Mechanical

American Airlines Flight 191

American Airlines Flight 191 As American Airlines Flight 191 took off from Chicago-O’Hare International Airport on May 25, 1979, "the left engine and pylon assembly separated from the aircraft, went over the top of the wing, and fell to the runway" (Failure Analysis, 2008, pg. 1). The McDonnell-Douglas DC 10-10 aircraft climbed 325 feet, rolled, and crashed to the ground as a result of contributing mechanical and structural failures. Everybody onboard the plane, as well as two people that were on the ground, were killed. At the time, it was the worst loss of life in aviation history.
The aircraft had a total of 19,871 flight hours. 341 of those flight hours had occurred after a maintenance procedure in Tulsa, OK that has since been tied to the crash. This maintenance procedure is believed to have led to the engine separating from the wing. The procedure was carried out because McDonnell-Douglas issued a service bulletin requiring that the "upper and lower spherical bearings that attached the pylon to the wing" (Failure Analysis, 2008, pg. 1) be replaced. Instead of carrying out the procedure according to the accompanying directions, American Airlines decided to replace the assembly via a cheaper method that involved less time and effort to undertake.
"Postaccident investigation revealed that a portion of the upper forward flange of
the aft bulkhead had been fractured by overload in the inboard-outboard direction
just forward of the radius between the flange and the bulkhead plane. The fracture
had been initiated by a downward bending moment at the center section of the flange
just forward of the fracture plane due to contact between the clevis and the flange.
As a result of this contact, the aft fracture surface of the upper flange was deformed
into a crescent shape that matched the shape of the lower end of the wing clevis.
The length of this overload fracture was 10 inches. Fatigue cracking was present at
both ends of the overload fracture, and the total length of the crack due to both overload
and fatigue was 13 inches" (Failure Analysis, 2008, pg. 1).
The aft bulkhead could have been brought into contact with the wing-mounted clevis via a number of different ways. Either during or after the hardware in the aft bulkhead fitting was removed, a load could have been applied that would have been sufficient enough to produce a crack. When attaching the pylon, the maintenance personnel had to be extremely careful because of the small distance between the pylon and wing attachments and the structural elements. It would only take a minor error for the forklift operator to damage the bulkhead and its upper flange (Failure Analysis, 2008).
"The structural separation of the pylon was caused by a complete failure of the
forward flange of the aft bulkhead after its residual strength had been reduced by
the fracture induced during the maintenance operation as well as by additional fatigue
crack growth in service. It is also clear the poor communications between engineering
and maintenance personnel, and between the FAA, the manufacturer, and
the airlines contributed to this accident" (Failure Analysis, 2008, pg. 1). Overall, an effort to maintain the aircraft in a cheaper fashion on the part of American Airlines was the ultimate cause of the disaster. Had the Tulsa, OK maintenance facility followed McDonnell-Douglas’ initial instructions for the procedure, the crash may never have happened in the first place. This has, thus, become a case study since then as to what can happen when the wrong corners are cut.
References
Failure analysis. (2008). Wiley. Retrieved July 6, 2008, from http://media.wiley.com/product_data/excerpt/60/04714143/0471414360.pdf

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