Investigation Summary
The investigation centered on the structural failure of the pylon's aft bulkhead. American Airlines had implemented a maintenance shortcut for engine changes, using a forklift to remove the engine and pylon as a single unit rather than separately as recommended by McDonnell Douglas. This procedure was difficult to execute with precision; slight misalignments during the re-installation process caused the forklift to exert upward pressure on the pylon, creating hairline cracks in the aft bulkhead. Over time, these cracks grew until the structure failed during the high-stress takeoff rotation of the accident flight.
Final Conclusions
The NTSB determined the probable cause was an asymmetrical stall caused by the uncommanded retraction of leading-edge slats following the separation of the No. 1 engine. The separation was the result of improper maintenance procedures (using a forklift for engine/pylon removal) which induced structural damage. Contributing factors included the vulnerability of the pylon design to maintenance damage and the loss of critical cockpit warnings (stall warning and slat disagreement) because the engine separation destroyed the electrical buses powering those systems.
Video Analysis
The Tragic Story of American Airlines Flight 191 — A detailed look at the maintenance shortcuts and design vulnerabilities that led to the deadliest aviation accident on U.S. soil.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Leading Edge Slats
High-lift devices on the wing. On the DC-10, they were held in place by hydraulic pressure without a mechanical backup lock, allowing them to retract if lines were severed.
Pylon Aft Bulkhead
The primary structural attachment point for the engine pylon to the wing; it failed due to fatigue cracks induced by a forklift.
Stick Shaker
The stall warning system that failed to activate because it was powered by the electrical bus associated with the lost engine.
Interesting Facts
- 01Deadliest aviation accident occurring in the United States (excluding 9/11).
- 02The engine separation destroyed the No. 1 AC electrical bus, disabling the Captain's flight instruments and the stall warning system.
- 03The maintenance shortcut saved approximately 200 man-hours per engine change.
- 04Two people on the ground were killed in addition to all 271 occupants.
- 05The DC-10 fleet was temporarily grounded by the FAA following the accident.