McDonnell Douglas MD-11 accident at Shemya, Alaska, 6 Apr 1993

Accident narrative

On April 6, 1993, at 0110 Hawaiian Standard Time, China Eastern Airlines flight 583, a McDonnell Douglas MD-11, experienced an inadvertent deployment of its leading edge wing slats while in cruise flight approximately 950 nautical miles south of Shemya, Alaska. The autopilot disconnected, and the captain was manually controlling the airplane when it progressed through several violent pitch oscillations and lost 5,000 feet of altitude. The captain regained stabilized flight, declared an emergency, and diverted to the U.S. Air Force Base in Shemya, Alaska. Of the 235 passengers and 20 crewmembers aboard, two passengers were fatally injured, and 149 passengers and seven crewmembers received various injuries. The airplane did not receive external structural damage, but the passenger cabin was extensively damaged.

### The flight

Flight 583 was a scheduled international passenger flight from Beijing, China, to Los Angeles, California, with an intermediate stop in Shanghai, China. The flightcrew consisted of a captain, a first officer, a flight engineer, and a radio operator. There was also a four-person international relief flightcrew and 12 flight attendants.

The captain, age 42, had accumulated approximately 8,535 hours of total flight time, of which 1,341 hours were in the MD-11. The first officer, age 43, had accumulated 9,714 hours of total flight time, of which 199 hours were in the MD-11. The flight engineer, age 41, had accumulated 9,892 hours of total flight time. At the time of the accident, the captain was the flying pilot but occupied the right seat because he was providing instruction to the first officer in the left seat.

The airplane's maximum gross takeoff weight was 618,000 pounds. The calculated takeoff weight for the departure from Shanghai was 591,416 pounds, and the approximate weight at the time of the accident was 495,000 pounds. The airplane had been modified in accordance with all applicable manufacturer's service bulletins and FAA airworthiness directives regarding the flap/slat handle.

### The sequence of events

The airplane had been airborne about 5 hours. The flight attendants had completed the meal service and dimmed the lights for a movie. The airplane was in cruise flight at 33,000 feet, above the clouds, at an indicated airspeed of approximately 298 knots (Mach 0.82) with the No. 1 autopilot engaged.

The captain stated that they had not experienced any unusual weather phenomenon until approximately 15 minutes prior to the event, when the airplane encountered what he described as "light" turbulence, and he turned on the seatbelt sign. He stated that shortly thereafter, the turbulence increased. However, the flight attendants and most passengers stated that the flight had been smooth and the seatbelt sign had been off until the initial pitch oscillation occurred.

The captain stated that prior to the turbulence encounter, he observed a second Mach speed indication (depicted by an open circle with the speed .728) below the selected flight management computer (FMC) command speed indication (.82) on the right side primary flight display airspeed indicator. He attempted to correct this by momentarily engaging the autopilot speed command and then disengaging the system. When this was unsuccessful, he attempted to correct the airspeed indication with inputs to the FMC through the No. 2 multifunction control display unit (MCDU). This action was also unsuccessful. The MCDU keypad is located on the right side of the center pedestal, forward and below the flap/slat handle.

The captain stated that when they experienced the increase in turbulence, he observed the white "SLAT" light with a down arrow illuminate on the primary flight display. He stated that the angle of attack (AOA) bars changed color to red, the stall warning stick shaker activated, and the slat overspeed warning chime sounded. The captain stated that he immediately verified that the flap/slat handle was in the retracted position by pushing the handle forward, and the flight engineer placed his hand on the handle twice to ensure it remained forward.

According to the flight data recorder (FDR), the airplane was in a slow right turn at 296 knots indicated airspeed (KIAS) with the No. 1 autopilot engaged when a slat disagree indication was recorded. The outboard ailerons began to move at this point. About 7 seconds later, the airplane began pitching nose up despite autopilot-commanded nose-down elevator deflections. Three seconds later, the stall warning system activated while the airplane was pitching nose up through 7.4 degrees at 296 KIAS and 1.37 G. One second later, the airplane reached a maximum nose-up pitch attitude of 9.5 degrees, the airspeed decreased to 293 KIAS, and the vertical acceleration peaked at 1.50 G. The airplane then began to pitch nose down.

After a nose-down pitch rate was established, the elevators began moving in the nose-up direction. Approximately 13 seconds after the slat disagree indication, the FDR recorded a rapid movement of the elevators in the nose-down and then nose-up directions, followed immediately by deactivation of the stall warning system and disengagement of the No. 1 autopilot. The airplane reached 5.6 degrees nose down at 286 KIAS and -0.29 G, and then started to pitch nose up.

The airplane completed a second, third, and fourth pitch oscillation over the next 13 seconds. The maximum and minimum pitch attitudes became increasingly nose down, reaching a maximum of 24.3 degrees nose down at the bottom of the fourth oscillation. Vertical acceleration oscillated divergently during the second and third pitch oscillations, reaching peak values of +1.53 G and -1.24 G during the third, and then began to converge during the fourth as airspeed increased through 320 KIAS.

