A black box is a device for assisting investigation into aircraft accidents. These are often the lone survivors of airplane accidents. A black box consists two parts, a cock pit voice recorder (CVR) and flight data recorder (FDR). It is fully automatic for fit-and-forget operation with a memory mechanism that would store hours of pilot voice and instrument readings up to the moment of any accident, but would automatically erase older records for the memory to be re-used.
Most of the black boxes these days use solid-state memory boards, considered much more reliable than their magnetic-tape counterparts. Solid state uses stacked arrays of memory chips, so they don't have moving parts. With no moving parts, there are fewer maintenance issues and a decreased chance of something breaking during a crash.
Data from both the CVR and FDR is stored on stacked memory boards inside the crash-survivable memory unit (CSMU). Using three layers of materials, the CSMU in a solid-state black box insulates and protects the stack of memory boards that store the digitized information. The memory boards have enough digital storage space to accommodate hours of audio data for CVRs and 25 hours of flight data for FDRs.
All of the data collected by the airplane's sensors is sent to the flight-data acquistion unit at the front of the aircraft. The flight-data acquisition unit is the middle manager of the entire data-recording process. It takes the information from the sensors and sends it on to the black boxes. Black boxes are powered by one of two power generators that draw their power from the plane's engines.
There may be up to four microphones in the plane's cockpit, each connected to the cockpit voice recorder (CVR). Any sounds in the cockpit are picked up by these microphones and sent to the CVR, where the recordings are digitized and stored. Black boxes are usually installed in the tail of the plane as it increases their chances of survival. Although called black boxes they are actually painted bright orange. This colour, along with the strips of reflective tape attached to the recorders' exteriors, help investigators locate the black boxes following an accident.
Black boxes fitted in a passenger aircraft contains a flight data recorder and a cockpit voice recorder. The flight recorder registers on tape or wire, information about the functioning of the aircraft and its systems such as speed, direction, altitude and other relevant details during the flight. The cockpit voice recorder contains a very long loop of tape as found in an answering machine. This tape runs continuously right from the take-off stage of the aircraft to its landing at the destination (or smashup in extreme situations) and records conversation at the cock-pit among crew on the flight deck and between crew and air traffic control. It is generally fitted with a homing radio beacon and flash light.
Each recorder is packed separately and kept in a crash proof, fireproof, floatable metal shell. It is painted bright yellow or orange for easy location in wreckage.
Source : The Hindu