Passenger aircraft have been fitted with cockpit voice recorders and flight data recorders for more than five decades now. The aircraft does not need these "black boxes" in order to fly. Their value becomes clear in the case of an accident, when investigators can recover a range of data from their memories and, consequently, identify the causes of the accident.
The general public generally only hears about these recorders in the event of an air crash, when the media report on the search for or finding of the black box. The boxes are not actually black in colour but orange, to make them easier to find. On the other hand the news from air crashes is usually black. It was the inability to explain the causes of an accident suffered by a DH 106 Comet, the first commercial jetliner, in 1954 that prompted Australian engineer David Warren to invent the black box. Australia was in fact the first country to introduce the compulsory fitting of these devices to all passenger aircraft.
The cockpit voice recorder (CVR) records all conversations between the pilot, co-pilot, crew and air traffic control and all sounds (movements of switches and levers, alarms, bangs, knocks, etc.) that are heard in the cockpit
The data collected are stored on magnetic tape or a data disk. Usually up to two hours of conversations are recorded. The memory medium also stores flight parameters. The flight data recorder (FDR) records various parameters of the aircraft's systems and its surroundings. Modern FDRs record several hundred different parameters, including time, course, altitude and airspeed, vertical speed, acceleration, throttle position, control stick position, aileron and flap positions, engine rpm, pressure and temperature outside the aircraft, angle of attack, etc. The data stored in the FDR are stored for up to 25 hours, after which they are overwritten by new data. The CVR and FDR are built into the tail of the aircraft, where there is the greatest likelihood of them surviving an impact of the aircraft with the ground with the smallest amount of damage. In this case the aircraft's fuselage acts as a kind of buffer zone.
Black boxes are the main source of information about what happened in the aircraft and to the aircraft before a crash.
Therefore are built to be able to withstand an acceleration of 3,400 g (33 km/s²) for 6.5 milliseconds. This is enough time for the data stored in the black box to survive such an impact. Because of the high probability of fire in the case of an accident, the black box must be able to withstand temperature of 1,100 °C for half an hour. In the event of an aircraft crashing into the sea, the black box must be able to withstand water pressure at a depth of 6,300 metres. In order for accident investigators to find the black box as easily as possible, it must emit a signal at 37.5 kHz from this depth for at least 30 days. The signal is triggered by the contact of the black box with the water.
The data obtained from black boxes and material evidence from accident sites are usually enough to enable investigators to decipher the causes of air crashes. The directives which accident investigators send to aircraft manufacturers and airlines on the basis of their findings help prevent similar tragedies that could otherwise come about from similar or identical causes.