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Aircraft emergency frequency

By Wikipedia,
the free encyclopedia,

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 tactical military 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.

Another reason for the choice of 121.5 MHz as a frequency is that it is in the exact middle of the original international VHF airband allocation of 118 - 136 MHz, thus minimising the number of physical control knob rotations that were likely to have to be made in an emergency situation when changing frequency from a standard non-emergency allocated frequency. This was particularly useful in the days before current digital frequency pre-selection, readouts and channel memory presets, when many standard legacy 360/720 channel COMM radio transceiver cockpit control panels only had the capability to operate on one primary frequency without a secondary switch-over option.

In the United States, 121.5 MHz is monitored by most air traffic control towers, FSS services, 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.

In the UK, 121.5 MHz is monitored by the Royal Air Force Distress and Diversion Cells (known as "D&D") at the London Terminal Control Centre and the Scottish Oceanic and Area Control Centre, from a nationwide network of antennas.

Older Emergency Locator Transmitters (ELTs) transmit on 121.5 MHz in case of impact. Newer ELTs transmit on 406 MHz, with a low power beacon on 121.5 MHz for local homing. Satellites listen for the signals and alert local personnel to the emergency, and the beacon allows search and rescue to find the scene of the accident faster. 406 MHz beacons are encoded, allowing the vessel of origin to be determined and false alarms to be quickly verified. Satellite support for the 121.5 MHz-only versions was discontinued in early 2009.

See also

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Published - July 2009

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