The "Beeper" is a nickname for an "Emergency Locator Transmitter" (
ELT) which transmits
homing signals on UHF/VHF
Emergency frequencies, guiding rescue aircraft and/or ground rescue personnel to an aircraft crash site, or parachute location.
All U.S. registered civil aircraft are required to be equipped with Emergency Locator Transmitters, which automatically activate when crash forces exceed 9 Gs, and transmit a
homing signal that can be tracked by special receivers. Most military fixed-wing aircraft have these ELTs installed as well, with the exception of certain fighter aircraft.
Fighters may crash several miles from the point of crew ejection, so
the parachute or seat frame containing the "Beeper" device is ideal for locating the crew, rather than the airframe. Large
tanker and cargo aircraft may contain jettisonable ELTs which are released by the crew prior to a crash, and are called "Crash Position Indicators" (
CPI).