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The engine separation<!--, though, had--> severed the hydraulic fluid lines that controlled the leading-edge slats on the left wing and locked them in place, causing the outboard slats (immediately left of the number-one engine) to retract under air load. The retraction of the slats raised the stall speed of the left wing to about {{convert|159|kn|mph km/h|abbr=on}}, 6 knots higher than the prescribed takeoff safety airspeed (V<sub>2</sub>) of 153 knots. As a result, the left wing entered a full [[stall (flight)|aerodynamic stall]]. With the left wing stalled, the aircraft began banking to the left, rolling over onto its side until it was partially inverted at a 112° bank angle (as seen in the [[:File:Aa191_ohare.jpg|Laughlin photograph]]) with its right wing over its left wing.
Since the cockpit had been equipped with a [[closed-circuit television camera]] positioned behind the captain's shoulder and connected to view screens in the passenger cabin, the passengers may have witnessed these events from the viewpoint of the cockpit as the aircraft dove towards the ground.<ref name=urldpld>{{cite news |url=https://news.google.com/newspapers?id=Sk5OAAAAIBAJ&sjid=KxQEAAAAIBAJ&pg=6631%2C5710196 |work=Sunday Star-News |location=(Wilmington, North Carolina) |agency=Associated Press |title=Did plane's TV show death dive? |date=May 26, 1979 |page=1A}}</ref><ref>{{cite news|title=Dive May Have Been Televised|agency=Associated Press|url=https://www.nytimes.com/1979/05/27/archives/dive-may-have-been-televised.html|access-date=March 21, 2018|work=The New York Times|volume=128|issue=44230|date=May 27, 1979|archive-url=https://web.archive.org/web/20170909180931/http://www.nytimes.com/1979/05/27/archives/dive-may-have-been-televised.html|archive-date=September 9, 2017|url-status=live}}</ref><ref>{{cite news |last1=Young |first1=David |title=...191, do you want to come back? |url=http://archives.chicagotribune.com/1979/06/03/page/16/article/191-do-you-want-to-come-back |access-date=May 27, 2017 |newspaper=Chicago Tribune |issue=154 |date=June 3, 1979 |page=16 |archive-date=September 9, 2017 |archive-url=https://web.archive.org/web/20170909181345/http://archives.chicagotribune.com/1979/06/03/page/16/article/191-do-you-want-to-come-back/ |url-status=dead }}</ref> Whether the camera's view was interrupted by the power loss from the number-one electrical bus is not known, or if it was turned on for this flight.<ref name=urldpld/> The aircraft eventually slammed into a field around {{convert|4600|ft|abbr=on}} from the end of the runway, the left wing contacting the ground first.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|2}} Large sections of aircraft debris were hurled by the force of the impact into an adjacent trailer park, destroying five trailers and several cars. The DC-10 had also crashed into an old aircraft hangar at the edge of the airport at the former site of Ravenswood Airport, which was used for storage. The aircraft was destroyed by the impact force and ignition of a nearly full load of 21,000 gallons of fuel; no sizable components other than the engines and tail section remained.<ref>{{cite book|last1=Macarthur|first1=Job|oclc=464170955|title=Air Disaster|date=1996|publisher=Airlife|location=Shrewsbury|isbn=9781875671199|page=49|edition=2}}</ref>
[[File:Aa191 ohare2.jpg|thumb|Flight 191 after impact]]
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The NTSB determined that the loss of one engine and the asymmetrical drag caused by damage to the wing's leading edge should not have been enough to cause the pilots to lose control of their aircraft; the aircraft should have been capable of returning to the airport using its remaining two engines.<ref name=aeat/><ref name="NTSB AAR-79-17 Final Report"/>{{rp|54}} The NTSB thus examined the effects that the engine's separation would have on the aircraft's flight control, hydraulic, electrical, and instrumentation systems. Unlike other aircraft designs, the DC-10 was not equipped with a separate mechanism that would lock the extended leading-edge slats into place, relying instead solely on the hydraulic pressure within the system.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|53,57}} The NTSB determined that the engine tore through hydraulic lines as it separated from the DC-10's wing, causing a loss of hydraulic pressure; airflow over the wings forced the left wing slats to retract, which caused a stall over the left wing.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|53}} In response to the accident, slat relief valves were mandated to prevent slat retraction in case of hydraulic line damage.<ref name="AD80-03-03">FAA [http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgAD.nsf/0/0664E2B3BEE25C0E862569F200697162?OpenDocument Airworthiness Directive 80-03-03] {{Webarchive|url=https://web.archive.org/web/20161204215429/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/0664E2B3BEE25C0E862569F200697162?OpenDocument |date=December 4, 2016 }}</ref>
