The U.S. Navy is contributing two types of equipment to the search effort. One is an unmanned, undersea drone. It's called the Bluefin autonomous underwater vehicle, and it comes with sonar systems designed to map and penetrate the seabed, as well as side-scanning sonar to detect objects nearby. The Bluefin-21, which is the system the Navy's using, can dive to nearly 15,000 feet and move at 4.5 knots.
The other system is essentially an underwater microphone. Known as a towed pinger locator or the TPL-25, it's a listening device designed to pick up the audio signals coming from the black box. The TPL-25 is towed behind a ship about 1,000 feet off the sea floor at around 3 knots, according to the Navy. It can hear up to a mile in two directions, and the Navy estimates that it can search about 150 square miles of ocean per day with the device. But compared to an overall search area of more than 90,000 square miles, the chances of finding Flight 370's black box this way seem slim.
For lack of alternatives, our search techniques are little more sophisticated than simple trial-and-error. The flight recorder isn't here? Move onto the next grid. The painstaking process is a grim reminder of how some technology tends to plateau once it's gotten good enough. The towed pinger locator, for example, is the same machine that helped locate the flight recorder from TWA 800, the 1996 crash that killed 230 people. After nearly two decades, our search and rescue techniques haven't changed much.
Some argue that the constraint lies in the black boxes themselves. If only they could stream data in real time back to the Earth, we wouldn't need to go searching for them in the event of a disaster. Fair enough. But as Michael Planey, a former air safety investigator for the Air Force, explained to The Atlantic, any data being streamed by the plane would likely get cut off in an emergency, anyway — meaning we'd still need to find the flight recorder to get the full picture:
In the case of the immediate, catastrophic event, data streaming would likely cease at the moment of the event. Either a complete loss of electrical power would disrupt the data stream or a mechanical break in the aircraft systems would prevent data transmission. Further, if an aircraft was in an out-of-control attitude such as a steep dive, a spin or a hard roll, maintaining a direct link with a satellite would be nearly impossible, thus again breaking the data stream and rendering the system incapable.
That still doesn't answer the question as to why we're still reliant on manually scouring the ocean after the fact, however.