Black boxes usually survive even the worst plane crashes

The black boxes on the Germanwings jet that crashed in France this week have been recovered, but both were damaged.

It remains to be seen whether investigators will be able to use what data they can recover the black boxes to confirm the now-dominant theory that the aircraft's co-pilot locked himself in the cabin and committed suicide by effectively flying the Airbus into a mountainside in the Alps.

Black boxes are pretty tough. The units on Air France 447, which crashed into the Atlantic Ocean in 2009, were found about two years later and worked fine.

The devices came into use in the 1950s, according to Boeing. They served the same role then as they do now: (1) to record what's happening on board and (2) be able to survive a crash so that investigators can discover the cause of an accident.

Today, airliners come with two units, the flight data recorder (FDR) and the cockpit voice recorder (CVR). The FDR keeps track of a wide variety of parameters, including speed, altitude, engine power, and flight controls. If something is happening that changes how the plane is flying, it's noted. The CVR records the pilots' voices, air traffic control, and noise in the cockpit.

On Germanwings 9525, the CVR enabled investigators to hear the attempts by the pilot to get back into the cockpit. Investigators were also reportedly able to hear the co-pilot's breathing and the screams of passengers when the plane crashed.

The FDR, however, was badly damaged. Various outlets reported that its memory card was dislodged during the crash and wasn't located.

The information contained on teh FDR and the CVR often makes the difference between being mystified by a plane crash and understanding what happened - and making sure it doesn't happen again.

The FDR and CVR are remarkably robust. According to the French Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA), both units can handle:

• acceleration up to 3,400 g (3,400 times the intensity of gravity on Earth)
• an hour in temperatures up to 1,100° C (2,012° F)
• a month in water six miles deep

No surprise the units survive nearly 100% of crashes.

Black boxes can't be disabled by the pilots, Steve Abdu, a 777 captain for a major carrier, told Business Insider.

The FDR and CVR are also designed to be found after an accident. That's why they're painted bright orange, not black. To handle ocean crashes, each is mounted with an underwater locator beacon, which emits a pulse that can be detected by sonar equipment up to two nautical miles away. It stays active for about 30 days, and works at a depth of 20,000 feet.

[An earlier version of this article was written by Alex Davies.]