An aircraft emergency locator transmitter (ELT) is a battery-powered radio beacon designed to help rescuers find an aircraft after an accident. When an armed unit detects crash-generated forces—or someone activates it manually—it transmits a distress signal. A 406 MHz ELT can alert the Cospas-Sarsat satellite system and provide encoded information; responders can then use its 121.5 MHz homing signal to locate the aircraft nearby. The alert still depends on the beacon and antenna surviving, the equipment working, and rescue coordinators receiving and assessing the signal.
What happens when an ELT activates?
The process runs from activation to a coordinated search. An ELT does not itself dispatch a specific rescue crew: it sends a signal into a system that detects, routes, assesses, and acts on the alert.
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- Activation: An armed ELT is designed to switch on under crash-generated forces. A pilot, passenger, or rescuer may need to activate it manually if it does not start automatically.
- Transmission: The beacon sends a radio signal. A 406 MHz model sends a digital alert, and it also transmits on 121.5 MHz for local homing.
- Detection and routing: Cospas-Sarsat satellites detect 406 MHz alerts. In the United States, NOAA registration links a 406 MHz beacon to its registration details, and the alert is forwarded to an appropriate rescue coordination center.
- Assessment and search: Coordinators assess the alert and can task search-and-rescue resources. Responders near the aircraft can use the 121.5 MHz signal to home in on it.
For U.S.-registered beacons, NOAA maintains the registration database; the FAA describes alerts being forwarded to an appropriate search-and-rescue center, which may be an Air Force or Coast Guard Rescue Coordination Center. Registration information can help coordinators identify the aircraft owner and check whether an alert may be inadvertent.
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An ELT is attached to the aircraft and has its own battery. When armed, it is designed to activate in response to crash-generated forces. Automatic activation is not certain in every accident, so pilots and passengers should know the unit’s manual activation procedure.
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The beacon must remain functional and able to transmit after the crash. Installation, maintenance, battery condition, and whether the transmitter and antenna survive the accident all affect whether an alert can be sent. The FAA says a properly installed, maintained, functioning ELT can expedite search and rescue and save lives if it survives the crash and is activated.
What is the difference between 121.5/243 MHz and 406 MHz ELTs?
Older 121.5 MHz and 243 MHz ELTs transmit an analog swept audio tone. A 406 MHz ELT sends a digital alert that can encode identifying information; some models can also include position data. The FAA says 406 MHz beacons transmit a stronger alert signal than older 121.5 MHz units.
| Feature | 121.5/243 MHz analog ELT | 406 MHz digital ELT |
|---|---|---|
| Signal | Analog swept audio tone. | Digital alert with encoded data. |
| Satellite detection | Cospas-Sarsat stopped satellite monitoring of these frequencies on February 1, 2009. A signal depends on a nearby aircraft or facility hearing and reporting it. | Detected by the Cospas-Sarsat satellite system. |
| Information in the alert | Does not send a 406 MHz-style encoded registration message. | Can include owner or aircraft information; position information is available on some models. |
| Local homing | May be heard by nearby aircraft or ground receivers. | Includes a 121.5 MHz homing transmission for responders nearby. |
| Registration | FAA guidance specifically emphasizes NOAA registration for 406 MHz beacons. | U.S. 406 MHz beacons must be registered with NOAA. |
The end of satellite monitoring for 121.5/243 MHz did not make those signals inaudible to nearby receivers; it means those frequencies no longer have Cospas-Sarsat satellite monitoring. On a 406 MHz ELT, 121.5 MHz serves a different, local purpose: helping responders home in during the search.
Why registration and maintenance matter
For a U.S. 406 MHz beacon, registration connects the alert to owner and aircraft details and contact information. Keep those details accurate, particularly after an aircraft or ownership change. The alert system can only help identify the beacon using the information registered to it.
The FAA describes at least 48 hours of continuous operation as the design duration under specified conditions. That is not a guarantee of runtime after any particular crash: actual operation depends on the unit’s condition, battery state, installation, and whether the equipment remains intact.
Quick Recap
- Follow current aircraft, beacon-manufacturer, and FAA instructions for testing and maintenance.
- Have a qualified aircraft maintenance or avionics professional address aircraft-specific installation, inspection, and battery-service questions.
- Check current FAA requirements for the aircraft and operation. Requirements vary by aircraft, operation, and jurisdiction; this explanation does not establish an upgrade requirement for every aircraft.
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