When a plane goes down in the ocean, the black boxes aboard have enough juice to ping for thirty days. The pingers on flight data recorders AKA “black boxes” last a minimum of 30 days. After 30 days, the devices are still active, but the sound on which searchers hone is expected to die out. The pinger is located by a “pinger locater,” a device that listens for the sound of the black box. It is towed within the search area but it’s listening radius is usually around 2 miles. The pinger’s sound is not very powerful, and the pinger is towed at 3 knots.
After AirFrance 447, legislation was underway to increase the battery life to 90 days. The technology exists, but because implementation of that transition has been slow, EgyptAir MS 804’s pinger battery is expected to expire at around 30 days.