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It is used to signal a life-threatening emergency primarily by mariners and aviators, but in some countries local organisations such as police forces, firefighters, and transportation organizations also use the term. The call is always given three times in a row ("Mayday Mayday Mayday") to prevent mistaking it for some similar-sounding phrase under noisy conditions, and to distinguish an actual Mayday call from a message about a Mayday call.
Mayday calls 
Making a false distress call in the United States is a federal crime carrying sanctions of up to six years imprisonment, and a fine of $250,000.
If a mayday call cannot be sent because a radio is not available a variety of other distress signals and calls for help can be used. A mayday can be sent on behalf of one vessel by another, using a convention called a mayday relay (see below).
Civilian aircraft making a mayday call in United States airspace are encouraged to use the following format (omitting any portions as necessary for expediency or where they are irrelevant):
MAYDAY, MAYDAY, MAYDAY; (Name of station addressed); Aircraft callsign and type; Nature of emergency; Weather; Pilot's intentions and/or requests; Present position and heading, or if lost then last known position and heading and time when aircraft was at that position; Altitude or Flight level; Fuel remaining in minutes; Number of souls on board; Any other useful information.
The Mayday procedure word originated in 1923 by Frederick Stanley Mockford (1897–1962). A senior radio officer at Croydon Airport in London, Mockford was asked to think of a word that would indicate distress and would easily be understood by all pilots and ground staff in an emergency. Since much of the traffic at the time was between Croydon and Le Bourget Airport in Paris, he proposed the word "Mayday" from the French m’aider, "venez m'aider" meaning "come help me."
Before the voice call "MAYDAY", SOS was the Morse Code equivalent of the MAYDAY call. In 1927, the voice call MAYDAY was adopted in place of the SOS Morse Code call.
Other urgent calls 
Mayday is one of a number of words used internationally as radio Code Words to signal important information. Senders of urgency calls are entitled to interrupt messages of lower priority. As with Mayday the use of these terms without proper cause could render the user liable to civil and/or criminal charges.
Each of these urgency calls is usually spoken three times; e.g., "Pan-pan, Pan-pan, Pan-pan."
Mayday relay 
A Mayday relay call is made by one vessel on behalf of a different vessel which is in distress. If a vessel makes a Mayday call and it is not acknowledged by the coastguard after a single repetition and a two-minute wait, then a vessel receiving the Mayday call should attempt to contact the coastguard on behalf of the Mayday vessel by broadcasting a Mayday relay.
A Mayday relay call should use the callsign of the transmitting vessel but give the name and position of the Mayday vessel.
Mayday relay calls can be used to summon help for a vessel which is either too far offshore to contact the coastguard directly or without radio capabilities (though most vessels above a certain size or crew complement are legally required to carry two-way radio equipment, which could have potentially been damaged or destroyed).
Pan-pan (from the French: panne – a breakdown) indicates an urgent situation of a lower order than a "grave and imminent threat requiring immediate assistance", such as a mechanical breakdown or a medical problem. The suffix medico used to be added by vessels in UK waters to indicate a medical problem (Pan-Pan medico, repeated three times), or by aircraft declaring a non-life-threatening medical emergency of a passenger in flight, or those operating as protected medical transport in accordance with the Geneva Conventions. "Pan-pan medico" is no longer in official use.
Declaring emergency 
Sometimes the phrase "declaring emergency" is used in aviation. This is the same as calling "Mayday". For example Swissair Flight 111 radioed "Swissair one eleven heavy is declaring emergency" on discovering their situation.
The following calls may be made only by the vessel in distress or the responding authority:
Seelonce Mayday or Seelonce Distress means that the channel may only be used by the vessel in distress and the coastguard (and any other vessels they ask for assistance in handling the emergency). The channel may not be used for normal working traffic until 'seelonce feenee' is broadcast.
The expressions Stop Transmitting — Distress and Stop Transmitting — Mayday are the aeronautical equivalents of Seelonce Mayday.
Seelonce Feenee (French: silence fini — silence finished) means that the emergency situation has been concluded and the channel may now be used normally. The word prudonce (prudence caution) can also be used to allow restricted working to resume on that channel.
Distress Traffic Ended is the aeronautical equivalent of seelonce feenee.
See also 
- Aircraft emergency frequency
- Call for help
- Distress signal
- French phrases used by English speakers
- Global Maritime Distress Safety System
- Vessel emergency codes
- Online Etymology Dictionary
- No Joke
- Aeronautical Information Manual, paragraph 6-3-2, "Obtaining Emergency Assistance", Federal Aviation Administration, 1999
- "Why Mayday?". Research Questions. National Maritime Museum Cornwall. Retrieved 1 May 2013.
- Naval Terms
- "Wikipedia SOS".
- ICAO Annex 10 V2 Section 5.3
- Swissair Flight 111 Transcript
- NTSB accident investigation report for Pinnacle Airlines Flight 3701
- Handling Distress and Help Calls
- ACP135(F): Communications Instructions: Distress and Rescue Procedures
- Audio recording of Mayday call made from the Sunshine Skyway Bridge Collapse, Tampa Bay, Florida, May 1980 (MP3 format) (updated URI, Jun 29 2005)
- Boating Safety: A VHF Primer, the use and misuse of the VHF
- ThomsonFly Passenger Aircraft calling Mayday after bird strike
- 9815 Lima loses control of aircraft, stuck in bad weather.
- Transport Canada: Radio Distress Procedures Card TP9878