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The aircraft emergency frequency (also known in the USA as Guard) is a frequency used on the aircraft band reserved for emergency communications for aircraft in distress.The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD), International Aeronautical Emergency Frequency, [1] or VHF Guard, [1] and 243.0 MHz—the second harmonic of VHF guard—for military ...
121.5 MHz is the civilian aircraft emergency frequency or International Air Distress frequency. It is used by some civilian emergency locator beacons; however, the Cospas-Sarsat system no longer monitors the frequency. 243 MHz for NATO military aircraft emergency frequencies
Mayday is an emergency procedure word used internationally as a distress signal in voice-procedure radio communications. It is used to signal a life-threatening emergency primarily by aviators and mariners, but in some countries local organizations such as firefighters , police forces, and transportation organizations also use the term.
Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz for air-to-air and air-to-ground, including air traffic control communication. This band has a designated emergency and guard channel of 243.0 MHz. [2] [8] Radio aeronautical navigation aids use other frequencies.
An emergency locator beacon is a radio beacon, a portable battery powered radio transmitter, used to locate airplanes, vessels, and persons in distress and in need of immediate rescue. Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers.
Pan-pan calls may be made on the aircraft emergency frequency, but they are more often made on the frequency already in use, or another appropriate frequency. ICAO Annex 10, Volume V, § 4.1.3.1.1 states "the emergency channel (121.5 MHz) shall be used only for genuine emergency purposes". However, ICAO member states can deviate from this rule.
The civilian aircraft frequency for voice distress alerting is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well). Aircraft can also signal an emergency by setting one of several special transponder codes, such as 7700.
Despite the switch to 406 MHz, pilots and ground stations are encouraged to continue to monitor for transmissions on the emergency frequencies, as most 406 MHz beacons are required to be equipped with 121.5 "homers." Furthermore, the 121.5 MHz frequency remains the official global VHF aircraft voice distress frequency.