What is a black box?
It's a device that helps air-crash investigators determine what went wrong, by providing a detailed record of a plane's final hours. Passenger jets normally carry two of these shoebox-size containers — one holding a voice recorder, which captures pilots' conversations and other cockpit noise, and another housing a data recorder, which logs the plane's speed, altitude, direction, fuel flow, hydraulic pressure, and hundreds of other metrics. These steel-and-titanium black boxes — which, despite their name, are actually painted orange to make them easier to spot — are fire resistant, waterproof, and engineered to survive being smashed into the ground at over 300 mph. Most flight recorders are stored in the tail section of the aircraft, which is usually the last point of impact in a crash.

When were they invented?
Devices that could log very basic flight data were introduced at the start of World War II, but none recorded cockpit audio. Then in 1953, Australian scientist David Warren was asked to investigate a spate of mysterious crashes involving the world's first jet-powered airliner, the de Havilland Comet. "I kept thinking if only we could recapture those last few seconds, it would save all this argument and uncertainty," he said. Warren developed a prototype machine that recorded pilots' voices as well as instrument readings — it was supposedly nicknamed the "black box" by a reporter who was mystified by its workings — and by the mid-1960s the equipment was mandatory for all commercial planes. Since then the technology has improved, with data now being stored on durable flash memory rather than magnetic tape.

How have cockpit recorders helped investigators?
The black box from United Airlines Flight 93, one of four planes hijacked on Sept. 11, 2001, confirmed investigators' suspicions that hijackers deliberately crashed the aircraft into a Pennsylvania field to stop passengers storming the cockpit from gaining control of the jet. "Shall we pull it down?" one terrorist can be heard saying, as the sound of fighting grows closer. "Yes," another terrorist replies, "pull it down." In other cases, background noise captured by recorders has provided vital clues. In 1987, for instance, a South African Airways 747 jet aircraft crashed into the Indian Ocean, killing all 159 people on board. When the voice recorder was recovered two years later, it revealed the sound of popping circuit breakers — a sign of an onboard fire.

How are the boxes recovered?
On land, search teams just look among the wreckage. Recovery is more complicated if a plane goes down in the sea — like missing Malaysia Airlines Flight 370, which is thought to have crashed in the Indian Ocean in March. Each black box is fitted with a water-activated beacon, which emits a sonar "ping" every second for at least 30 days. But search teams have to come within a couple of miles of the box to pick up those pings. In the case of Flight 370, the initial search area was more than 660,000 square miles, making the odds of finding the boxes quite slim. "We're not searching for a needle in a haystack," said Mark Binskin of the Australian Defense Force. "We're still trying to define where the haystack is." Once a beacon's batteries run out, the chances of recovering a black box decrease even further. Recorders from at least seven planes that crashed in the sea have never been found.

Are there other problems with black boxes?
Apart from being hard to find, they also don't provide a complete record. The devices store about 25 hours of flight data, but only the last two hours of cockpit audio. In most crashes, that's enough to determine what happened. With Flight 370, it might not be. Investigators suspect the crew was probably incapacitated during the flight — by a fire, loss of cabin pressure, or a climb to a very high altitude — and that the autopilot flew the plane until it ran out of fuel and dropped into the ocean. If that's the case, the audio recording would just consist of two hours of silence.

Can the technology be improved?
Some experts have suggested that airliners introduce black boxes that eject on impact and float on water — technology already used by the U.S. Navy. The United Nations is also examining proposals to wirelessly stream cockpit recordings and flight data to computer servers on the ground, so investigators would immediately know why and where a plane went down. Pilots' groups have opposed this technology, arguing that constant monitoring would violate their privacy, but the bigger issue is cost: about $100,000 per plane for installation, and much more to keep transmitting the data. "[Airlines] are very cost-sensitive," said Mary Schiavo, former inspector general of the Transportation Department. "They simply will not add additional safety measures unless mandated by the federal government." Following the disappearance of Flight 370, though, many governments might look more favorably at streaming. "Navies and airplanes from around the world are searching for this plane," said Mark Rosenker, a retired U.S. Air Force general. "That's not cheap either."