The examination into the causal factors of an aircraft mishap or incident.
Those of the flying public who are not airline pilots, and even some pilots who fly as passengers, are sometimes nervous while doing so. Although airline accidents occur infrequently, those that do occur can be catastrophic events involving a great loss of life. Media coverage of airline accidents is usually extensive, fueling the uneasy feelings many people have about airline travel. However, the periods after aviation accidents are often the safest times in which to fly. Further, contemporary aviation accident rates are very low, in relation to other types of accidents, such as automobile accidents.
Because aviation always involves the risk of an accident, accident investigation is an important element of aviation education. By studying the accidents of other pilots, less experienced aviators can avoid making similar mistakes.
A commonly noted pattern in aviation accidents is that there is rarely only a single reason for the accident. Experienced pilots refer to the events leading up to an accident as an error chain. Individual links of the chain, when combined, cause an accident to happen. For instance, bad weather alone might not cause an accident, but bad weather combined with darkness and the fact that the pilot became lost might. The error chain is weather, darkness, and becoming lost. The elimination of any one of these factors may prevent the mishap. In other words, if one link of the chain were broken, the accident would not happen.
The key to accident investigation, then, is to determine the error chain leading up to the event. Rectifying the situation regarding an accident under investigation is impossible, because the accident has already happened. However, accident investigators can study each link of the chain and prepare documents to report their findings. In turn, other flight crews can study the reports and avoid the same fate.
The vast majority of aviation accidents result from human-factor errors. All those who have roles in launching an aircraft, including pilots, mechanics, air traffic controllers, cabin attendants, and baggage handlers, can make mistakes that may cause an airplane accident.
In the study of aviation accident investigation, an important statistic is the number of accidents compared to the hours flown. The number of accidents per 100,000 hours determines a figure known as the accident rate. With the exception of the mid-1990’s, the accident rate has been declining since 1982. The number of accidents has declined, as has the number of fatalities. The Federal Aviation Administration (FAA) and others in the industry attribute better pilot education and technological advances for improvement of aviation safety statistics.
According to statistics compiled by the industry, approximately 70 percent of air carrier accidents are the results of flight crew error. Maintenance error constitutes another 5 percent, while air traffic control or other airport issues account for about 4 percent. Human error is responsible for a total of almost 80 percent of the commercial airline accidents worldwide. Of the remaining 20 percent, mechanical failures make up 11 percent, weather factors account for about 4.5 percent, with the remainder categorized as miscellaneous or other.
Pilots and first officers are responsible for most of the human error accidents. Reasons for the flight crew’s mistakes are many, including loss of situational awareness, flight crew fatigue, and training and operational issues.
Accidents can occur during all phases of flight, from pushback to arrival at the destination gate. However, the majority of accidents, almost 56 percent, happen during the approach and landing phase of the flight.
The reasons for approach and landing accidents vary. If a flight has been a particularly long one, crew fatigue can play a significantly greater role at the end of a flight than at the start. Being tired or fatigued can impair a crew’s decision-making process. Poor destination weather combined with a tired flight crew could be a recipe for disaster.
During an accident investigation, there are many simultaneous issues requiring attention. The first and most important consideration is to assist the injured. Medical personnel are needed immediately to administer to those on site, and provisions are needed to transport patients to the nearest medical facility as quickly as possible. The rescuers also need to determine where the injured were sitting in the aircraft and where they ended up after the accident. The next task is dealing with the survivors of anyone killed in the accident; if there is even one fatality, the loss touches many people.
After the first officials arrive on the scene, their first order of business is to secure the area. Another important task is to observe evidence that is transient in nature. For instance, a popular twin-engine aircraft seemed to be crashing for no reason. It took four such crashes of a similar nature before investigators arrived at the wreckage quickly enough to determine that ice forming on the aircrafts’ horizontal stabilizers had caused the accidents. After the previous accidents, the ice had melted before anyone could see or record its presence.
After first seeing to the injured, personnel guarding the accident scene have several responsibilities. They must make certain the wreckage is not disturbed, because if someone moves the wreckage, aviation accident investigators will no longer be able to see the parts of the aircraft as they came to rest after the accident. Consequently, investigators will lose many clues that may help them determine a probable cause of the accident. Another essential task is to determine whether hazardous materials were being transported and are present at the scene. If so, personnel must take measures to protect everyone on scene from the dangers of the hazardous materials.
It is important for the accident investigators to photograph the scene. Photographs of the wreckage can preserve the visual evidence of the accident for later analysis and should include all aspects of the accident: from individual parts of the aircraft to any fatalities where they lie. Marks caused by the craft’s hitting the ground and aerial photos are important to show the path of the aircraft as it crash-landed.
