"Mayday" is a call sign used by vessels, aircraft, or any such carrier of people that is transmitted by a person when they are in grave and imminent danger. It is a sign to all listeners of the Mayday call that immediate assistance is required for the transmitter's safety, or that it is too late for such assistance. It was originated in 1923 by a senior radio officer at Croydon Airport in London, called Frederick Stanley Mockford. He was asked to think of a distress call that could be easily understood by all pilots and ground personnel in the event of an emergency. The word "Mayday" was chosen from the French word "m’aide," meaning "come help me" because of the high level of air traffic between London and France at the time. The one advantage of a Mayday call over all other distress calls is that when being transmitted over a frequency, no other radio traffic is permitted unless it is directed at the distressed vessel in order to help.
Though the Mayday distress signal can be transmitted over any frequency, aircraft have special frequencies that are specifically for distress calls. The frequencies 121.5 MHz and 243 MHz are left unused by air traffic for the purpose of distress calls and are constantly monitored by air traffic control. A pilot's ELT, or emergency locator transmitter, also broadcasts over those frequencies in the event it is activated. The Mayday call is the verbal equivalent of an SOS.
When air traffic control receives a Mayday transmission, they must launch search and rescue operations in search of the aircraft sending it. Any aircraft in the area are asked to keep their eyes peeled for any activity in order to assist with search and rescue. In the United States, as well as many other countries, it is a criminal act to broadcast a Mayday distress signal when not actually in danger. The cost of running a search and rescue operation is extremely costly and puts many people's lives at risk, allowing for the bending of Federal Aviation Regulations in order to successfully locate the distressed aircraft. In a case where the Mayday call was falsely sent, the person responsible must pay for the search and rescue operations costs and can be subject to up to 6 years in prison as well as a $250,000 fine.