Douglas DC-10-10 accident at Chicago-O'Hare International Airport, Illinois, 25 May 1979

Accident narrative

About 1504 c.d.t. on May 25, 1979, American Airlines Flight 191, a McDonnell-Douglas DC-10-10, crashed into an open field just short of a trailer park about 4,600 feet northwest of the departure end of runway 32R at Chicago-O'Hare International Airport, Illinois. During the takeoff rotation, the left engine and pylon assembly separated from the aircraft. The flight climbed to about 325 feet above the ground before rolling to the left and crashing. The aircraft was destroyed by the impact, explosion, and ground fire. All 271 persons on board were killed, along with two persons on the ground. Two other persons on the ground were injured.

### The flight

Flight 191 was a regularly scheduled passenger flight to Los Angeles, California, with 258 passengers and 13 crewmembers on board. The aircraft was powered by three General Electric CF6-6D engines.

The flightcrew was properly certificated and qualified. Captain Walter H. Lux had flown 22,500 hours, including 3,000 hours as captain in the DC-10. First Officer James R. Dillard had flown about 9,275 hours, with 1,080 hours in the DC-10. Flight Engineer Alfred F. Udovich had flown about 15,000 hours, including 750 hours in the DC-10.

The weather at the time of departure was clear, with a visibility of 15 miles and a surface wind of 020 degrees at 22 knots. The takeoff gross weight was 379,000 pounds. The calculated critical engine failure speed (V1) was 139 knots indicated airspeed (KIAS), rotation speed (VR) was 145 KIAS, and takeoff safety speed (V2) was 153 KIAS. The stabilizer trim setting was 5 degrees aircraft noseup, and the takeoff flap setting was 10 degrees.

### The sequence of events

Flight 191 was cleared to taxi into position on runway 32R and hold. At 1502:38, the flight was cleared for takeoff. The captain acknowledged, "American one ninety-one under way."

The takeoff roll was normal until just before rotation. The digital flight data recorder (DFDR) showed the elevator began to deflect up at VR, and the aircraft began to rotate upward at a rate of 1.5 degrees per second. Just before rotation, sections of the left (No. 1) engine pylon structure came off the aircraft, and witnesses saw white smoke or vapor coming from the vicinity of the pylon. During rotation, the entire No. 1 engine and pylon separated from the aircraft, went over the top of the wing, and fell to the runway.

One second before liftoff, the word "damn" was recorded on the cockpit voice recorder (CVR), and then the CVR ceased operating. Simultaneously, the DFDR ceased recording the positions of the left inboard aileron, left inboard elevator, lower rudder, and several left wing leading edge slats. The electrical power for the CVR and the sensors for the lost DFDR functions were derived from the aircraft's No. 1 alternating current (a.c.) generator bus.

Flight 191 lifted off about 6,000 feet down the runway at V2 + 6 KIAS and a 10-degree pitch attitude. The loss of the engine and pylon caused the aircraft's center of gravity to move aft and 11.9 inches to the right. The aircraft climbed out in a slight left wing-down attitude, but application of right wing-down aileron and right rudder restored the flight to a wings-level attitude. The flight maintained a steady climb of about 1,150 feet per minute at a 14-degree noseup pitch attitude, which was the target pitch attitude displayed by the flight director for a two-engine climb. During the initial climb, the aircraft accelerated to a maximum speed of 172 KIAS at about 140 feet above the ground.

The aircraft then began to decelerate at an average rate of about 1 knot per second. At 20 seconds after liftoff, at 325 feet above the ground and 159 KIAS, the flight began to roll to the left. The left roll was accompanied by increasing right-wing-down aileron deflection. The flight began to turn to the left, and the roll increased despite increasing right rudder and right-wing-down aileron deflections. The pitch attitude began to decrease from 14 degrees even though the elevator was being increased to the full aircraft noseup deflection. The aircraft reached a maximum altitude of 350 feet.

Three seconds before the end of the DFDR tape, the aircraft was in a 90-degree left bank and a zero-degree pitch attitude. The recording ended with the aircraft in a 112-degree left roll and a 21-degree nosedown pitch attitude. The aircraft crashed into the open field and scattered into the adjacent trailer park.

### What the investigation found

**The wreckage** The disintegration of the aircraft structure was extensive. The No. 1 engine and pylon were found off the right side of runway 32R. A 3-foot section of the left wing's leading edge, just forward of where the pylon joined the wing, was torn away when the assembly separated.

The pylon is attached to the wing using spherical ball joints in three structural elements: a forward bulkhead, a thrust link, and an aft bulkhead. The pylon forward bulkhead and portions of the flange from the pylon aft bulkhead fell to the runway. The pylon's aft clevis attach assembly and portions of the pylon aft bulkhead remained with the wing.

