ELT

An Emergency Locator Transmitter is a radio beacon mounted in the aircraft that activates automatically on impact (typically a G-load above threshold) and transmits a distress signal that satellites and search-and-rescue aircraft can use to locate the crash site.

The current standard for new installations is 406 MHz. Older 121.5 MHz ELTs still exist in the GA fleet but are no longer monitored by satellite; they're only picked up by aircraft within local radio range.

FAR 91.207 requires the ELT to be inspected within 12 calendar months of the previous inspection, and the battery to be replaced when the transmitter has been used for more than one cumulative hour or when 50% of the battery's useful life has expired (whichever comes first).