Each flight recorder carries an Underwater Locator Beacon, a small cylinder bolted to the front of the unit. It activates automatically on immersion in water and emits an acoustic pulse — the "pinger" — at 37.5 kHz, roughly once per second.
The old standard: 30 days
For decades the requirement was a minimum of 30 days of transmission from a battery that also had to survive years of dormancy, deep-sea pressure and the impact that put the recorder in the water.
Thirty days is a long time in most accidents. If an aircraft goes into water near a coast, searchers are on scene within hours and the beacon is found comfortably inside its life.
Twice it was not enough
Air France 447. The wreckage was in the mid-Atlantic at around 3,900 metres. Surface debris was found quickly, but the search area on the seabed was large and imprecisely defined. Vessels towing hydrophones searched, and the 30 days expired without a detection. The recorders were eventually found nearly two years later by autonomous underwater vehicles running a systematic survey.
MH370. Far worse. It took most of a month simply to work out which ocean to search, using the Inmarsat handshake analysis. By the time assets were in the right region, the beacons were at or beyond the end of their life. Acoustic signals detected in April 2014 raised hopes and were later assessed as not coming from the aircraft.
In both cases the limiting factor was not the beacon's design but the time needed to define where to listen.
What changed
Two requirements followed.
Ninety days. Beacon life for flight recorders was extended from 30 to 90 days, tripling the window in which a located search area can still produce a detection.
A second, longer-range beacon. Aircraft operating long over-water routes must carry an additional beacon mounted on the airframe itself, transmitting at a much lower frequency — 8.8 kHz. Low-frequency sound propagates far further through seawater: the 37.5 kHz recorder pinger is detectable at a few kilometres, while the low-frequency airframe beacon can be heard at tens of kilometres.
That difference matters enormously when a search vessel is towing a hydrophone through thousands of square kilometres. It widens the effective search swath by an order of magnitude.
The practical takeaway
A beacon is only useful if someone is listening within its range while it is still transmitting. Both parts of the improvement address that: more days, and a much wider circle. Neither helps if nobody knows which ocean — which is why the tracking requirements introduced alongside them matter more than the beacons themselves.