McDonnell Douglas MD-11 accident at Anchorage, Alaska, 7 Dec 1992

Accident narrative

On December 7, 1992, about 1036 Coordinated Universal Time, China Airlines flight CI-012, a McDonnell Douglas MD-11 (Taiwan registration B-150), encountered moderate turbulence at flight level 330 about 20 miles east of Japan. The airplane departed controlled flight and sustained damage to its left and right outboard elevators, portions of which separated in flight. The airplane recovered and continued to an uneventful landing in Anchorage, Alaska. None of the 265 people on board were injured.

### The flight

Flight CI-012 was a scheduled passenger flight from Taipei, Taiwan, to Anchorage. There were 246 passengers, three flightcrew members, two additional crewmembers, and 14 cabincrew members on board. The captain, who was the flying pilot, had 18,241 total flight hours, including 401 hours in the MD-11. The relief captain had 14,934 hours (481 in the MD-11), and the first officer had 1,509 hours (279 in the MD-11).

### The sequence of events

The captain stated that the crew received a weather briefing before takeoff indicating forecast light-to-moderate turbulence and windshear. The flight was normal until about 18 minutes from Kushimoto, Japan, when the airplane was cruising at 33,000 feet and 290 knots indicated airspeed with the autopilot engaged.

Flight data recorder (FDR) information showed the airplane entered an area of light turbulence about four minutes before the upset, followed by moderate turbulence. At the start of the upset, a lateral gust from the left side of the airplane produced a rightward acceleration. Within two seconds, the airplane rolled 30 degrees right wing down, and the autopilot disconnected.

The pilot applied left-wing-down wheel deflection to counteract the roll. However, the FDR indicated that as the right roll was arrested, the left deflection was not reduced rapidly enough, causing the airplane to roll up to 32 degrees left wing down. Concurrently, the pilot commanded excessive control column deflections. The elevators moved to an airplane-nose-up position, and the pitch angle increased to about 10 degrees. The angle of attack increased, activating the stall warning system, and normal acceleration reached about 1.75 G.

The captain later described fighting to keep control amid vibration so severe the instruments could not be read. He stated that altitude fluctuated rapidly between flight level 350 and 310, and he made manual throttle changes to prevent a stall. He requested a descent to flight level 290 from Tokyo Center and reported the turbulence.

FDR data showed the airplane climbed to about 35,800 feet and its airspeed decreased to 160 knots. During the climb, the pilot applied nose-up stabilizer trim. The airplane stalled and pitched down. As the nose dropped, the pilot continued to increase nose-up elevator deflection. The data indicated the airplane stalled and pitched down at least four times during the recovery, with the stall warning activated for most of the time between 4 and 170 seconds following the initial upset. Airplane control was established about three minutes after the initial upset when the elevators were returned to neutral and airspeed increased.

### What the investigation found

Postflight inspection revealed damage limited to the left and right outboard elevators, which were constructed of carbon epoxy composite material. Portions of the upper right and lower left outboard skin panel assemblies had separated. Detailed examination showed the failure mode was predominantly adhesive, occurring between the adhesive and the composite structure.

The Safety Board concluded that the flight encountered moderate turbulence that preceded the violent motions. However, analysis of the FDR data revealed that the airplane's pitch attitude closely followed the pilot's elevator deflections. The Board concluded that the pilot used more control than desirable or necessary during the initial upset and throughout the recovery. Because the pilot stated he was experiencing severe turbulence, the Board concluded it was reasonable to believe he did not recognize the motion cues were the result of a stall buffet he had induced.

The investigation examined the MD-11's flight characteristics. To improve aerodynamic efficiency, the MD-11 was designed to operate with a smaller horizontal stabilizer and an aft center of gravity, resulting in relaxed longitudinal stability compared to older transport airplanes. Consequently, the control column forces required to maneuver the airplane are lighter, particularly at high altitudes and speeds. The Board concluded that these light control forces make recovery from high-altitude, high-speed upsets difficult, and that the initial overcontrol was a result of this design.

The Board found that neither the manufacturer nor the airline had specific training procedures addressing high-altitude upsets. The Board noted that the pilot's response to the stall warning was inappropriate, as he ignored the warning and overrode the control column forces to maintain the airplane in a stalled condition.

Regarding the structural damage, the Board found that the airplane exceeded its design buffet boundary. The Board concluded that the elevators were loaded above their design limit load during the stall buffet, and that the separation of the elevator skin most likely occurred during this period. The Board noted that the skin ruptures were benign failures that decoupled the balance weights, preventing high loads from transferring to the structure, and did not create an unsafe condition for continued flight.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this incident was the light control force characteristics of the MD-11 airplane in high altitude cruise flight. The upset was induced by a moderate lateral gust and was exacerbated by excessive control deflections. Contributing to the incident was a lack of pilot training specific to the recovery from high altitude, high speed upsets in the MD-11.