Examination of the left wing's leading edge showed that the No. 1 and No. 3 hydraulic system's extension and retraction lines and the followup cables for the left wing's outboard slat drive actuators were severed. At impact, the left wing's outboard slats were retracted, while the left wing's inboard slats and the right wing's slats were extended to the takeoff position.

**Metallurgical examination** Examination of the pylon aft bulkhead disclosed a 10-inch overload fracture of the upper forward flange. Fatigue cracking was evident at both ends of the fracture, making the total length of the overstress fracture and fatigue cracks about 13 inches. The aft fracture surface contained a crescent-shaped deformation that matched the shape of the lower end of the wing clevis. This deformation indicated that the overstress crack was introduced during removal and installation of the pylon during maintenance. The accident aircraft was the only one found with shims installed on the upper surface of the flange, which reduced the clearance between the fastener heads and the lower surface of the wing clevis fitting.

**Maintenance procedures** About eight weeks before the accident, the No. 1 pylon and engine had been separated from the wing of the accident aircraft to replace the spherical bearings in compliance with manufacturer service bulletins. The manufacturer recommended performing this maintenance during an engine removal, removing the engine before removing the pylon. A McDonnell-Douglas field service representative advised American Airlines that the manufacturer did not encourage removing the engine and pylon as a single unit due to the risk involved in remating the assembly.

However, American Airlines devised an engineering change order (ECO) to remove the engine and pylon as a single unit using a Hyster forklift. The pylon alone weighs about 1,865 pounds, while the pylon and engine together weigh about 13,477 pounds.

Mechanics found it difficult to adhere to the ECO's sequence of removing the forward bulkhead's attach assemblies first, so they reversed the procedure, removing the aft bolt and bushing first. This permitted the forward bulkhead to act as a pivot. Any advertent or inadvertent loss of forklift support—such as a pressure leak causing the forks to drift down—would produce an upward movement at the aft bulkhead's upper flange, bringing it into contact with the wing clevis. Postaccident tests showed that a movement of 0.4 inch or less at the center of gravity could produce a 7-inch fracture of the flange. The Board concluded that the structural separation of the pylon resulted from a complete failure of the forward flange of the aft bulkhead after its residual strength had been critically reduced by a maintenance-induced crack.

The investigation also found that Continental Airlines had damaged the upper flanges of two DC-10s during similar forklift maintenance procedures in December 1978 and February 1979. The carrier classified the damage as maintenance error, repaired the aircraft, and returned them to service. Neither the carrier nor the manufacturer interpreted regulations as requiring them to report the damage to the Federal Aviation Administration (FAA).

**Systems and aerodynamics** When the engine and pylon separated from Flight 191, the hydraulic lines and position feedback cables in the wing leading edge were severed. Hydraulic fluid was lost, and air loads forced the left outboard slats to retract. This uncommanded retraction reduced the lift of the left wing and increased its stall speed to 159 KIAS.

The separation also severed electrical wire bundles, causing the loss of the No. 1 a.c. generator bus. This disabled the captain's flight director instrument, the stickshaker motor (stall warning), and the slat disagreement warning light system.

The first officer, flying the aircraft, maintained the 14-degree pitch attitude commanded by the flight director for an engine-out climb. This decelerated the aircraft to 159 KIAS, at which point the left wing stalled and the roll began. Because the flightcrew could not see the wings from the cockpit and the warning systems were inoperative, they had no indication of the slat retraction or the impending stall. Simulator tests showed that the aircraft could have been flown successfully at higher speeds, but participating pilots agreed it was not reasonable to expect the flightcrew to recognize the roll as a stall or to recover without the warning systems.

**Design and certification** The Board noted that the pylon design satisfied the 1965 fail-safe requirements, but the clearances at the pylon-to-wing attachment points were unnecessarily small, making the structure vulnerable to maintenance damage. The Board also noted that the regulations in effect during certification did not require the manufacturer to account for multiple malfunctions resulting from a single failure, such as the structural loss of a pylon.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "the asymmetrical stall and the ensuing roll of the aircraft because of the uncommanded retraction of the left wing outboard leading edge slats and the loss of stall warning and slat disagreement indication systems resulting from maintenance-induced damage leading to the separation of the No. 1 engine and pylon assembly at a critical point during takeoff." The Board found that the separation resulted from damage by improper maintenance procedures which led to the failure of the pylon structure.

Contributing to the cause were the vulnerability of the design of the pylon attach points to maintenance damage; the vulnerability of the design of the leading edge slat system to the damage which produced asymmetry; deficiencies in Federal Aviation Administration surveillance and reporting systems which failed to detect and prevent the use of improper maintenance procedures; deficiencies in the practices and communications among the operators, the manufacturer, and the FAA which failed to determine and disseminate the particulars regarding previous maintenance damage incidents; and the intolerance of prescribed operational procedures to this unique emergency.