Why do aeroplanes need a black box?

How important aircraft data is designed to survive the worst disasters

Black box recorders are used to retrieve data about an aeroplane and its operating environment in the event of a crash. There are two types of black box: the CVR or Cockpit Voice Recorder, and the FDR or Flight Data Recorder. Both record different types of information, and when combined this information can be used to build up a picture of what happened during a crash.

The Cockpit Voice Recorder picks up sound from inside the cockpit, including the pilot and copilot’s headset microphones and those of any other cockpit staff. There’s also a microphone placed centrally in the cockpit to record any other ambient sound, such as conversations with other crew members, radio, and even the noise of switches and dials. They used to be magnetic tape recorders but are now more reliable solid state devices akin to flash drives. These record around two hours of information at a time, recording over and replacing older audio. The CVR allows listeners to find out what the cockpit staff were doing in the event of the crash; what they observed and reacted to among the circumstances that caused it, if they sent out a Mayday message or signal or recorded any grid co-ordinates. 

The Flight Data Recorder, on the other hand, records important information about what the plane was doing at the time. There are several areas of the plane it takes data from, including the wings, engines, landing gear and rudders. This information is aggregated in the Flight Data Acquisition Unit at the front of the plane and fed into the FDR at the back. Typically it includes factors like speed, altitude, engine performance and the positions of the wings, rudder and landing gear. 

Inside the black box

Where is it kept?

Sensors that feed data to black box recorders are located in key areas of the plane, such as the engine, wings, rudder and landing gear. Microphones and data recording and processing units are stored around the cockpit to record voices, ambient noise and data from the cockpit instruments. The actual black boxes themselves though – the units in which the recorded data is stored – are located at the back of the plane beneath the tail. 

This location lessens the chance of the black box being destroyed, as it’s not in an area which would take the impact of a head-on or belly-down crash, and it’s as far away as possible from the most combustible areas of the engine.

This article was originally published in How It Works issue 5

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