BLACK BOXES EXPLAINED

Editor's note: After a China Eastern Airlines jet crashed in South China's Guangxi Zhuang autonomous region, rescue teams have managed to find the two black boxes, which may provide the keys to understanding why the plane crashed.

What are black boxes?

Black boxes are flight recorders placed in an aircraft for the purpose of helping investigations of aviation accidents.

An airplane has two black boxes. The flight data recorder captures information such as altitude, airspeed, direction and engine thrust, which can reveal whether human error or instrument problems contributed to an accident. The cockpit voice recorder records pilots' voices and cockpit sounds.

What do they look like?

Black boxes are not black. They are painted bright orange to make it easier for searchers to find them after accidents. Strips of reflective tape are also attached to the exterior.

Where are they placed?

Black boxes are usually placed in the tail of a plane where they are more likely to survive a crash.

How much data can a black box store?

A flight data recorder is required to store a minimum of 25 hours of flight information. A cockpit voice recorder is required to record a minimum of two hours of audio information.

How strong are they?

For black boxes to survive a crash, they are built to be waterproof, fireproof, shockproof and heat, pressure and corrosion resistant.

The black box must be able to withstand an acceleration of 3,400 Gs (3,400 times the force of gravity), which equals an impact velocity of about 500 km/h.

It must also survive flames of temperatures up to 1,100 C for one hour, and the beacon should be able to emit a signal once per second while submerged in 6,000 meters of saltwater for at least 30 days.

How is data from black boxes retrieved?

Special techniques are required to physically restore the black boxes. Investigators may need to clean and vacuum dry the black boxes to download data from them.

Does it take long to decode the data?

It takes time to download and decipher data. If the data storage units are damaged, it will take longer. Interim reports into a crash are published after a month, but often have sparse information. Deeper investigations take a year or more to complete.

Who made the black boxes?

The crashed China Eastern Airlines jet is a Boeing 737-800. Its black boxes were made by Honeywell in the United States.