[Selanne_200]

I stand corrected.

If a plane crashes into the water, an underwater locator beacon sends out an ultrasonic pulse that cannot be heard by human ears but can be detected by sonar and acoustic-locating equipment.
There is a submergence sensor on the side of the beacon, and when this sensor touches water the beacon activates.
The beacon then sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep as 14,000 feet (4267m).
Once the beacon begins “pinging,” it pings once per second for 30 days. In rare and severe crash instances, the beacon may get snapped off during a high-impact collision.
Search teams must be about 25km in range of the beacon to locate it. Underwater terrain such as deep trenches could also affect how easily the beacon’s signal can be detected.