In addition to photographs of the crash scene, the investigators should also draw sketches and maps of their observations. By using the photographs, sketches, and maps, they will be able to create a diagram of the last moments of the flight. This diagram of the flight’s last moments is essential to understanding the decisions made by the crew.
Guarding the wreckage is an important responsibility for those tasked with the job. Guards should be somewhat familiar with aviation. They must protect the property, the wreckage, and the crash site from being disturbed. They have the difficult duty of making sure that people do not wander through the area. They also collect the names, addresses, and telephone numbers of anyone who may have witnessed the crash. If an accident occurs in a remote location and all aboard sustain fatal injuries, eyewitness statements may be nonexistent. Anyone with knowledge of the accident must be located because witnesses can be very important in helping to determine the cause of an aviation accident.
The best witnesses to an aviation accident are not other pilots, or those involved in the industry. In fact, they are not even adults. Children often provide the most accurate and unbiased statements about aviation accidents. Adults often tend to put their own spin on an accident. Pilots who witness accidents may inject far too much opinion into their account of what happened. Children very simply report what they see.
Once notification of a major aircraft mishap reaches the authorities, the National Transportation Safety Board (NTSB) launches a go team. The team originates from Washington, D.C., where the members of the NTSB rotate the duty of being on the team. While the go team is en route, it is the job of the local authorities on scene to organize the agencies to start the rescue or recovery procedures.
The accident investigator is indeed a detective. Typically, it is very difficult to determine exactly what caused an aircraft accident. The investigator’s first order of business is to sort through the pieces of wreckage, cataloging the more and less obvious clues leading to the most probable cause of the mishap. In many cases, it is much like looking for the proverbial needle in a haystack.
Once the team is on site, it will survey the wreckage to determine where the aircraft initially struck the ground. Damaged shrubbery and trees may mark the path of the airplane into the crash scene. The investigators will note the general condition of the wreckage while trying to account for all the parts. If parts or components of the aircraft are missing from the accident scene, this may be indicative of an in-flight structural failure. If the empennage, or tail, or the wing of the aircraft came off the airplane at a high altitude, that could well be the cause of the accident. Those parts, once separated from the main aircraft, will fall to the ground sometimes miles away from the main site of the wreckage.
The investigators begin with the physical aspects of the accident. They collect parts, examine each one, and map its final placement against the original installation on the aircraft. They also have to be certain that the aircraft is complete; parts of the aircraft missing from the wreckage site suggest an in-flight breakup in which other essential components of the airplane have landed elsewhere.
From the physical evidence, the investigators can then determine the aircraft’s approximate angle and speed of impact. They can also determine whether the engines were working at the time of impact. They map and photograph the wreckage to preserve as many of the clues as possible. After this work is complete, they will begin their true detective work.
This detective work begins with a review of the pilot’s qualifications and an examination of the pilot’s training and certification documents and medical records. The investigators conduct interviews with the pilot’s friends, peers, and relatives. They review autopsy results. They create a pathological history for a seventy-two-hour time period leading up to the accident and conduct weather data analysis, among other things. The investigators check into the pilot’s physical and psychological makeup and try to determine the pilot’s state of mind at the time of the accident. They question the pilot’s aeronautical decision-making abilities, look into recent history of the pilot’s judgment, and even evaluate the pilot’s training and experience.
Investigators also try to determine whether the weather was a factor in the accident, relying upon official weather reports and forecasts. They look for indications of low visibility, turbulence, extreme wind shear, or heavy rains that may been contributors.
Finally, investigators examine the aircraft wreckage to determine whether mechanical malfunctions may have caused the crash. This is one of the more intense segments of the investigation. The aircraft will undergo reconstruction in a secure hangar or other facility. Plans of the aircraft are helpful in determining that all parts of the aircraft have been retrieved from the wreckage site and adjacent areas.
The aircraft’s reconstruction helps the examiners find signs of structural failure. The key to determining structural failure involves asking whether engine failure caused the accident or whether it caused the breakup of other parts of the airplane. Investigators try to figure out where such a breakup first occurred. This is the most intriguing part of accident investigation, and it may go well beyond the expertise of the investigators. On many occasions, expert witnesses, such as metallurgists, are necessary to assist in finding the answers.
Every aspect of the aircraft is under dissection during the accident examination. Disassembly of each component and system of the aircraft will follow for investigation of any possible failures. Examination of the flight controls may reveal a frayed cable or a broken bearing; a control pushrod may have become bent, allowing aerodynamic flutter to start. That aerodynamic flutter may have caused an actual structural failure of the elevator or rudder, hastening the accident.
