A continuous sounding with any sound apparatus (See Note (2 ).
NOTE (1).--When the signal II (a) above is sent by radiotelegraphy on 500 kc/s (600 m.), it shall, when possible, in order to be received by automatic maritime apparatus, be followed by the automatic alarm signal consisting of a series of twelve dashes of four seconds each, separated by an interval of one second.
NOTE (2).--The signals II (c) (3), II (c) (4) and II (d) (2) above are normally for use by seaplanes on the surface of the water, but they may also be used by aircraft in the air.
III. Urgency signals.
(d) When an aircraft wishes to give notice of difficulties which compel it to land without requiring immediate assistance, the following signals shall be used, either together or separately, before the sending of a message: