The Emergency Locator Transmitter (ELT) in most airplanes is located on the tail section of the airplane. This is where you also find the data recorders and voice recorders because the tail is the safest place in a crash. You would be able to see the ELT antennae on the air frame of an airplane and it is the only thing you would see from outside the aircraft.
The insides of ELT are contained in a fire and impact resistant box which is also waterproof. It has its own batteries and thus is independent from airplane electrical system. If it remains intact, even when the airplane gets completely destroyed it will work.
ELT controls are located in the cockpit. A pilot can either manually turn it on or turn it off. In normal flights it is kept in armed position. When armed, the ELT is an auto mode. Most ELTs are designed to automatically activate when a certain longitudinal G force is exceeded. Something which happens only in a high impact crash.
The ELT, when activated transmits on two frequencies. One is on civil distress frequency 121.50 Mhz and the other is on the military distress 243.0 Mhz. Most modern ELTs also have another module which transmits on 406.025 Mhz. This is a satellite based frequency. Information given on 406.025 Mhz are:
- Aircraft country code.
- Aircraft identification.
- Aircraft ID code.
The ELT battery life depends on the type of ELT. In our airplanes, it goes on transmitting for about 72 hours from the moment of activation. The transmission on satellite lasts for 24 hours. But transmission continues on distress frequencies until the battery dies out. The distress signal on these frequencies is an aural signal, much like an ambulance siren. This signal can be used to pin point aircraft position but in case of satellite transmission it is much easier to locate the airplane. One of the many reasons why SAR teams try to locate the airplane as soon as possible.
Author – Anas Maaz (Airline pilot. Airbus A320/A321)
