Should the pilots have attempted to stop before takeoff? American Airlines Flight 191 was a regularly scheduled passenger flight from O'Hare International Airport in Chicago to Los Angeles International Airport. The McDonnell Douglas DC-10-10 crashed on May 25, 1979, moments after takeoff from Chicago. All 258 passengers and 13 crew on board were killed, along with two people on the ground. It is the deadliest aviation accident to have occurred in the United States. Just as the aircraft hit takeoff speed, the number one engine and its pylon assembly separated from the left wing, ripping away a 3-foot (0.91 m) section of the leading edge with it. The combined unit flipped over the top of the wing and landed on the runway. Robert Graham, supervisor of maintenance for American Airlines, stated, "As the aircraft got closer, I noticed what appeared to be vapor or smoke of some type coming from the leading edge of the wing and the No. 1 engine pylon. I noticed that the engine was bouncing up and down quite a bit and just about the time the aircraft got opposite my position and started rotation, the engine came off, went up over the top of the wing, and rolled back down onto the runway. Before going over the wing, it went forward and up just as if it had lift and was actually climbing. It didn't strike the wing on its way down, rather the engine followed the clear path of the airflow of the wing, up and over the top of it, and then down below the tail. The aircraft continued a fairly normal climb until it started a turn to the left. And at that point, I thought they were going to come back to the airport." It is not known what was said in the cockpit in the 50 seconds leading up to final impact, as the cockpit voice recorder lost power when the engine detached. The only crash-related audio collected by the recorder is a thumping noise followed by First Officer Dillard exclaiming "Damn!", at which point the recording ends. This may also explain why Air Traffic Control was unsuccessful in their attempts to radio the crew and inform them that they had lost an engine. This loss of power did, however, prove useful in the subsequent investigation, serving as a marker of exactly what circuit in the DC-10's labyrinthine electrical system had failed. In addition to the engine's failure, several related systems failed. The number one hydraulic system, powered by the number one engine, failed but continued to operate via motor pumps that mechanically connected it to hydraulic system three. Hydraulic system three was also damaged and began leaking fluid, but maintained pressure and operation up until impact. Hydraulic system two was undamaged. The number one electrical bus, whose generator was attached to the number one engine, failed causing several electrical systems to go offline, most notably the captain's instruments, his stick shaker, and the slat disagreement sensors. While a switch in the overhead panel would have allowed the captain to restore power to his instruments, it was not used. It might have been possible for the flight engineer to reach the backup power switch, in an effort to restore electrical power to the number one electrical bus. That would have worked only if electrical faults were no longer present in the number one electrical system. Furthermore, to reach the switch the flight engineer would have needed to rotate his seat, release his safety belt, and stand up. Since the aircraft never got higher than 350 feet (110 m) above ground, and was in the air for only 50 seconds between when the engine separated and when it crashed, there was not sufficient time to take such an action. In any event, the first officer was flying the airplane and his instruments continued to function normally. As the wings and engines were not visible from the cockpit, the crew likely had no idea that an engine had fallen off, only that one had failed. Since it was no longer possible to abort the takeoff at this point, they followed standard operating procedure for an "engine out" situation, which meant throttling down the two remaining units. In addition, the power failure meant that, while the "engine out" light in the cockpit still worked, the stall warning did not, or the slat retraction indicator. Had the crew known about the slats on the left wing retracting, they likely would not have throttled down Engines 2 and 3. In the event, the loss of power in the cockpit left them without proper information to handle the situation they were in and in all likelihood, the aircraft was doomed to crash from the moment the engine separated. Recovery of the cockpit instrument panels from the wreckage did not provide any useful information.

• published: 10 Mar 2015
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