One of the first things that is asked when any airplane crashes, is what caused the accident and this is when investigators looking into the accident turn to the black box, which is made up of a flight data recorder and cockpit voice recorder. The black box is able to reveal details of events happening just before the accident.

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[Courtesy of Image: L-3 Communication Aviation Recorders]

While many of the black boxes make use of magnetic tape, many airlines are moving towards the use of solid state memory. The difference between the two is that the former works along the lines of a tape recorder while the latter are now considered to be the more reliable and use stacked arrays of memory chips and don’t have parts that are moving. This means there is less chance that the black box will be damaged in an accident. Typically memory boards can store two hours of audio data and 25 hours of flight data from the FDR.

The data collected by the sensors of the airplane goes to the flight data acquisition unit which is located towards the front of the aircraft, and it sends the data to the black boxes.

The black boxesare powered by two generators which take their power from the engine of the aircraft.

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[Courtesy of Image: L-3 Communication Aviation Recorders]

Several microphones are built into cockpits in aircraft and these can track the conversations of the flight crew. They also pick up any ambient noise, including bangs, knocks or thuds. Up to four microphones are located in the cockpit of the plane and sounds picked up by them are sent to the CVR and stored. Magnetic CVRs are able to store the last 30 minutes of sound and they continue to cycle every 30 minutes. Solid state recorders also record over previous sounds. The information from these can be very helpful in case of the plane crashing as it offers sound just before the accident.

The flight data recorder has been designed to record operating data from the system of the aircraft, with sensors being wired to various parts of the aircraft to the flight data acquisition unit, which in turn is wired to the FDR. When the switch is turned on or off the operation is recorded by the FDR.

Solid state recorders are able to track various parameters; these include time, pressure altitude, airspeed, magnetic heading, rudder pedal position, fuel flow and horizontal stabilizer. They are able to store up to 25 hours of flight data and each of the parameters is able to offer clues about what may have been the cause behind the accident.

Often in the case of accidents the only thing to survive intact is the crash-survival memory unit of the flight data recorders and cockpit voice recorders. Usually the inner components, along with the chassis of the recorder are mangled. The CSMU has been designed to withstand extreme heat along with many tons of pressure and violent crashes, using three layers of materials. The first is the housing which is made of aluminium, the second is high temperature insulation and the third is a stainless steel shell.

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[Courtesy of Image: National Transportation Safety Board]

The name Black Box can be misleading as generally they are painted bright orange. They also have reflective tape on the exterior and the bright colourings help investigators to find the black box following an accident. The black box also holds an underwater locator beacon which allows it send out an ultrasonic pulse in the event of the aircraft being submerged underwater. The beacon is automatically activated when water touches it.

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When investigators find the black box it is taken to a lab and the information from it can be downloaded and the final events of the aircraft recreated to get an idea of what caused the accident. This can sometimes take many weeks or months. In some cases the black box is dented and burned when retrieved but usually the memory boards can be removed and information got from them after cleaning them.

The black box is a very valuable tool in determining what went wrong and what caused the accident. It often provides clues which would otherwise be impossible to find. Black boxes will continue to be an important role in investigations of accidents.

The black box is the key to finding out what happened to the missing Malaysian flight 370 that vanished recently. However at the moment the box has disappeared too, taking evidence with it. This has started a debate about whether the black box should be replaced with a satellite based system. The jet disappeared with 227 passengers and 12 crew.

Via [How Stuff Works]