On 25 May 1979, American Airlines flight 191 lost its left engine during the takeoff roll from Chicago O'Hare. The engine and its pylon separated from the wing, went up and over it, and fell onto the runway. The DC-10 lifted off, rolled left and crashed about thirty seconds later. All 271 aboard died, along with two people on the ground. It remains the deadliest aviation accident on United States soil.
Why the engine came off
The cause was a crack in the pylon's rear bulkhead, created weeks earlier during maintenance.
The manufacturer's procedure for removing an engine for servicing called for the engine to be detached from the pylon first, then the pylon separately. American Airlines — and some other operators — had developed a faster method: lift off the engine and pylon together as one assembly using a forklift. It saved many hours of labour.
The problem was precision. The combined assembly weighs several tonnes, and positioning it with a forklift required extremely fine control. Any misalignment loaded the pylon attachment structure in a way it was not designed for. On this aircraft the procedure had cracked the rear bulkhead. The crack grew over subsequent flights until the pylon failed on takeoff.
Why the aircraft could not be saved
Losing an engine on takeoff is a trained, survivable event. What made this different was what the separating pylon destroyed on its way out.
It severed hydraulic lines in the leading edge of the left wing, which caused the left wing's leading edge slats to retract. The right wing's slats stayed extended. The aircraft now had asymmetric lift, and the left wing stalled at a speed at which the right wing was still flying.
It also cut electrical power to the captain's instruments and to the stall warning system on the left side. The crew, flying the standard engine-failure procedure, reduced to the correct speed for a normal engine-out climb — which was below the stall speed of the damaged left wing. They had no stall warning to tell them, and no indication that the slats had retracted.
The NTSB concluded that no crew, flying the procedure they were trained to fly with the information available, could have recovered.
What changed
The DC-10 fleet was grounded, and the maintenance procedure was prohibited. Inspection of pylon structures across the fleet found cracks in other aircraft.
Structurally, the accident forced a re-examination of how flight-critical systems are routed and protected against damage from a separating engine — the same class of problem as United 232 ten years later. Design requirements were changed so that slat retraction could not follow from a single hydraulic failure, and asymmetry protection was added.
There was also an institutional consequence. The accident exposed that an operator could adopt a maintenance procedure that departed from the manufacturer's without the regulator evaluating it. Oversight of operator-developed maintenance methods was tightened substantially afterwards.