• 1979 – During takeoff from O'Hare International Airport in Chicago, an engine detached from American Airlines Flight 191, causing a crash that killed 273 people, the deadliest aviation accident in United States history.

American Airlines Flight 191

Coordinates: 42°0'35"N 87°55'45"W


Flight 191 after takeoff, missing its left engine and leaking fuel and hydraulic fluid

Accident
Date: May 25, 1979; 45 years ago
Summary: Loss of control caused by engine detachment and hydraulic failure, due to improper maintenance[1]
Site: Des Plaines, Illinois, U.S.; 42°0'35"N 87°55'45"W
Total fatalities: 273
Aircraft
Aircraft type: McDonnell Douglas DC-10-10

American Airlines Flight 191 was a regularly scheduled domestic passenger flight in the United States from O'Hare International Airport in Chicago, Illinois to Los Angeles International Airport in California. On the afternoon of May 25, 1979, the McDonnell Douglas DC-10-10 operating this flight was taking off from runway 32R at O'Hare when its left engine detached from the wing, causing a loss of control, and the aircraft crashed less than one mile (1.6 km) from the end of the runway. All 258 passengers and 13 crew on board were killed, along with two people on the ground. With 273 fatalities, it is the deadliest aviation accident to have occurred in the United States.

The National Transportation Safety Board (NTSB) found that, as the aircraft was beginning its takeoff rotation, engine number one (the left engine) separated from the left wing, flipping over the top of the wing and landing on the runway. As the engine separated from the aircraft, it severed hydraulic fluid lines that lock the wing's leading-edge slats in place and damaged a 3-foot (0.9 m) section of the left wing's leading edge. Aerodynamic forces acting on the wing resulted in an uncommanded retraction of the outboard slats. As the aircraft began to climb, the damaged left wing—with no engine—produced far less lift than the right wing, which had its slats still deployed and its engine providing full takeoff thrust. The disrupted and unbalanced aerodynamics of the aircraft caused it to roll abruptly to the left until it was partially inverted, reaching a bank angle of 112°, before crashing in an open field by a trailer park near the end of the runway. The engine separation was attributed to damage to the pylon structure holding the engine to the wing, caused by improper maintenance procedures at American Airlines.

Background

Aircraft


N110AA, the aircraft involved in the accident, photographed at O'Hare five years prior

{snip}

Investigation

The disaster and investigation received widespread media coverage. The impact on the public was increased by the dramatic effect of an amateur photo taken of the aircraft rolling that was published on the front page of the Chicago Tribune on the Sunday two days after the crash.

Engine separation


An FAA diagram of the DC-10 engine and pylon assembly indicating the failed aft pylon attach fitting

Witnesses to the crash were in universal agreement that the aircraft had not struck any foreign objects on the runway. In addition, no pieces of the wing or other aircraft components were found along with the separated engine, other than its supporting pylon; this would lead investigators to conclude that nothing else had broken free from the airframe and struck the engine. Hence, the engine/pylon assembly separation could only have resulted from a structural failure. The cockpit instrument panels were damaged so badly that they did not provide any useful information.

During the investigation, an examination of the pylon attachment points revealed some damage done to the wing's pylon mounting bracket that matched the bent shape of the pylon's rear attachment fitting. This meant that the pylon attachment fitting had struck the mounting bracket at some point. This was important evidence, as the only way the pylon fitting could strike the wing's mounting bracket in the observed manner was if the bolts that held the pylon to the wing had been removed. The engine/pylon assembly was supported by something other than the aircraft itself. Therefore, investigators could now conclude that the observed damage to the rear pylon mount had been present before the crash occurred rather than being caused by it.

The NTSB determined that the damage to the left-wing engine pylon had occurred during an earlier engine change at the American Airlines aircraft maintenance facility in Tulsa, Oklahoma, between March 29 and 30, 1979. On those dates, the aircraft had undergone routine service, during which the engine and pylon had been removed from the wing for inspection and maintenance. The removal procedure recommended by McDonnell-Douglas called for the engine to be detached from the pylon before detaching the pylon itself from the wing. However, American, as well as Continental Airlines and United Airlines, had developed a different procedure that saved about 200 working hours per aircraft and "more importantly from a safety standpoint, it would reduce the number of disconnects (of systems such as hydraulic and fuel lines, electrical cables, and wiring) from 79 to 27." This new procedure involved the removal of the engine and pylon assembly as a single unit rather than as individual components. United's implementation involved the use of an overhead crane to support the engine/pylon assembly during removal and re-installation. The method chosen by American and Continental relied on supporting the engine/pylon assembly with a large forklift.

If the forklift had been positioned incorrectly, the engine/pylon assembly would not be stable as it was being handled, causing it to rock like a see-saw and jam the pylon against the wing's attachment points. Forklift operators were guided only by hand and voice signals, as they could not directly see the junction between the pylon and the wing. Positioning had to be extremely accurate, or structural damage could result. Compounding the problem, maintenance work on N110AA did not go smoothly. The mechanics started disconnecting the engine and pylon as a single unit, but a shift change occurred halfway through the job. During this interval, even though the forklift remained stationary, the forks supporting the entire weight of the engine and pylon moved downward slightly due to a normal loss of hydraulic pressure associated with the forklift engine being turned off; this caused a misalignment between the engine/pylon and wing. When work was resumed, the pylon was jammed on the wing, and the forklift had to be re-positioned. Whether damage to the mount was caused by the initial downward movement of the engine/pylon structure or by the realignment attempt is unclear.  Regardless of how it happened, the resulting damage, although insufficient to cause an immediate failure, eventually developed into fatigue cracking, worsening with each takeoff and landing cycle during the eight weeks that followed. When the attachment finally failed, the engine and its pylon broke away from the wing. The structure surrounding the forward pylon mount also failed from the resulting stresses. 

Inspection of the DC-10 fleets of the three airlines revealed that while United's hoist approach seemed harmless, several DC-10s at both American and Continental already had fatigue cracking and bending damage to their pylon mounts caused by similar maintenance procedures.  The field service representative from McDonnell-Douglas stated the company would "not encourage this procedure due to the element of risk" and had so advised American Airlines. McDonnell-Douglas, however, "does not have the authority to either approve or disapprove the maintenance procedures of its customers." 

{snip}

[1] Aircraft Accident Report: American Airlines, Inc. DC-10-10, N110AA, Chicago O'Hare International Airport, Chicago, Illinois, May 25, 1979 (PDF) (Report). National Transportation Safety Board. December 21, 1979. NTSB-AAR-79-17. Archived (PDF) from the original on April 22, 2019. Retrieved September 6, 2016.

{snip}