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The aircraft emergency frequency (also known as guard) is a frequency used on the aircraft radio band reserved for emergency communications for aircraft in distress. The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD) and 243.0 MHz for military use, also known as Military Air Distress (MAD). Both are in use at the international level.
The choice of 121.5 MHz was made by the ICAO in conjunction with ARINC and the ITU as a result of its third harmonic frequency relationship with the 40.5 MHz military tactical low band ground-to-air channel. Similarly 121.5 MHz is itself a sub harmonic of the military UHF distress frequency at 243 MHz. This choice gave a number of technical and operational compatibility and efficiency gains in the context of design and proximity interference.
121.5 MHz is monitored by most air traffic control towers, national air traffic control centers, and other flight and emergency services, as well as by many airliners. Separate frequencies exist for military and other government emergency frequencies. If an aircraft violates or is on a trajectory that will violate Restricted or Prohibited airspace, it will be warned of military interception on 121.5 MHz.
The frequency may also be used by ATC to establish contact with an aircraft that has inadvertently switched to an incorrect frequency. As pilots are strongly recommended to monitor 121.5 MHz at all times, a common practice is to set a secondary communications radio in the aircraft (often COMM 2) to 121.5 MHz in order to monitor, but not transmit on, 121.5 MHz. A pilot accidentally transmitting on 121.5 MHz will often hear a reply stating that they are "on guard", i.e., that they are on the guard frequency and should switch to the appropriate frequency instead.