WESTERLY, RI, USA
N1768D
McDonnell Douglas MD-11
The first officer (F/O) was making a descent from FL350 to FL240 in smooth air in VMC conditions with the autopilot (A/P) engaged. As the flight (flt) neared FL250, the captain became concerned that the airplane would not level off at FL240. He instructed the F/O to slow the rate of descent. The F/O attempted this by using the pitch thumbwheel on the A/P control panel. The captain then took control, attempted to overpower the A/P, and pulled back on the control yoke. With back pressure on the control yoke, he disengaged the A/P. With the reduced control column resistance after the A/P was disengaged, there was further excursion of the elevators to the up position. According to the flt data recorder, the airplane was subjected to a +2.28 G-load. A passenger in the aft lavatory suffered a fractured ankle. A 2nd passenger and 2 flt attendants received minor injuries. The MD-11 Flight Crew Operating Manual (FCOM) advised against attempting to overpower the A/P; however, this was contained under the title, 'SEVERE TURBULENCE AND/OR HEAVY RAIN INGESTION.' Each use of pitch thumbwheel interrupted the automatic level off, and the system would wait for 2 seconds after release of the thumbwheel before initiating a level off sequence again. Continued use of the thumbwheel precluded the A/P from performing the level off at FL240. The manufacturer has issued FCOM changes to warn pilots about the hazards of applying force to the control wheel or column while the A/P is engaged and adjusting the pitch thumbwheel during a level off.
On July 13, 1996, about 2040 eastern daylight time, an McDonnell Douglas MD-11, N1768D, operated by American Airlines as flight 107, experienced an abrupt maneuver during a descent, while operating near Westerly, Rhode Island. The airplane was not damaged. There were 180 occupants onboard the airplane, of which, one passenger received serious injuries, and one passenger and two flight attendants received minor injuries. Visual meteorological conditions prevailed, and flight 107 which had departed London, England, at 1354, was operated on an Instrument Flight Rules (IFR) under 14 CFR Part 121. Flight 107 was inbound to John F. Kennedy Airport, Jamaica, New York. In a telephone interview the Captain reported that the flight had crossed Boston at FL350 (35,000 feet), and was then instructed by ATC to cross 20 miles northeast of PARCH intersection at FL 240 (24,000 feet). At that time the flight was 45 miles from PARCH. The flight was operating in visual meteorological conditions, and the air was smooth. The first officer was performing the duties of the operating pilot. The descent was initiated with the auto-pilot engaged in the vertical profile (PROF) mode. The speed brakes had been extended full, and as the airplane neared FL 250 (25,000 feet), he became concerned that the airplane would not level off at FL 240. He instructed the first officer to slow the rate of descent. The first officer complied by using the pitch thumbwheel on the auto-pilot control panel, but no effect was observed. The Captain then took control of the airplane, retracted the speed brakes, and a few seconds later, disconnected the auto-pilot. This was followed by an immediate pitch up, which the captain described as similar to flying through a jet wake. The captain then applied forward pressure to the yoke, and the airplane leveled off at FL237 (23,700 feet), after which it was hand flown back to FL 240. One passenger in the aft lavatory received a fractured ankle (serious injury). One passenger received minor injuries and refused treatment. Two flight attendants on duty in the aft portion of the airplane received minor injuries. Following the incident, the flight continued into John F. Kennedy Airport where an uneventful landing was made. The Digital Flight Data Recorder (DFDR) was forwarded to the NTSB laboratory in Washington, DC for readout. According to the recorder, the maximum vertical "G" loading was +2.28 Gs, at an indicated airspeed of 354 knots, and at a pressure altitude of 23,781 feet. Additionally, on the descent between 25,900 feet and 23,900 feet, there were 7 excursion of the pitch thumbwheel. As the airplane descended between 25,700 feet and 25,300 feet, there was a momentary increase of g loading on the vertical axis. As the airplane passed through 24,200 feet, there was an increase in the elevators to the airplane nose up position. As the airplane passed through 23,800 feet, there was a momentary further increase of the elevator displacement which corresponded to the peak g load of +2.28 gs observed on the DFDR. Following this the elevators immediately went to a negative position and then returned to their pre-incident position through a series of oscillations. According to the American Airlines MD-11 Operating Manual, Auto-Flight section, when the auto flight system is engaged in the vertical profile (PROF) mode, rotation of the pitch thumbwheel will disengage the PROF mode. When the thumbwheel is released, the system reverts to the PROF mode. According to MDC personnel, the auto-pilot was programmed to initiate a level off at a maximum g loading of 0.2 gs. Use of the pitch thumbwheel would interrupt the automatic level off process, and once released, the system would wait for two seconds prior to initiating the level off process again. Each time the pitch thumbwheel was released, the pre-programmed level-off routine would re-initiate after a 2 second pause. The continued use of the pitch thumbwheel allowed the airplane to continue its descent to an altitude whereby a level off at FL240 would exceed the maximum g load of 0.2 gs that the system was designed to handle. Auto-pilot 2 was engaged and was driving the right inboard elevator. The other three sections of the elevator were being driven by the right inboard elevator through mechanical linkage. When the auto-pilot was overpowered by force to the control yoke, the three elevator sections not connected directly to the auto-pilot responded. The amount of force required for the deviation exceeded the control force necessary to move the elevators with the auto-pilot dis-connected. Additionally, when the auto-pilot was disconnected while control force was applied, two actions took place simultaneously. The auto-pilot driven section moved to match the position of the other three control surfaces and the control yoke, and the force that was restricting control movement before was removed. According to MDC personnel, it would be extremely difficult for a person applying control force and releasing the auto-pilot to avoid having a further excursion of the elevator in the direction of control force applied due to the reduction in control force when the auto-pilot was dis-connected. Examination of the MD-11 Flight Crew Operating Manual found the following under the title of "SEVERE TURBULENCE AND/OR HEAVY RAIN INGESTION" : "...Do not attempt to overpower the auto-pilot with control forces. This can cause the auto-pilot to disengage with too much control input, which could result in over control during recovery. Every attempt should be made not to over control. Longitudinal control forces at high altitude will be lighter than those which the pilot experiences at low altitude due to altitude effect as aft CG...."
insufficient information from the manufacturer in the airplane flight manual and flightcrew operating manual regarding the hazards of applying force to the control wheel or column while the autopilot is engaged and adjusting the pitch thumbwheel during a level off. Also causal was the flightcrew's lack of understanding of these items and the captain's improper decisions to overpower the engaged autopilot and then to disconnect the autopilot while holding back-pressure on the control yoke.
Source: NTSB Aviation Accident Database
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