Aircraft emergency frequency

[1] Earlier emergency locator transmitters (ELTs / EPIRBs) used the guard frequencies to transmit.

The notice to airmen FDC 4/4386 requires "…all aircraft operating in United States National Airspace, if capable, shall maintain a listening watch on VHF GUARD 121.5 or UHF 243.0.

[3] In the United Kingdom, pilots may make "Practice PAN" or "Training Fix" calls.

Satellites listen for the signals and alert local personnel to the emergency, and the beacon allows search and rescue to find the scene of the accident faster.

Beacons operating at 406 MHz are encoded, allowing the vessel of origin to be determined and false alarms to be quickly verified.

Cessna radio on 121.5 standby
A Swiss Air Force F/A-18 Hornet fighter with the order STBY 121.50 written on its external tank