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Kegworth Air Crash Investigation
Kegworth 1989: An accident waiting to happen?
On 8 January 1989, 092 was a scheduled domestic flight from London Heathrow Airport to Belfast, Northern Ireland. It was the second flight taken by a British Midland Boeing 737-400 that day, and the disaster was caused by mechanical and human error as the aircraft neared its landing destination.
While preparing to land at East Midlands Airport, the aircraft (tail marking G-OBME) crashed into an embankment of the M1 motorway near Kegworth, Leicestershire, killing 47 people and seriously injuring 74 others, including seven members of the flight crew.
Summarizing the cause of the accident, The Aircraft Accident Report stated that “the cause of the accident was that the crew shut down the No. 2 engine after a fan blade fractured in the No. 1 engine. This engine was subsequently hit hard. Secondary fan damage occurred after power was increased on final approach to land. resulting thrust loss” (AAIB 1980, 35). It is certainly true, however, that it was a combination of mechanical, procedural and cognitive errors that ultimately caused the aircraft to fail in its final landing phase.
Extrapolating the events of that day requires examining the chain of events rather than studying the error or malfunction of each component. As with an airplane crash investigation, a sequence of human and operational errors creates a domino effect that has inertia beyond one incident leading to a catastrophic conclusion (Job, 1996; 173). The chronology of these events is therefore particularly important in helping to analyze the failure chain leading up to the crash.
G-OBME was engaged on a double shuttle between London Heathrow Airport and Belfast Aldergrove Airport. The first leg of the journey was uneventful. During the second stage of the shuttle the aircraft initially climbed to 6,000 feet where it leveled off for about two minutes before being cleared to climb to a flight level of 12,000 feet. At 7.58 pm clearance was given to climb to thirty five thousand feet. At 8.05pm, the crew noticed a strong vibration and smell of fire while the plane was climbing from flight level 283. No fire warnings, visual or audible, were given by the instruments on the flight deck. Later replays of the flight data recorder showed that severe vibrations had occurred in the No. 1 (left) engine, as well as erratic fan speed, increased exhaust temperatures and low, variable fuel flow (AAIB, 1980; 145).
Captain Hunt took control of the aircraft and disengaged the autopilot. He later claimed that the engine instrumentation gave him no clear indication of the source of the malfunction. He also later stated that he thought smoke was coming from the passenger cabin which, based on his understanding of the 737’s air conditioning system, led him to believe that the smoke was actually coming from the No. 2 (right) engine. As a result an order was issued to throttle back the No. 2 engine. As a result of this procedure the aircraft slowly banked sixteen degrees to the left but the commander made no corrective movements of the rudder or ailerons.
The commander later claimed that reducing the throttle of the No. 2 engine reduced the smell and signs of smoke, and he later noticed that significant vibration continued after the No. 2 throttle was closed.
After No. 2’s engine was throttled back, London Air Traffic Control was immediately notified of the emergency and found the engine on fire. Forty-three seconds after the vibration began, the commander ordered First Officer McClelland to “shut down.” The shut down was delayed as the first officer responded to radio messages from London Air Traffic Control indicating which alternate airport they wanted to land at. Shortly after shutting down the No.2 engine BMA Operations requested that the aircraft be diverted to East Midlands Airport (AAIB,1980; 40).
As the No. 2 engine shut down, all evidence of smoke rose from the flight deck which convinced the commander that he had made the right decision, at least that No. 1 engine showed no signs of failure, and continued. To operate at lower power and with increased fuel flow.
Passengers were aware of smoke and an “oily” or “rubbery” smell in the cabin. Some passengers saw evidence of a fire from the left engine, and several cabin attendants saw fire from the No. 1 engine as well as light-colored smoke in the cabin.
Neither the passengers nor the cabin crew alerted the flight crew to this fact despite indications of fire from other engines. This may have been due to the general confusion of the time, related to the belief that the pilot finally knew what he was doing.
At 8.20 pm power was increased on No.1 engine at a height of three thousand feet. The aircraft was then cleared to descend to two thousand feet and after joining the center line at two thousand feet above ground level (AGL) the commander asked to lower the landing gear and apply fifteen degrees of flaps. At nine hundred feet, No. 1 engine suddenly lost power. When the aircraft dipped below the glidepath and the Ground Proximity Warning System (GPWS) played the commander broadcast “Prepare for crash landing” over the cabin address system. The plane hit the ground at 8.24 pm at a speed of 115 knots.
Gareth Jones, a survivor, described the moment the plane hit the ground as follows: “A shudder, crash, crunch, blackness, like a motor car crash, and I was by the emergency hatch.” (BBC, 1989).
The AAIB report (AAIB, 1980; 35) focused on the failure of the flight crew to respond correctly to the failure of the No. 1 engine and highlighted the following operational errors:
1. The combination of engine vibration, noise and the smell of fire was beyond their training and expertise.
2. They reacted to the initial engine problem prematurely and in a manner that was contrary to their training.
3. They did not absorb the indications on the engine instrument display before throttle back the No. 2 engine.
4. As the No. 2 engine was throttled back, the noise and shudder associated with the No. 1 engine surge stopped, convincing them that they had correctly identified the faulty engine.