The wreckage was too severely fragmented to determine the exact position of the rudders, elevators, flaps, and slats before impact. An examination of eyewitness photographs showed only that the right wing slats were fully extended as the crew tried unsuccessfully to correct their steep roll. The position of the left wing slats could not be determined from the blurry color photographs, so they were sent to a laboratory in [[Palo Alto, California]], for digital analysis, a process that was pushing the limits of 1970s technology and necessitated large, complicated, and expensive equipment. The photographs were reduced to black-and-white, which made distinguishing the slats from the wing itself possible, thus proving that they were retracted. In addition, the aircraft's tail section was verified to be undamaged, and the landing gear was down. The right and center engines were found in the main debris field; both had damage from impact and overspeed. None of the three engines showed any indication of in-flight malfunction.<ref name=vatz />{{rp|20–21}}
Wind-tunnel and flight-simulator tests were conducted to help understand the aircraft's trajectory after the engine detached and the left wing slats retracted. Those tests established that the damage to the wing's leading edge and retraction of the slats increased the stall speed of the left wing from {{Convert|124|kn|mph km/h|lk=in|abbr=on}} to {{Convert|159|kn|mph km/h|abbr=on}}.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|23}} The DC-10 incorporates two warning devices that might have alerted the pilots to the impending stall: the slat disagreement warning light, which should have illuminated after the uncommanded retraction of the slats, and the [[stick shaker]] on the captain's control column, which activates close to the stall speed. Both of these warning devices were powered by an electric generator driven by the number-one engine. Both systems became inoperable after the loss of that engine.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|54,55,67}} The first officer's control column was not equipped with a stick shaker; McDonnell Douglas offered the device as an option for the first officer, but American Airlines chose not to have it installed on its DC-10 fleet. Stick shakers for both pilots became mandatory in response to this accident.<ref name="ad80-03-10">FAA [http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgAD.nsf/0/83D4FF29A77B501C862569F200686555?OpenDocument Airworthiness Directive 80-03-10] {{Webarchive|url=https://web.archive.org/web/20161204215539/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/83D4FF29A77B501C862569F200686555?OpenDocument |date=December 4, 2016 }}</ref>
As the aircraft had reached V1, the crew was committed to takeoff, so they followed standard procedures for an engine-out situation. This procedure is to climb at the takeoff safety airspeed ([[V speeds|V<sub>2</sub>]]) and attitude (angle), as directed by the [[flight director (aeronautics)|flight director]]. The partial electrical power failure, produced by the separation of the left engine, meant that neither the stall warning nor the slat retraction indicator was operative. Therefore, the crew did not know that the slats on the left wing were retracting. This retraction significantly raised the stall speed of the left wing. Thus, flying at the takeoff safety airspeed caused the left wing to stall while the right wing was still producing lift, so the aircraft banked sharply and uncontrollably to the left. Simulator recreations after the accident determined that "had the pilot maintained excess airspeed the accident may not have occurred." What would have happened if the crew throttled the remaining two engines up to overcome the stall is unknown but given the damage to the left wing, which was trailing fuel and had possible exposed, live electrical wires, it may have accomplished nothing except allow a few more more minutes of flight before an explosion occurred, possibly bringing the plane down over a populated area. Attempting to shut the engines down and abort the takeoff likely would have resulted in a similar sequence of events, and in all likelihood Flight 191 was doomed the moment the engine separated.<ref name="NTSB AAR-79-17 Final Report"/>{{rp|54}}
===Probable cause===
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