The FDR data became unrecoverable at this point. It became recoverable approximately 5 seconds later with the airplane pitching nose down through 15 degrees, vertical acceleration decreasing through +2.0 G, and airspeed increasing through 337 KIAS.

The airplane began to pull out of its oscillating descent, reported by the crew to have ended at approximately 28,000 feet. An overspeed warning was recorded as the airplane was pulling out with airspeed increasing through 348 KIAS. Airspeed peaked at 364 KIAS before beginning to decrease. Pitch attitude stopped increasing at approximately 7 degrees nose up and oscillated between 5 and 8 degrees nose up as it climbed. The No. 2 autopilot was engaged approximately 94 seconds after the initial slat disagree indication, after which the elevator position, pitch attitude, and vertical acceleration oscillations stopped. The airplane maintained stabilized flight during the remainder of the flight.

At 0123, the radio operator requested a deviation to the nearest airport because of an emergency. At 0125, the radio operator reported injured passengers onboard and declared an emergency. The flight diverted to Shemya, dumped fuel en route, and made an uneventful landing at 0329.

### What the investigation found

#### Aircraft damage and survival aspects No external structural damage was found on the airplane. The passenger cabin interior sustained extensive damage. Damage to the passenger seats in the coach class section ranged from slight deformation to complete collapse of the seat arm rests and seat backs. Several passenger service units sustained impact damage when struck by passengers, causing 20 oxygen masks to deploy. Ceiling panels were damaged and displaced upward, and about 80 percent of the ceiling cross beams in the aft section were crimped, separated, or bent.

The circumstances that precipitated the pitch oscillations did not allow for a prewarning to the flight attendants and passengers. The most serious injuries occurred to unrestrained passengers located in the aft cabin. Passengers described the pitch oscillations as a series of cycles in which unrestrained passengers were alternately lifted to the ceiling and dropped to the floor, striking the ceiling, armrests, seats, and other passengers.

#### Weather According to forecast charts, satellite photographs, and pilot reports, no evidence of turbulence was forecast or present in the geographical area. The FDR data were consistent with flight in undisturbed air. The Board concluded that no turbulence existed before and during the accident sequence, and that the vibrations caused by the initiation of the slat extension above the maximum design speed were likely perceived as turbulence by the flightcrew.

#### Flight recorders The cockpit voice recorder provided no useful information due to a mechanical malfunction. Examination revealed an anomalous "run-on" of the unit, consistent with the continuous-loop tape not being erased prior to recording new audio. The manufacturer determined that a capacitor on the bias generator card had opened internally, a defect never before observed or reported in over 20,000 units.

#### The slat system The MD-11 has eight leading edge slat segments on each wing. The slats are actuated by a series of cables, hydraulic valves, and mechanical linkages operated by an integrated flap/slat control handle. To meet certification requirements, the slat input system incorporates an extend bias that pulls the flap/slat handle aft toward the slat extend position. Due to this bias, the handle will move aft if it is not securely held in the selected detent position.

The time for a normal slat extension cycle is 11 to 13 seconds. The extension cycle can be interrupted to prevent full extension if the flap/slat handle is immediately returned to the full forward position within 2 to 3 seconds. Once the outboard slat extension cycle has begun, it cannot be interrupted.

Ground and flight tests of the accident airplane's slat system revealed no mechanical malfunction or failure that would have resulted in the slats extending without the flap/slat handle being moved out of the retracted position. However, flight tests demonstrated that the handle could be inadvertently dislodged from the stowed position by normal crew movements in the cockpit, such as striking the handle on the right rear corner or catching the handle knob with a shirt sleeve cuff while moving a hand to program the No. 2 MCDU.

The centralized fault display system recorded several system malfunctions approximately 13 minutes prior to the pitch oscillations. The Board believes that these faults would have required a flightcrew member to perform manual inputs through the No. 2 MCDU keypad to restore navigation data. The Board believed that the captain, in the right seat, would have been the most likely crewmember to reenter the data because of his close proximity to the keypad. The Board concluded it is most likely that the captain inadvertently contacted the handle while moving his hand in the area of the MCDU keypad. Once the handle was displaced, the slat extension cycle began.

The Board noted 12 previous incidents of inadvertent or uncommanded in-flight slat extensions and two events on the ground involving MD-11 airplanes. Douglas Aircraft Company had issued four service bulletins recommending modifications to the flap/slat handle to prevent uncommanded movement. The Board noted that although these modifications decreased the probability of an uncommanded extension, they did not eliminate the potential for further inadvertent slat extensions due to contact with the handle.

#### Flight characteristics and pilot control The MD-11 is designed to obtain improved aerodynamic efficiency by reducing the aerodynamic download on the horizontal stabilizer during cruise flight. This is achieved by operating the airplane at an aft center of gravity maintained by carrying fuel in the horizontal stabilizer. Because of the aft center of gravity and reduced area of the stabilizer, the MD-11 operates in the cruise regime with less stability margin than some other transport-category airplanes, referred to by t