5. They were not informed of the flames coming from the No. 1 engine and which were seen by many on board, including 3 cabin attendants in the rear cabin.
Many accident reports cite human error as the primary cause (Johnson, 1998).
However, before looking at the glaring failure of Captain Hunt’s inability to determine which of the 737’s engines actually failed, the faulty engine must be addressed. The actual cause of the malfunction was a broken turbine, itself a result of metal fatigue from excessive vibration.
The upgraded CFM56 engine used on the 737-400 model was subject to excessive vibration when operating at power settings above twenty-five thousand feet. Because it was an upgrade to an existing engine, the engine was only tested in a laboratory, not in actual flight conditions. When this fact was later discovered, about a hundred 737-400s were grounded and the engines were subsequently modified. All significantly redesigned turbofan engines since the Kegworth crash have been required to be tested under actual flight conditions. Then, the inadequately tested CFM56 engine on Flight 092 may have been an “accident waiting to happen” (Owen, D. 2001; 132).
The AAIB report concluded that engine vibration, noise and the smell of fire were beyond the flight deck crew’s area of expertise. (AAIB, 1980). This may or may not be an accurate assessment as some pilots and first officers fortunately experience the actual effects of smoke and fire while in command.
While simulators can help train for emergency procedures, it is questionable how valuable such procedures can be, especially if the crew is not fully trained in the unique procedural and technical requirements involved in flying a particular aircraft type. Importantly, the flight crew of 092 had little confidence in the accuracy of key instruments, including vibration meters.
Dr Denis Besnard of Newcastle University, analyzing the Kegworth plane crash, concluded “the pilots of the B737 were caught in what is known as a confirmation bias where, instead of looking for contrary evidence, humans tend to overestimate consistent data. People ignore and sometimes unconsciously ignore the data to explain it.” cannot” (Besnard D, 2004; 117).
A research team from the University of York and the University of Newcastle upon Tyne also established “confirmation bias”, the overloading of consciousness by the amount of confusing or conflicting data, as a primary cause of the crash. The argument that people oversimplify complex situations is both well documented and significant (Besnard. D., Greathead, G. & Baxter, G., 2004; 117-119).
In particular, Captain Hunt had not trained on the new model 737-400 as no simulators for this type existed in the UK at the time. This is both shocking and critical when considering the following points. The captain believed that the right engine was malfunctioning due to the smell of smoke, possibly because air for the air conditioning system in earlier Boeing 737 models was taken from the right engine.
However, starting with the Boeing 737-400 variant, Boeing redesigned the system to use bleed air from both engines. Captain Hunt was unaware of this fact, which formed a significant part of the decision to shut down the wrong engine. This would prove disastrous.
Apart from the coincidence of the smoke escaping when the auto-throttle was closed, pilots must have been accustomed to ignoring the vibration warning meter readings, as they were considered unreliable in the early days. However, the crew of G-OBME does not know that the new ones are more reliable. Therefore, had more attention been paid to the vibration issues than to the smell of smoke and fire, the events of the evening of 8 January (Owen, 2001; 131-2) would have played out very differently.
Subsequent research has critically concluded that “organizational failures create the necessary preconditions for human error” and that “organizational failures also exacerbate the consequences of those errors” (Stanton, 1994; 63). The Kegworth plane crash was therefore the result of a series of failures caused by a mechanical fault.
In addition, cognitive error exacerbated by insufficient flight training of the flight crew increased the error chain. Ultimately, the flight crew did not verify their explanation of events by consulting with the cabin crew or passengers, even though information was available at the time indicating that another engine on the plane had failed.
BBC (1989) On this day: Dozens dead after plane crashes on motorway. [online] Available from http://news.bbc.co.uk/onthisday/hi/dates/stories/january/8 [accessed 2 March 2007]
Besnard, D. (2005) International Aviation and Fire Protection Association. [online] Available from http://www.iafpa.org.uk/news-template.php?t=4&id=1312 [accessed 1 March 2007]
Besnard, D., Greathead, G., & Baxter, G., (2004) International Journal of Human-Computer Studies. When mental models go wrong. Dynamics, Co-occurrence in Critical Systems, Vol. 60, p. 117-128.
Job, M. (1996) Air Disasters Volume 2. pp. 173-185. Aerospace Publications Pty Ltd
Johnson, D. 1988; University of Glasgow Department of Computing Science (1980) Visualizing the Relationship Between Human Error and Organization [online] University of Glasgow, 1980. http://www.dcs.gla.ac.uk/~johnson/papers/fault_trees/organisational_error.html [accessed 2 March 2007]
Owen, D. (2001) Air Accident Investigation, 1st Edition, Ch. 9, p. 132-152. Sparkford, Patrick Stephens Limited
Stanton, NA, (1994) The Human Factors of Alarm Design, Ch. 5, p. 63-92. London, Taylor & Francis Ltd
United Kingdom. Air Accidents Investigation Branch (1990) Boeing 737-400, G-OBME, near Kegworth, Leicestershire 8 January 1989, No. 4/90. London, HMSO.
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