What is a mayday signal, who uses it, and how was it invented?

Mayday: it's the international signal for an emergency made by planes and boats which use radio communications, which was recently used on Qantas flight QF144 from Auckland to Sydney, which ended up landing safely.

Airservices Australia, a government-owned organisation responsible for the management of Australia's skies, describes such a call as an indication "an aircraft is in grave and imminent danger and requires immediate assistance".

But the history of the mayday call dates back to 1920s England.

READ MORE: Anxious landing after Qantas plane engine fails over the Pacific

While SOS had been used to signal an emergency via morse code, with the increase in air travel it was less practical to use over radio.

Frederick Stanley Mockford came up with a new word for signalling an emergency: "mayday".

He was in charge of radio at Croydon Airport near London.

Planes flew to Le Bourget Airport in Paris, France, just across the channel.

Mockford proposed the word as it was the phonetic equivalent of "m'aidez" which means "help me" or "m'aider" which means "come help me" in French.

It is repeated three times to avoid confusion.

It was approved in February 1923.

Mayday calls in the USA should, according to the nation's Federal Aviation Authorities, follow the signal with more details, including the aircraft call sign, nature of emergency, weather, pilot's intentions or requests, present position and heading, altitude, fuel remaining and number of people on board.

READ MORE: 'Cancel takeoff': Flight radar and audio shows terrifying near-miss

Firefighters and police calls also use the word.

However, if there is an urgent issue but not a mayday, another signal can be used.

"PAN-PAN" from the French panne, means "a breakdown", and also stands for possible assistance needed.

Examples of PAN situations include a medical emergency or faulty instrument.

However, calling in a misleading emergency has consequences: it is an offence to declare a false mayday.