The short answer
An ELT is a crash-activated radio beacon required in most US civil aircraft that transmits a distress signal to aid search and rescue. US regulations require inspection every 12 calendar months and battery replacement after 1 hour of cumulative use or when 50% of the battery's useful life has expired.
Maintenance items owners forget
The ELT carries two recurring clocks that live outside the engine and airframe intervals: the 12-calendar-month inspection (usually folded into the annual) and the battery expiration date, which is a hard calendar item stamped on the battery itself. Expired ELT batteries are among the most common squawks found at annuals.
Modern 406 MHz ELTs transmit a registered, identifiable signal to satellites; registration with NOAA (free, renewed every two years) is what connects the beacon's signal to the aircraft and its emergency contacts.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.