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.