The most important pieces of wreckage to be recovered are usually the two recorders carried on all civil airliners.
The flight data recorder provides a record of the movements of key instruments, such as airspeed and altitude indicators, and the positions of rudders and spoilers. The information is recorded as electronic pulses on a tape. When the tape is played back, it will give a computer printout of the aircraft’s movements. A computer screen can also be programmed to look like the main instruments, giving a more realistic playback.
In the future it may be possible to increase the items recorded and to play them back through a flight simulator, which is normally used for training pilots, to re-create the cockpit of the aircraft in the hours before it crashed. Flight data recorders can record up to 200 hours of flying time.
Cockpit voice recorders pick up conversations and sounds of the crew. They work on a continuous tape lasting 30 minutes, so at any time only the last 30 minutes is recorded. A weakness of the system is that if a crash does not stop the recorder working, it can remain switched on and erase the vital section.
The recorders are installed in the rear of aircraft – the area most likely to survive a crash. They are housed in a case made of two shells of stainless steel with heat protective material between. They must be able to survivor a temperature of 2000ºF (1100ºC) for 30 minutes.
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