An ELT, or Emergency Locator Transmitter, is a radio beacon installed in an aircraft that sends a distress signal after a crash to help search and rescue teams find the wreckage. Most ELTs activate automatically on impact, transmitting on 406 MHz to a global satellite network that can pinpoint the aircraft’s location and relay that information to rescue authorities within minutes.
How an ELT Works
An ELT is a self-contained unit with a transmitter, battery, antenna, and an impact-sensing switch. The impact switch is calibrated to trigger when it detects a force of roughly 5 g’s or greater lasting at least 11 milliseconds along the aircraft’s length. That threshold is high enough to ignore hard landings and turbulence, but low enough to activate reliably in a crash. Pilots can also activate the ELT manually if they’re able to do so before or after an emergency.
Once activated, a modern 406 MHz ELT sends a digital signal that carries encoded data: the beacon’s unique ID, the owner’s contact information, and in newer models, GPS-derived position coordinates. Satellites in the Cospas-Sarsat system, operated internationally with the U.S. portion managed by NOAA, pick up this signal and relay it to ground stations. Those ground stations decode the alert, compute the beacon’s location, and forward everything to the nearest Rescue Coordination Center. The entire chain from signal to rescue notification happens almost instantaneously.
Why the 406 MHz Frequency Matters
Older ELTs broadcast on 121.5 MHz or 243.0 MHz, but the Cospas-Sarsat satellite system stopped monitoring those frequencies in 2009. Aircraft still carrying only a 121.5 MHz ELT depend entirely on a nearby air traffic control facility or an overflying aircraft happening to pick up the signal. That’s a much less reliable path to rescue.
The differences go beyond satellite coverage. The 406 MHz signal is stronger, digital rather than analog, and tied to a registration database. When a 406 MHz ELT goes off, rescue authorities can immediately look up who owns the aircraft, pull contact information, and verify whether the alert is real. With older analog ELTs, there was no quick way to confirm an alert, and false activations were extremely common. The practical result: search and rescue teams can respond to a 406 MHz alert in minutes, while a 121.5 MHz alert could mean a delay of hours.
Which Aircraft Must Carry One
Under FAA regulations (14 CFR 91.207), nearly all U.S.-registered civil airplanes must carry an approved, operable ELT. This applies to general aviation flights, charter operations, and commercial flights governed by various operating rules. The ELT must be an automatic type for airline and charter operations, meaning it activates on impact without pilot input. For other operations, either an automatic or personal-type ELT satisfies the requirement.
There are narrow exemptions. You can ferry a newly purchased airplane to a shop to have an ELT installed, or ferry an airplane with a broken ELT to a facility that can repair it, as long as only required crew members are on board. New ELT installations must meet current technical standards; units built to the older TSO-C91 spec have not been allowed for new installations since 1995.
Registration Requirements
If you own a 406 MHz ELT, U.S. law requires you to register it with NOAA. This is the registration that links your beacon’s unique digital code to your name, phone number, address, and aircraft details. Without current registration data, rescue authorities who receive your distress signal have no way to verify the alert or contact your emergency contacts.
Registration expires every two years and must be renewed. Any time your contact information changes, or you sell the aircraft or beacon, you’re required to update NOAA. If you buy a used beacon, registration responsibility transfers to you. This database is the reason 406 MHz ELTs trigger such faster rescue responses: it gives search and rescue teams something to act on immediately instead of chasing an anonymous signal.
ELTs vs. PLBs and EPIRBs
ELTs are sometimes confused with two related devices. A PLB (Personal Locator Beacon) is a handheld unit carried by an individual rather than installed in an aircraft. A PLB transmits on the same 406 MHz frequency and works through the same satellite system, but it’s manually activated and isn’t tied to a specific aircraft. Some pilots carry a PLB as a backup, but it does not satisfy the FAA’s ELT carriage requirement.
An EPIRB (Emergency Position Indicating Radio Beacon) serves the same basic function for maritime use. EPIRBs are designed to activate on contact with water and are built to float. ELTs, by contrast, use an inertial impact switch tuned to crash forces and are certified under aviation-specific standards. All three device types now require 406 MHz capability. Units operating only on 121.5 MHz or 243.0 MHz can no longer be manufactured, imported, or sold.
Maintenance and Testing
ELT batteries have a limited shelf life and must be replaced by the date stamped on the unit, or after any activation (including accidental ones) that may have drained the battery. The pilot in command is responsible for the ELT’s operation. One recommended practice: before shutting down the engine after each flight, tune the radio to 121.5 MHz and listen briefly for an ELT tone. This catches accidental activations from hard landings or rough taxiing before the signal generates a false alert.
Testing an ELT requires care to avoid broadcasting a false distress signal. Any bench or ground testing done outside a shielded enclosure must use a dummy load instead of the actual antenna, so the signal stays contained. Inadvertent activations are a real problem. They waste search and rescue resources and, for older 121.5 MHz units, were one of the main reasons satellite monitoring of that frequency was abandoned entirely.
How Rescue Actually Happens
When a 406 MHz ELT activates in a crash, a satellite detects the signal and forwards it to a ground station called a Local User Terminal. That station decodes the alert, computes a location, and sends the data to a national Mission Control Center. In the U.S., the Mission Control Center then routes the distress message to the appropriate rescue authority: the U.S. Air Force for inland incidents, or the U.S. Coast Guard for coastal and maritime locations.
If the ELT includes GPS encoding, rescue teams get coordinates accurate enough to go directly to the crash site. Without GPS encoding, the satellite system still narrows the search area considerably compared to older analog signals, but ground teams may need to home in on the 121.5 MHz signal (which modern 406 MHz ELTs also transmit simultaneously as a close-range homing frequency) to find the exact location. The combination of satellite detection on 406 MHz and local homing on 121.5 MHz is what makes the current system effective, even in remote terrain where cell service and radar coverage don’t exist.