Basically there are three types of emergency locator beacons in use today to transmit distress signals: the maritime EPIRBs - Emergency Position Indicator Radio Beacon. The aviation community uses ELTs - Emergency Locator Transmitters, the PLBs - Personal Locator Beacons are used for land-based applications. Although Cat II EPIRBs and PLBs can be used by the pilot of an aircraft as these are manually activated beacons whereas the CAT I EPIRBs are housed in a special bracket, aviation ELTs are automatically activated and manually tested.
Installation of an ELT in an aircraft is subject to certain rules and accepted methods so as to assure that in the unlikely event of an off-airport landing the beacon does activate.
Just to make sure: these beacons NEED to be registered!
Regulations require that aircraft remain in their original or properly modified condition. The installation of an ELT is a modification on certified aircraft and this will require that the proper documentation is filed. On homebuilt experimental aircraft we can do the job ourselves following the general acceptable standards for certified aircraft but it is wise to have the installation checked and signed off by a licensed engineer.
The acceptable standards will produce a reliable and effective ELT installation and keep unwanted activation to a minimum. The following sources are used for that:
The notes below represent an useful addition to these requirements.
In an upright position an antenna located on the top rear fuselage gives a good overall efficiency. When the aircraft is inverted (as one third is after a crash) an internal antenna exhibits the best efficiency in a high wing aircraft. In a low wing aircraft neither location (internal or external) has an real advantage.
One must select a location in the aircraft where the ELT is protected by the structure in case of a crash. Make sure that the ELT will not get separated from the antenna by using a high tensile cable between the two.
These installations need to take into account that helicopters generate more vibrations than aeroplanes, sometimes leading to premature fatigue failures. Parts separating the helicopter could hit the tail rotor with exciting results. Mount the antenna as close as possible to the ELT in a location where crash events would likely have a minimal impact.
The mount must provide a load path from the aircraft structure to the automatic activation system. It may also NOT vibrate under any normal flight condition of the aircraft which could lead to false alarms. Be sure to follow the manufacturer installation instructions to the letter.
A number of aircraft crashes lead to the aircraft being on fire. This indicates that the antenna cable should be protected with fire resistant materials. Preferably the complete installation should be protected with this. Make sure the cable doesn't pass over any fuselage joint (which might break in a crash) and that the ELT and antenna are mounted on the same frame structure to minimize transverse separation.
The presence of the ELT antenna close to another antenna can influence its performance, cause detuning or alteration of its radiation pattern. High power VHF transmissions (COM) can cause interference of receiving GPS signals and the injection of this VHF signal can excite the output stage of the ELT transmitter generating harmonics resulting in interference with the GPS receiver. So make sure to keep these antennas several feet apart.
The interaction of components is often critical to arrive in an overall acceptable performance of the installation. Obtaining parts from questionable sources leads to inferior installations. Stay with good brand names for proper result!
The battery expiration date must be recorded in the aircraft maintenance logbook in accordance with your local aviation law. Every test cycle must also be recorded in the logbook. Replacing these batteries when due ensures that they will work when it is needed the most.