Aircraft Accident Investigations

Although flying is generally a safe method of transportation, accidents occasionally happen—whether through human error, mechanical failure, or criminal activity. Over the last two decades (1985–2005), there have been between 30 and 65 fatal aircraft accidents per year worldwide. These, and lesser accidents, have to be investigated scientifically in order to gain important lessons about aircraft performance and safety.

The International Civil Aviation Organization (ICAO) requires that a civil aircraft accident be investigated by an independent body belonging to the country where the accident took place. Each country has its own organization taking responsibility for this: in the United States, it is the National Transportation Safety Board (NTSB ); in the United Kingdom, it is the Air Accidents Investigation Branch (AAIB). The NTSB investigates around 2,000 accidents and incidents a year. The purpose of the investigation is to find out why the accident happened and how similar events might be avoided in the future, rather than to apportion blame. The police will be involved in the investigation if sabotage or some other form of criminal activity is suspected, and the military generally looks into accidents involving service aircraft.

The ICAO sets guidelines on how an aircraft investigation is to be carried out. The first step is to report the aircraft accident or incident. An accident is an event involving death or serious injury, or severe damage to an aircraft. Missing or out-of-contact aircraft must also be reported. Incidents are not as serious; they are events best described as "near misses" involving problems such as forced landings, near collisions, or fires.

The investigation team consists of a permanent core group and outside scientific experts who are called on when needed. They respond immediately to the accident and go to the accident site. Each member will carry flashlights, tape recorders, camera and film, as well as any specialist tools. The investigation of the accident site and the wreck itself may take from several hours to a few days. During this time, the team works in small groups and gathers a wide range of data which may be relevant to the inquiry.

The history of the flight and the crewmembers' duties leading up to the accident are noted. Careful documentation is made of the wreckage and accident scene, with calculation of the impact angles so that the pre-accident flight path can be determined. The experts will also examine the engines and propellers, together with all details of the electrical, hydraulic, and pneumatic systems of the craft and the flight control instrumentation. The investigation is extended beyond the aircraft itself, to the weather prevailing at the time of the accident and the air traffic control instructions given to the plane. Crew-members are interviewed to look at possible human error factors, such as medical history, fatigue, training , workload, working environment, and drug and alcohol abuse. If survivors are involved, the team will document injuries, offer support, and arrange evacuation and rescue efforts.

A vital part of the aircraft accident investigation in the case of larger planes is recovery and examination of the flight recorder, also known as the black box. Airplanes usually have two types of flight recorders, the flight data recorder (FDR) and cockpit voice recorder (CVR).

The FDR is a digital recorder which may use magnetic tape to record, although solid-state devices are now available which can store data on memory chips. The flight data recorder records various flight parameters; the basic ones include: altitude, airspeed, direction, acceleration, and microphone keying, that is, the timing of radio transmissions made by the crew. Modern jet aircraft can record far more data than this with the most advanced logging up to a thousand different parameters, covering every aspect of the flight. The FDR records on an endless loop principle and contains data from the last 24 hours of the flight.

The cockpit voice recorder (CVR) records not only the voices of the crew in conversation or making radio transmissions, but any noise within the cockpit such as alarms, control movements, switch activations, and engine noise. Data from the CVR is correlated with that from the FDR by the microphone keying mentioned above. The CVR may be an analogue recorder using magnetic tape, although, as with FDRs, there are now solid-state devices that store the audio data in digitized form on memory chips. The CVR also operates on an endless loop principle and carries the last 30 minutes of audio information.

Analysis of data from the flight recorders may be the only way of establishing what happened in some aircraft accidents, particularly if they occur at night or where there is little recoverable wreckage. They are also very useful when a sudden event, such as a change in wind speed or direction, is the prime cause of an accident.

Flight recorders are built to protect the data from both high-speed impact and any fire that may occur after impact. They are usually located near the aircraft's tail, because past experience has shown that this is the area that generally suffers least damage in an accident.

Recovering the flight recorders after an accident can, however, be challenging. In the VH-IWJ West-wind 1124 accident, which occurred on October 10, 1985, an aircraft crashed into the sea off Sydney, Australia, shortly after takeoff. The pilot and co-pilot, who were the plane's only occupants, were killed. Three months passed before the flight recorders were discovered on the seabed. Analysis of the CVR at the Australian Transport Safety Bureau (ATSB) labs showed that the pilot was testing the co-pilot and simulating emergency instrument flight conditions. The FDR revealed a loss of control at 5,000 feet which caused the plane to crash.

The flight recorders are analyzed in specialized audio laboratories. The black box data can be integrated with that from other sources, including ground-based radar recorders, wreckage analysis, and eye-witness reports. Advanced computer graphics can be applied to create a graphical reconstruction or video of the sequence of events leading up to the accident or incident.

The mission of the investigative team is to produce an interim report within four to eight weeks that will be sent to all the interested parties for their comments. Safety recommendations are always given priority, so that any changes can be put into place as soon as possible. A final report will appear some time after the investigation and will be published on the website of the investigative organization. This final report includes all the factual information collected in the course of the investigation, an analysis of the information, and a conclusion which gives the cause of the accident or incident and any safety recommendations arising.

Aircraft accident investigations cover events from the crash of light aircraft flown by one person to major disasters such as the terrorist attacks of September 11, 2001. One example, which illustrates many of the aspects of accident investigations described above, is the investigation into the loss of Pam Am Flight 103 over Lockerbie, Scotland, on December 21, 1988. None of the 259 crew members and passengers on board the Boeing 747 survived and eleven people on the ground were also killed. The US-based plane was bound for New York, but it was the United Kingdom's AAIB which carried out the investigation—starting on the day of the crash and reporting in 1990. The plane was brought down by a bomb that had been placed in one of the overhead luggage lockers. The criminal aspects of the event were investigated separately, as were issues of airport security.

The flight recorders were found just east of Lockerbie, about 15 hours after the accident. These showed nothing unusual, stopping suddenly at the moment when the bomb exploded. Investigation of the crew and the aircraft's technical history ruled out human error or technical failure. Weather conditions were unremarkable and the air traffic control records were consistent with the sudden loss of the plane as shown by the flight recorders.

The plane disintegrated in mid-air, creating 1,200 significant items of debris requiring investigation. Larger items, such as the engines and the aircraft wings, fell on the town of Lockerbie, producing a fireball. Lighter debris was scattered for many miles.

Forensic scientists discovered traces of explosive material in the debris and were able to reconstruct the explosion and the impact it had on the plane. Postmortem examination of the victims revealed they died of multiple injuries consistent with a mid-air explosion followed by impact on the ground.

The loss of Pam Am Flight 103 was a tragedy with far reaching legal and political implications. But for AAIB, the priority was aircraft safety. The report recommends developing the facility for a flight data recorder to record the pressure changes associated with explosions. It also suggests that aircraft should be designed to better withstand the impact of an explosion.

see also Explosives; Flight data recorders.