Investigators also check switch positions at the time of the crash. They are especially interested in the positions of the switches and controls and the relative positions of the associated components. These indicate whether the pilot may have done something improper to cause the accident, such as raising the flaps at the wrong time or unintentionally dumping fuel, resulting in fuel exhaustion. A component failure may be indicated if a switch was properly set and the component discovered is not positioned per the switch selection.
Investigators are also intensely interested in the instrument readings at the time of impact. After removal from the crash site, each instrument is sent to an appropriate laboratory for intense post-accident analysis. At the time of impact, each needle on the face of an instrument leaves impact marks that allow the technicians to determine exactly what measurement the instrument was indicating at the time of the crash. With this technique, investigators can determine the speed of the aircraft. They can also make corroborations between engine operations and other instrument indications that may help in explaining the accident.
The investigators have a high interest in the navigational instruments and radios, along with everything else in the cockpit. This is particularly true if the accident happened in poor weather. By careful analysis of the frequencies selected, the switch positions of the units, and the readings at impact, examiners can determine if a navigational error factored into the cause of the accident. All other systems in the airplane, such as the generators or alternators, the vacuum systems, pneumatics, and hydraulics, are also scrutinized during the investigation.
Flight recorders and cockpit voice recorders, carried in the tail of an aircraft, are important elements in accident investigation. They provide critical clues in solving the mysteries associated with many of the world’s air disasters and are invaluable in helping to prevent future accidents. Although they are known as black boxes, they are actually painted bright orange to aid in their recovery following an accident.
Aircraft flight recorders record many different operating conditions of a flight and provide information that may be difficult or impossible to obtain by any other means. Cockpit voice recorders record the flight crew’s voices, as well as other sounds within the cockpit, including communications with air traffic control, automated radio weather briefings, and conversation between the pilots and ground or cabin crew. Sounds of interest to an investigation board, including engine noise, stall warnings, landing gear extension and retraction, and any clicking or popping noises, are also typically recorded. Based on these sounds, important flight parameters, such as speed, system failures, and the timing of certain events can often be determined.
In the event of an accident, an investigation committee creates a written transcript of the cockpit recorder tape. Local standard times associated with the accident sequence are determined for every event on the transcript. This transcript contains all the pertinent portions of the cockpit recording. Due to the highly sensitive nature of the verbal communications inside the cockpit, a high degree of security is provided for the cockpit recorder tape and its transcript. The timing of release and the content of the written transcript are strictly regulated.
The examiners find the pilot’s autopsy results particularly important in determining whether pilot incapacitation caused the accident. The pilot may have experienced a heart attack, stroke, or some other medical factor that caused incapacitation. The pilot may have passed out due to hypoxia or carbon monoxide poisoning. The results of the autopsy enable the investigators to assign probable cause to the pilot or rule out incapacitation.
The airplane also undergoes a mechanical autopsy of sorts. After an aviation accident, the maintenance records and logbooks of the accident aircraft are collected. NTSB examiners examine the records in search of evidence of possible material defects or mechanical malfunction. The inspectors may determine that there were metallurgical or manufacturing defects in the history of the aircraft. They may uncover improper maintenance procedures, such as that a life-limited part has been allowed to exceed its time in service. These are only a few of the possible explanations for the accident.
These observations and examinations comprise the bulk of the investigators’ work. The compilation of information on the accident and the examination of evidence may take months, or even years. Many people in the agency are involved in the search for answers about the cause of the accident. As the work is completed, many more people await the results, some patiently, others less so.
The meticulous work of the accident investigators takes time, however. Although this challenging work is sometimes tedious and demanding, it must always be thorough. Its reward is the promise of the prevention of future accidents.
Ellis, Glenn. Air Crash Investigation of General Aviation Aircraft: With Emphasis on the Crash Scene Aspects of the Investigation. Greybull, Wyo.: Capstan, 1984. An explanation of the investigative process followed by the Federal Aviation Administration and National Transportation Safety Board regarding the general aviation industry. Hawkins, Frank H. Human Factors in Flight. 2d ed. Brookfield, Vt.: Ashgate, 1987. The definitive volume on the study of humans as they relate to aviation and aviation accidents and required reading for anyone considering a position with the NTSB. Panas, John. Aircraft Mishap Photography: Documenting the Evidence. Ames: Iowa State University Press, 1996. An excellent guide to aircraft accident investigation. Although the emphasis is on photography, the text remains an excellent source of information regarding aviation accidents and inquest. Wells, Alexander T. Commercial Aviation Safety. 2d ed. New York: McGraw-Hill, 1997. A reference work that covers all aspects of aviation safety, including the management of human error, aircraft safety technology, the nature of accidents, and flight standards and rule making.
National Transportation Safety Board
After an airplane crash, investigators arrive at the scene to inspect the debris. Comparisons of where different parts originated on the craft and where they landed on the crash site are often key to discovering the cause of the accident.