Aircraft emergency frequency
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 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
.
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 9) 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. Depending on the aircraft's altitude and location, the personnel in the centres may be able to use Auto Triangulation
to determine its exact position which can be useful to the pilot if the aircraft is lost or 'temporarily unsure of position'.
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.
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
used on the aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
band reserved for emergency communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
for aircraft in distress
Emergency
An emergency is a situation that poses an immediate risk to health, life, property or environment. Most emergencies require urgent intervention to prevent a worsening of the situation, although in some situations, mitigation may not be possible and agencies may only be able to offer palliative...
. 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
ARINC
Aeronautical Radio, Incorporated , established in 1929, is a major provider of transport communications and systems engineering solutions for eight industries: aviation, airports, defense, government, healthcare, networks, security, and transportation...
and the ITU
Itu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
as a result of its third harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
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
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
.
In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, 121.5 MHz is monitored by most air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
towers, FSS
Flight service station
A Flight Service Station is an air traffic facility that provides information and services to aircraft pilots before, during, and after flights, but unlike air traffic control , is not responsible for giving instructions or clearances or providing separation...
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
Restricted airspace
Restricted airspace is an area of airspace in which the local controlling authorities have determined that air traffic must be restricted for safety or security concerns...
or Prohibited airspace
Prohibited airspace
Prohibited airspace refers to an area of airspace within which flight of aircraft is not allowed, usually due to security concerns. It is one of many types of special use airspace designations and is depicted on aeronautical charts with the letter "P" followed by a serial number...
, it will be warned of military interception on 121.5 MHz. The frequency may also be used by ATC
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
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 9) 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
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...
Distress and Diversion Cells (known as "D&D") at the London Terminal Control Centre
London Terminal Control Centre
The London Terminal Control Centre was an air traffic control centre based in West Drayton, in the London Borough of Hillingdon, England, approximately 2.5 miles north of London Heathrow airport...
and the Scottish Oceanic and Area Control Centre, from a nationwide network of antennas. Depending on the aircraft's altitude and location, the personnel in the centres may be able to use Auto Triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
to determine its exact position which can be useful to the pilot if the aircraft is lost or 'temporarily unsure of position'.
Older Emergency Locator Transmitters
Emergency Position-Indicating Radio Beacon
Distress radio beacons, also known as emergency beacons, ELT or EPIRB, are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. Strictly, they are radiobeacons that interface with worldwide offered service of Cospas-Sarsat, the international...
(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
- Air-ground radiotelephone serviceAir-ground radiotelephone serviceAir-ground radiotelephone service is a method of telephone communications used by aircraft pilots and passengers of commercial airline jets flying over the United States....
- Call for helpCall for HelpCall for Help, also known as CFH, was a computer-themed television program that first aired exclusively on TechTV , a cable and satellite television network focused on technology, and then aired on G4techTV Canada and the HOW TO Channel in Australia...
- Distress radiobeacon
- Mayday (distress signal)Mayday (distress signal)Mayday is an emergency procedure word used internationally as a distress signal in voice procedure radio communications. It derives from the French venez m'aider, meaning "come help me"....
- Transponder code