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The first priority after a major accident is of course taking care of passengers and crew of the crashed vehicle, train, vessel or aircraft. But shortly after search-and-rescue operations are completed the question arises as to what happened, how and why. Teams of investigators assembled by the country in which the accident occurred, that from which the vehicle originated, and its manufacturer, gather at the crash site.

The first thing the teams try to recover is the black box. This expression does not refer to colour, which is bright orange, but to the fact that only the output of the box is of interest, without regard to its internal mechanism or structure. An aircraft carries two of these crash-impact-protected devices; a flight data recorder that records all operational variables during the flight and a cockpit voice recorder that records all conversation among crew members and theirs with the air-traffic control centre.

The debris from an accident is scattered over a wide area. Authorities are urged to remove this as soon as possible, but investigators may derive valuable information from the position of debris. A wide variety of survey instruments may thus be found at a crash site: scanners to produce a 3D-model of the wreck, total-stations for positioning small remains spread over the area, GNSS systems to establish control points, and photogrammetric cameras to focus on details.

The photo on this page shows the remains of a Boeing 737-800 which crashed recently during its approach to Amsterdam airport (Netherlands). Clearly visible is a scanner on a tripod gathering information to help investigators determine what happened, how, and, most important for avoiding future accidents, why. In this case the cause of the accident was traced to moisture leading an altitude meter to report erroneous data to the autopilot system concerning the height of the aircraft during the approach.Nine of the 128 passengers and nine crewmembers lost their lives. It is scant consolation to know that air traffic safety will be increased as a result of this accident.