Hello,
This signal can be heard and located by a variety of receivers. The beacon will operate
for 30-90 days (depending on the battery fitted) to a depth of 20,000 feet
So .. 30 days is not a limitation .. it's a choice made by the airline (I suppose)
Source:
Underwater locator beacon - Wikipedia, the free encyclopedia
See also:
Dukane Underwater Beacon (Pinger) - Science, Helderberg, and ZSSAS
And:
NTSB - CVR & FDR Note:
The NTSB do not mention that there are batteries that can run 90 days.
Is that 30 days is a minimum or a limit not to exceed imposed by the NTSB?
Regards.