Emergency Locator Transmitter is a device installed as far aft as practicable. Fixed type ELT is mounted aft in an airplane and ELT antenna is located on top of the aft fuselage or on the upper skin of empennage. ICAO defines ELT as equipment that broadcasts distinctive signals on designated frequencies: 406 MHz ELT sends distress signal on 406 MHz for satellite detection and transmits low-power 121.5 MHz homing transmitter for terminal search, while 406-MHz ELTs transmit GPS position for search and rescue. ELT is an emergency transmitter designed to be triggered upon impact. ELTs are either attached to the airplane structure or are portable.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is ELT in aircraft?
Emergency Locator Transmitters (ELTs) are emergency transmitters which are carried aboard most general aviation aircraft. Emergency Locator Transmitters may be automatically activated by impact or manually activated by survivors. ELTs transmit a distress signal and broadcast distinctive signals. A removable ELT is stowed to facilitate its ready use in an emergency.
An Emergency Locator Transmitter is a self contained radio transmitter that serves as a safety device in aviation. It is a self-contained battery-operated transponder that is fixed in several forms on the aircraft or carried by people on board. The fixed installations are the Automatic Deployable ELT (ELT(AD)) and the standard fixed unit. The ELT(AD) is attached to the airframe and is automatically activated on impact. It is also activated by hydrostatic sensors if the aircraft enters water. All fixed ELTs can be manually triggered from the cockpit through a remote switch and control panel indicator.
When the beacon is triggered, the 406 MHz ELT, a digital transmitter, sends a coded burst that is picked up by Cospas-Sarsat, the international satellite search and rescue programme. Cospas-Sarsat forwards the alert to the appropriate rescue centre, giving global coverage. Older analog units still in service simultaneously emit a distinctive downward-swept audio tone on the operating frequencies 121.5 MHz and 243.0 MHz to aid homing by search aircraft.
For smaller aircraft certified for six or fewer passenger seats, a Survival ELT (ELT(S)) - a removable personal locator beacon - is carried instead of a fixed unit. This Survival ELT is manually activated by survivors and provides the same digital 406 MHz signal, assuring that the distress message reaches the same satellite network.
What is the purpose of an ELT in aircraft?
An ELT is a piece of flight safety equipment designed to transmit distress signals on 121.5 MHz and 406 MHz, aiding search and rescue operations. These devices are integral components of the international satellite system known as the COSPAS-SARSAT constellation. When activated - typically upon impact or manually by survivors - the ELT sends highly accurate location data to appropriate local search and rescue organizations' command centers. The transmitted signal provides GPS position information that rescuers can use to expedite the location of aircraft and people in distress following an aircraft impact with terrain.
ELTs are used primarily in severe emergencies where the aircraft has crashed or has made an unplanned landing in remote areas. An impact with terrain, for example, will trigger the device automatically due to its design. Manual activation is also possible; survivors in distress can manually trigger the ELT after a crash if it survives. This dual activation capability ensures rescuers receive alerts regardless of the specific circumstances of the crash scenario. The device broadcasts distinct emergency frequencies including 121.5 MHz and 243.0 MHz along with GPS position details.
Where is an ELT located in an aircraft?
For fixed-wing airplanes, the main unit of an ELT is usually located in the aft fuselage. It can be accessed through a small access plate on the right side of the aft fuselage. The transmitter itself is placed as far aft as practicable to minimize the chance of impact damage, while remaining clear of flight-control cables and vital structure. Fixed-type installations are permanently attached in a remote area so the device will not be harmed if the airframe is struck.
The antenna of an ELT (flexible whip about 14 inches long) is typically located on top of the fuselage, on the right side just aft of the rear window, along the dorsal fin on the aft upper skin of the empennage. It must be at least 30 inches from other VHF antennas to prevent efficiency losses. If an external mount is not practical, the antenna will be installed only inside the fuselage, never in a luggage compartment that is prone to being bumped or damaged.
For rotorcraft, ELT antenna placement follows the manufacturer's instructions. All installations must be parallel to the longitudinal axis of the airplane and the transmitter must carry an external label indicating its presence.
What are the types of ELTs used in aircraft?
Two general types of ELTs are used in aircraft: Automatic Fixed (AF) and Automatic Portable (AP). General Aviation pilots are most familiar with these AF and AP ELTs, both designed to trigger in the event of a crash. A survival ELT (ELT(S)), powered by water-activated batteries, is carried on overwater flights. These batteries furnish power and have virtually unlimited shelf life. Two types of ELT now in service are the 406 MHz ELT and the 121.5 MHz ELT. The 406 MHz unit is a digital transmitter that can be encoded with the aircraft data, while the 121.5 MHz device is an analog signal no longer monitored by the COSPAS-SARSAT system.
How does an aircraft ELT work?
An Emergency Locator Transmitter is automatically activated by impact, immersion in water, or hydrostatic sensors. It is also manually activated using the remote switch and control panel indicator in the cockpit. When armed and when subject to crash-generated forces, digital ELTs continuously emit their respective signals on 406 MHz, while older analog units simultaneously broadcast distinctive downward swept audio tones on 121.5 MHz and 243.0 MHz. For the system to work properly, the ELT must first begin transmitting and someone must be listening. The COSPAS-SARSAT system of non-geostationary satellites detects these distress signals, locates the ELT by GPS trilateration or doppler triangulation, and relays the data to a command center. Pilots are encouraged to monitor 121.5 MHz and/or 243.0 MHz in flight, and inspection includes verification that a sufficient signal is radiated from its antenna. The 406Test automatically notifies operators when the annual ELT test is due.
An aircraft ELT works by continual function: an inner G-switch within the unit observes abrupt shift in impulses and automatically triggers the distress alert. Once activated, the instrument is already transmitting recognition and whereabouts to search and rescue spacecraft. The continuous bleep is proof that the place is being communicated and becomes the real link to the rescue team.
What are the specifications of an aircraft ELT?
A modern Type AF ELT transmits signals at 121.5 MHz, 243.0 MHz and 406.025 MHz. RF output is 250 mW typical on 121.5 MHz and 243.0 MHz on 406.025 MHz. The 406.025 MHz carrier is phase-modulated whereas 121.5 MHz and 243.0 MHz use AM. Frequency tolerance is 0.005% for 121.5 MHz and 243.0 MHz, and 2 kHz for 406.025 MHz. Each 50 seconds the beacon sends a 10 ms digital burst on 406.025 MHz. The 121.5 MHz homing tone sweeps 300-1600 Hz at 700 Hz, 2-4 times per second.
Physical size for single or dual antenna ELT models is 6.0 in length, 2.5 in width, and 2.5 in height. The float-option housing is 6.0 in length ,6.0 in width, and 10.25 in height. Weight is 1.69 lbs (single antenna), 1.81 lbs (dual antenna), and 2.69 lbs (with float pack). Operating temperature range is -20°C (-4°F) to 55°C (131°F). Power comes from an internal lithium battery rated for 5 years or 50 hours continuous operation, whichever comes first. The battery must be replaced after 1 hour of cumulative transmission or when 50% of battery health is expired. The antenna is a flexible, omnidirectional, vertically polarized whip.
Approval is granted under FAA TSO-C126, TSO-C142, TSO-C91a, C/S T.001 and JTSO 2C126. Each 406 MHz ELT must carry a Cospas-Sarsat type-approval certificate and be registered with NOAA (or the Canadian Beacon Registry). 14 CFR 91.207 requires inspection every 12 calendar months for proper installation, battery corrosion, control and crash-sensor operation, and adequate radiated signal.
