The Emergency Locator Transmitter is a piece of safety equipment designed to help search and rescue (SAR) aircraft find a downed aircraft. Although there may be differences in individual models, most of ELTs will automatically activate in the event of an accident. When an ELT is activated, it sends a pulsing tone on the emergency radio frequency, 121.5 Mhz - commonly called "Guard". Search and Rescue aircraft can hone in on an ELT signal and quickly find an aircraft that is down. With the exceptions of items listed in 14 CFR 91.207(e) and (f), all aircraft are required to be equipped with an operable ELT.
Because ELTs are designed to work in the event of an accident, they nearly always are powered by batteries independent of the aircraft's electrical system. To ensure that the ELT will work in an emergency situation, the FAA sets specific times in which those batteries must be replaced. As per 14 CFR 91.207(c), the batteries must be replaced when either -
(1) The transmitter has been in use for more than 1 cumulative hour; or
(2) When 50 percent of their useful life has expired
In addition to keeping the batteries charged, 14 CFR 91.207(d) specifies that an ELT must be inspected within the previous 12 calendar months in order to be airworthy.
Lastly, ELTs should be tested in accordance with the manufacturer's instructions. Ideally, this should take place in a screened room that prevents the broadcast of signals that could lead to a false alert. However, if this cannot be done, tests may be conducted during the first 5 minutes after the top of the hour. A test should be no longer than three audible sweeps.