The aircraft emergency frequency (also known in the USA as Guard) is a

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

used on the aircraft band reserved for emergency

communications Communication is commonly defined as the transmission of information. Its precise definition is disputed and there are disagreements about whether Intention, unintentional or failed transmissions are included and whether communication not onl ...

for aircraft in distress. The frequencies are 121.5

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

for civilian, also known as International Air Distress (IAD), International Aeronautical Emergency Frequency, or VHF Guard, and 243.0 MHz—the second harmonic of VHF guard—for military use, also known as Military Air Distress (MAD), NATO Combined Distress and Emergency Frequency, or UHF Guard. Earlier emergency locator transmitters (ELTs / EPIRBs) used the guard frequencies to transmit. As of February 1, 2009 satellite monitoring of the 121.5 and 243 MHz ELT (EPIRB) frequencies ceased, whereas an additional band from 406.0 to 406.1 MHz is now used exclusively by modern emergency locator transmitters (EPIRB).

History

The choice of 121.5 MHz was made by

ICAO The International Civil Aviation Organization (ICAO ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sch ...

in conjunction with ARINC and the ITU.

Monitoring

In the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, the emergency frequency is monitored by most

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled air ...

towers, FSS services, national air traffic control centers, military air defense and other flight and emergency services, as well as by many commercial aircraft. The notice to airmen FDC 4/4386 requires "…all aircraft operating in United States National Airspace, if capable, shall maintain a listening watch on VHF GUARD 121.5 or UHF 243.0." In the UK, 121.5 MHz is monitored by the

Royal Air Force The Royal Air Force (RAF) is the Air force, air and space force of the United Kingdom, British Overseas Territories and Crown Dependencies. It was formed towards the end of the World War I, First World War on 1 April 1918, on the merger of t ...

Distress and Diversion cells (known as "D&D") at the London Terminal Control Centre and the Shanwick Oceanic Control, from a nationwide network of antennas. Depending on the aircraft's altitude and location, the personnel in the centres may be able to use

triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...

to determine its exact position which can be useful to the pilot if the aircraft is lost or "temporarily unsure of position".

Use

Both guard frequencies can be used by any aircraft in distress or experiencing an emergency and in addition it can be used by

to warn aircraft if they are about to fly into restricted or prohibited airspace. Guard frequencies can be used for distress calls, such as

Mayday 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 organiz ...

calls, and urgency calls, such as Pan-pan calls. Aircraft will also be contacted on 121.5 MHz when intercepted by air defence aircraft, to ask for identification and intentions and to pass on instructions.

Misuse

Annex 10, Volume V, § 4.1.3.1.1 states "the emergency channel (121.5 MHz) shall be used only for genuine emergency purposes". A misuse of the frequency can result in punishment. In the United States,

Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States government that regulates communications by radio, television, wire, internet, wi-fi, satellite, and cable across the United States. The FCC maintains j ...

(FCC) rules prohibit false distress calls and superfluous communications. If the FCC Enforcement Bureau identifies an individual breaking these rules, they can be subject to a fine of up to $19,246 for a single violation and up to $144,344 for an ongoing violation. FCC may also confiscate radio equipment and file for criminal charges. In the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, pilots may make "Practice PAN" or "Training Fix" calls. Practice

calls are not permitted.

Locating beacons

Older emergency locator transmitters 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. Beacons operating at 406 MHz 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 (and 243 MHz) versions was discontinued in early 2009.

References

External links

{{DEFAULTSORT:Aircraft Emergency Frequency Air traffic control Airbands Emergency communication International telecommunications Rescue ja:航空無線機#周波数および出力