Activity Title: American Airlines Flight 191 Case Study
Date: Oct 20, 2010
Cause(s) of Accident
The National Transportation Safety Board (NTSB) determined that the probable cause of this accident was the asymmetrical stall and the ensuing roll of the aircraft because of the uncommanded retraction of the left wing outboard leading edge slats and the loss of stall warning and slat disagreement indication systems resulting from maintenance-induced damage leading to the separation of the number 1 engine and pylon assembly at a critical point during takeoff. The separation resulted from damage by improper maintenance procedures which let to failure of the pylon structure.
Structural and Mechanical Factors
After a thorough examination of the pylon attachment points, fractures and deformations at the separation points in the forward bulkhead and thrust link were all characteristic of overload. Testimony indicated the forklift was not powered for a period of time because it ran out of fuel. Post accident forklift tests showed that under these conditions leakage would allow a drift down of 1 inch in 30 minutes. Movement of 0.4 inch or less would produce a 7 inch fracture at the flange.
The design and interrelationship of the essential systems as they were affected by the structural loss of the pylon contributed to this accident. Flight control, hydraulic, and electrical systems in the aircraft were all affected by the pylon separation. When the engine separated from the pylon hydraulic pressure and fluid were lost and not recoverable. The separation also severed the electrical wire bundles inside the pylon which included the main feeder circuits between the generator and the No 1 a.c. generator bus. The flight crew was unable to restore power to the aircraft.
The failure of engineering to ascertain the damage-inducing potential of a procedure which deviated from the...