NTSB Identification: NYC96LA148.
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Scheduled 14 CFR operation of AMERICAN AIRLINES
Accident occurred Saturday, July 13, 1996 in WESTERLY, RI
Probable Cause Approval Date: 03/17/1999
Aircraft: McDonnell Douglas MD-11, registration: N1768D
Injuries: 1 Serious,3 Minor,176 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
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.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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. Full narrative available
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