"MAYDAY" is the emergency radio call sign used in international aviation. It may only be used when there is an urgent and major threat to the safety of a flight and immediate assistance is required. When used, this call sign gives absolute precedence to radio communications from the affected flight over all others on the same frequency.
It was introduced to aviation in 1923 by Frederick Stanley Mockford (1897-1962) who was at the time Senior Radio Officer at Croydon Airport in London. He had been asked to think up a word that would be easily memorable and comprehensible to signify an emergency. Moreover, it had to be easy to pronounce and understand, both by pilots and by ground staff. As this was a time when there was a high level of traffic between Croydon and Le Bourget in Paris, he suggested an English derivative of the French phrase "aidez moi" ("help me"), transformed it to "moi aidez", then "m` aidez" and further to "mayday" so that English as well as French pilots would feel equally comfortable using it.
When repeated three times ("MAYDAY, MAYDAY, MAYDAY"), this call sign, just like the equivalent "SOS" (…---…) in telegraphy, proved to be clearly audible and unmistakable, even under poor transmission or reception conditions.
"SOS" had been agreed upon as an emergency call sign at the first International Conference for Wireless Telegraphy in Berlin on 3rd October 1909. Four years after its introduction by Mockford, "Mayday" was adopted at the 1927 International Radiotelegraph Convention in Washington as the emergency call sign for radio communications. At the 1932 Telecommunications Convention in Madrid, agreement was reached on standards and frequencies for emergency transmissions and, at the 1948 conference in Montreal, "Mayday" was incorporated into the standard phraseology of international aviation.
An emergency call should contain the following information:
The message should preferably be relayed via the following emergency frequencies:
If a transmission occurs continuously on the 121,500MHz or 406.0MHz frequency for longer than 25 seconds, the SARSAT/COPAS satellite system will automatically trace and identify the location of the transmitter.
Analog data transmissions use the 121,500MHz and 243.0MHz frequencies (Voice and Beacon).
The relevant emergency transponder code for flight safety is A 7700. Activation of this produces an extra-large signal on all radar screens within range of the affected aircraft giving far more information to air traffic controllers than would normally be displayed.
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