One of two black boxes installed in the China Eastern Airlines flight that crashed on Monday has now been retrieved, however, in a heavily damaged condition. The black box’s condition has confused officials about whether the device is a cockpit voice recorder or a flight data recorder. 

Mao Yanfeng, the director of the accident investigation division of the Civil Aviation Authority of China, told reporters on Wednesday that officials are trying to locate the other black box. 

After a plane crash, recovering black boxes from the wreckage is essential to determine the events that possibly led to the crash. 

What is a black box? 

A black box is an electronic device fitted inside an aircraft to record flight data that can be used in case of an accident or malfunction. 

Types of black box recorders

Generally, commercial airplanes have two types of black box recorders located near the tail or the back of the aircraft. 

The CVR (Cockpit Voice Recorder) is used to tape conversations and announcements from the cockpit. The device can record up to two hours of data. 

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The FDR (Flight Data Recorder) is used to record technical details including temperature, altitude, speed, etc. The device can record data for up to 25 hours. 

How resilient are these devices? 

Black box recorders are constructed to withstand extreme impact, temperatures and underwater submersion. 

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How is a black box traced? 

A black box can emit signals even if an aircraft crashes into water, thanks to its built-in underwater locator beacon (ULB). The beacon is responsible for transmitting ultrasonic pulse that can be detected by audio and sonar equipment.

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The beacon of a black box runs on a battery with a shelf life of six years. After a crash, the beacon begins pinging each second for 30 days until the battery runs out.