How does a black box work?

If you want to establish why a plane crashed, you need to retrieve the black box. This virtually indestructible orange device records all relevant flight data and conversations in the cockpit.

For specialists at the German Federal Bureau of Aircraft Accident Investigation (BFU) in Braunschweig, evaluating data recorded by a black box is routine.

"I think we receive something like this every other week or so," said Jens Friedemann, spokesman for the BFU. "But such evaluations are also for incidents that aren't so spectacular, so-called serious incidents." And by serious incidents, Friedemann means events where a flight narrowly averted disaster.

Essentially, a black box flight recorder is heavily protected recording device, similar to a hard disk or a memory card. The black box records all relevant flight data, in addition to conversations in the cockpit. Previously, this data had to be recorded on two different devices. But today there are also units that can do both. According to regulations, however, every airplane must have two of these devices on board.

Robust, easy to find

A black box must be able to withstand many accident scenarios without sustaining damage. Before being put into use, they are tested to see if they can withstand an impact with a concrete wall at 750 kilometers per hour (about 466 miles/hour), a static load of 2.25 tons for at least five minutes, a maximum temperature 1,100 degrees Celsius (2,012 Fahrenheit) for one hour and water pressure found in depths of up to 6,000 meters (about 19,700 feet).

In order to be easier to find at sea, the devices send out a signal on contact with salt water that can be picked up within a radius of about two kilometers (1.2 miles). At such a short range, the location of the wreck should already be more or less pinpointed in order to find the device. Everything recorded

The voice recorder logs all sounds in the cockpit. In addition to discussions between the pilots, it also records automatic computer announcements, radio traffic, discussions with the crew and announcements to the passengers. The sounds of switches and engine are also recorded by the device.

Private conversations between the pilots are also stored on the black box - which is why the captured audio files must be handled carefully, from a data protection point of view. Discussions can only be evaluated in order to clarify accidents or malfunctions. For this reason, the recordings are overwritten after a maximum of 120 minutes; older devices only record 30 minutes. Technically, it's even possible for pilots to stop or delete a recording. In practice, however, BFU's Friedemann said pilots don't make use of that feature.

Ever increasing data quantities

But when it comes to the flight recorder, the second component of the black box, pilots are not able to directly access stored files. In older aircraft, they need to switch on the devices before flight; in modern aircraft, it's automatic.

The amount of data collected has increased significantly in recent years. "Today, hundreds, sometimes thousands of parameters are recorded there," says Friedemann. This includes information on things like the flight path, altitude, aircraft location, speed, temperature of the engine and exhaust, as well as flap positions, among many others.

The data helps experts investigate the cause of an accident or serious incident and reduce the potential sources for error. However, investigators do not fully reconstruct a flight.

"We don't use a flight simulator or animation - we're able to get information from the parameters themselves," said Friedemann.

There are only a few specialized agencies worldwide capable of evaluating a black box, and not every agency is able to examine the various models. The BFU can evaluate both Western and Russian devices. But with some models, the experts in Braunschweig must turn to foreign labs for help with the data.

In the future, Friedemann believes that video devices will record certain displays in the cockpit, and that the transmission power of the locator signal through water will be improved.

Incidentally, the so-called black box has never actually been black. The color is predetermined: bright orange.