Introduction
This case study reviews the loss of American Airlines Flight 191 a McDonnell-Douglas DC-10. An article in the Chicago Tribune on the same day (Young, 25 May 1979) reported that after losing an engine from the left wing following takeoff from O’Hare International Airport, the aircraft rolled over and crashed within a mile of O’Hare, resulting in the deaths of all 271 on board and causing two fatalities on the ground. The case study looks at the accident causes, structural and mechanical factors involved, contributory factors, and the Investigation Board findings.
The Research
Causes of the Accident. According to a recent postscript to Young’s article, the subsequent NTSB investigation found that just two months prior to the crash, mechanics had “taken a shortcut” when de-mounting the engines for maintenance, and in so doing had damaged an aluminum part in the engine-mounting pylon. Normal wear and tear had increased the damage, until the day of the crash when the part had failed completely, allowing the engine to tear loose from the aircraft.
Mitchell (1979) writing for AvStop, an online aviation magazine, corroborated the issue of incorrect maintenance (by American Airlines) and reported that the procedure for engine removal used by American Airlines maintenance personnel was not approved by McDonnell Douglas, but was nonetheless used by AA because it save circa 200 man-hours on each aircraft and reduced the number of items to be disconnected from 72 to 27.
Structural and Mechanical Factors. “SO3E15 Chicago Air Crash: American Airlines Flight 191” (Feb 2013) stated that when the engine tore free, it damaged the leading edge of the wing caused a loss of hydraulic pressure that allowed the port wing slats to retract, which meant that the wing provided no lift. As a consequence, the DC-10 rapidly rolled over to the left, until it was almost upside down before the crash.
Another structural factor involved was that the American Airlines engine change procedure had not (according to “Prevailing Cultural / Organizational Factors” (n.d.) – an FAA document) taken into account the very precise forklift movements essential to ensure an efficient and safe engine replacement process, as a consequence damaging the pylon fittings, which were also not thoroughly inspected afterwards.
Contributing Factors. A report of the Flight 191 accident, entitled “Investigation: American Airlines 191” (n.d.) itemized these contributing factors to the accident.
1) The vulnerability of the pylon attachment points subject to accidental damage during maintenance (had happened on six other DC-10s using the “forklift” procedure). Also the design of the wing slat system – had caused lift asymmetry in the Flight 191 accident.
2) The FAA’s monitoring and reporting processes failed to detect these improper methods of maintenance.
3) Poor communication between the FAA, McDonnell Douglas, and the aircraft operators, which meant incidences of similar previous damages had not been shared.
4) The aircrew failing to cope with this “unique” emergency.
Investigation Board Findings. The “NTSB Aircraft Accident Report NTSB-AAR-79-17” (Dec 1979) found (p. 55) that the crew lost control of the doomed aircraft due to three events occurring in combination: the leading edge slats on the port wing retracting, and the “slat disagreement” and stall warning systems not functioning. All three events were caused by the left engine pylon assembly separating from the aircraft.
According to a NASA Case Study entitled “Deadly Efficiency” (July 2010), later investigations found that a total of 76 DC-10 engines + pylon assemblies had been removed using forklift trucks, of which 9 had suffered cracks to the pylon attachment points.
Recommendations. The first recommendation was to immediately stop changing engines and pylons as a unit (the “forklift” method) (Vatz, Sep 2004, p. 21). Secondly, to alter the standard procedure in the event of engine loss, so that airspeed is not reduced so much, in order to prevent a stall. In addition, the NTSB recommended building better redundancy provisions into the stall warning system, as well as requiring all aircraft manufacturers to incorporate slat locking measures (p. 22) into the aircraft design.
Other more general recommendations reported in the Vatz paper (p. 22) were to implement better quality control and monitoring of maintenance activities; to consider carefully before modifying maintenance procedures; and to improve communications between manufacturers, operators and the FAA.
Outcomes. As a direct result of this terrible accident – then the worst in America’s history – the FAA issued “14 CFR 25.1529, Instructions for Continued Airworthiness” (“Resulting Safety Initiatives”, n.d.), together with other requirements, all to ensure better definition of procedures to change engines or to perform other crucial maintenance tasks.
Within the same overall document as above, but under the heading “Lessons Learned” the FAA also itemized lessons learned from the accident, categorized as either “Technical Related Lessons” or “Common Theme Related Lessons.”
In the first category the FAA highlighted the need to consider catastrophic damage (e.g. hydraulic systems) that could occur when an engine separates. The second item in this category was the need to ensure that slat asymmetry could not occur in such a situation, and the third lesson learned in this technical category was that warning systems should incorporate greater redundancy so that their function is not lost.
Under the category of common theme lessons the paper drew attention to the need to properly validate any changed maintenance procedures and to review flawed assumptions regarding safety made during aircraft design.
References
“Deadly Efficiency.” (July 2010). National Aeronautics and Space Administration. Retrieved from http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=9&ved=0CGUQFjAI&url=http%3A%2F%2Fnsc.nasa.gov%2FSFCS%2FSystemFailureCaseStudyFile%2FDownload%2F157&ei=7n_iUYrLDMS3hQf5jYDICQ&usg=AFQjCNEdnnSgxqaA0v2k6mGcznSjQaeEBw
“Investigation: American Airlines 191.” (n.d.). Air Disaster.com. Retrieved from http://www.airdisaster.com/investigations/aacrash.shtml
“Lessons Learned.” (n.d.). Federal Aviation Administration. Retrieved from http://lessonslearned.faa.gov/ll_main.cfm?TabID=1&LLID=14&LLTypeID=12
Mitchell, M. (1979). “Engine Separation Causes American Airlines Flight 191 To Crash Killing All Onboard.” AvStop Online Magazine. Retrieved from http://avstop.com/news/engine_separation_causes_american_airlines_flight_191_to_crash.htm
“NTSB Aircraft Accident Report NTSB-AAR-79-17.” (Dec 1979). National Transportation Safety Board. Retrieved from http://www.airdisaster.com/reports/ntsb/AAR79-17.pdf
“Prevailing Cultural / Organizational Factors.” (n.d.). Federal Aviation Administration. Retrieved from http://lessonslearned.faa.gov/ll_main.cfm?TabID=3&LLID=14&LLTypeID=6
“Resulting Safety Initiatives.” (n.d.). Federal Aviation Administration. Retrieved from http://lessonslearned.faa.gov/ll_main.cfm?TabID=1&LLID=14&LLTypeID=10
Vatz, M., E. (Sep 2004). “Knowing When to Stop: The Investigation of Flight 191.” MIT. Retrieved from http://dspace.mit.edu/bitstream/handle/1721.1/39430/59008054.pdf?seque
Young, D. (25 May 1979). “The crash of American Airlines Flight 191 near O'Hare.” Chicago Tribune. Retrieved from http://www.chicagotribune.com/news/politics/chi-chicagodays-flight191-story,0,4407421.story
