The mayday distress signal was officially adopted as an international radiotelephone distress signal in 1927.
The spoken call was created for voice radio, where Morse code SOS could not be used conveniently. Frederick Stanley Mockford, a radio officer at Croydon Airport in England, proposed “mayday” in the early 1920s because much cross-Channel air traffic connected Croydon with Paris. The term echoed the French m’aider, meaning “help me.”
The call was tested for aviation and introduced on cross-Channel flights in 1923. The 1927 International Radiotelegraph Convention in Washington, D.C., formally adopted mayday as the voice distress call while retaining SOS for Morse-code transmissions.
In an emergency, standard procedure repeats the word three times: “Mayday, mayday, mayday.” It signals grave and imminent danger, unlike “pan-pan,” which indicates urgency without the same immediate threat to life.