ADELT: Crash Position Indicator with Deployable ELT
Two frequencies, two purposes — the swept tone that just says "here" and the digital message that says "here, precisely, from GPS."
Source: AW169 / AW169 EP RFM Supplement 15 (Crash Position Indicator with Deployable ELT) and Section 7 (System Description, OES).
What it is
A locator beacon on the left side of the tail cone, containing its own transmitter and antenna, that ejects from the aircraft in a crash — automatically, or manually at the pilot's discretion. Once deployed it transmits on two channels for two different purposes. The standard swept tone on 121.5/243.0 MHz — the classic analog distress tone — runs until battery power is exhausted, typically around 48 hours. Separately, 406.025 MHz carries an encoded digital message of the aircraft's position, sourced from the GPS/FMS via ARINC, and runs for about 24 hours.
The system comprises the beacon itself, a Beacon Release Unit, a System Interface Unit, a Cockpit Control Panel in the interseat console, a water-activated switch, and an Aircraft Identification Device.
Manual control
Two separate manual actions, not one: lifting the guard and operating the TRANSMIT switch starts transmission from the beacon without releasing it — useful for a self-test or if activation is needed without an actual beacon separation. Lifting the guard and operating DEPLOY on the cockpit control panel releases the beacon itself, confirmed by both the TX/TEST and BEACON GONE lights. Transmission alone can be stopped by moving TRANSMIT to off and pressing TEST/RESET.
Self-test discipline
The self-test should not be run more often than monthly — running it more frequently shortens the battery's life. Notify the nearest ATC facility before testing, since the 121.5 MHz tone the test actually transmits will be treated as a valid distress signal by anyone monitoring it, test or not.