ANCHORAGE, AK, USA
N681MA
BOEING 737-3M8 (300)
THE FLIGHT EXPERIENCED AN UNCOMMANDED PITCH UP DURING AN EN ROUTE CLIMB WITH THE B SYSTEM AUTOPILOT ENGAGED. A FLIGHT ATTENDANT WORKING IN THE REAR CABIN AREA SUSTAINED A FRACTURE TO HER RIGHT ANKLE. REPLACEMENT OF THE B SYSTEM CHANNEL AUTOPILOT ELEVATOR ACTUATOR ULTIMATELY FIXED THE PROBLEM.
HISTORY OF FLIGHT On December 11, 1994, at approximately 1435 Alaska standard time (AST), a Boeing model 737, series 3M8 airplane, U.S. registration N681MA, SN:24376, registered to and operated by MARKAIR Inc., as flight No. 308, with a flight crew of two, a cabin crew of three, and 116 revenue passengers, experienced an uncommanded pitch up during a cruise climb from the Anchorage International Airport, Anchorage, Alaska. As a result of the aberrant maneuver, the first flight attendant, standing in the rear of the airplane, sustained a fracture to a bone in her right ankle. The airplane was not damaged, and no additional injuries were reported. The scheduled 14 CFR Part 121 flight departed Anchorage on an instrument flight plan at 1425 and was en route to Seattle, Washington when the accident occurred. Both the Captain and the First Officer (F/O) filed trip reports with the airline and each were interviewed by the Safety Board investigator-in-charge (IIC). The F/O reported that he was the flying pilot. At 12,000 feet, he directed the captain to engaged the "B" system autopilot. Passing 17,000 feet, the aircraft abruptly pitched up then down. The captain reported that the flight control yoke came back a "good eight inches". At the time of the event, he believed that the plane's indicated airspeed was around 300 knots and that there was some atmospheric turbulence. The F/O believed that the flight was experiencing a relatively smooth ride. The captain reported that he immediately disengaged the "B" autopilot and engaged the "A" autopilot. Neither the F/O or the Captain were interacting with the airplane at the time the event occurred. The flightcrew likened the short history of the event to the blinking of an eye, and that they did not have time to absorb the total kinematics of the event. The flight experienced no further problems and continued on to Seattle. The injured flight attendant initially believed that she had only bruised her right ankle. It wasn't until she went to a medical facility several days afterwards that she learned the extent of her injury. She subsequently notified the airline, who in turn notified the Safety Board on December 14, 1994. FLIGHTCREW INFORMATION The captain and first officer, both of whom were airline transport certificated, were medically and operationally qualified to conduct the flight. Neither pilot had experienced a pitch anomaly in a Boeing 737 previously. The captain had a total of about 21000 flight hours, of which about 5600 were in the B737. The First Officer had a total of approximately 9000 flight hours and had been assigned to the B737 for about two years. He had accrued about 1800 flight hours in the B737, of which approximately 300 were in the model 300. AIRCRAFT INFORMATION The airplane was built in May of 1989 and subsequently delivered to Trans European Airlines (TEA). MARKAIR began leasing the airplane from the ITOCHU Corporation in November of 1991. At the time of the accident, the airplane had accrued a total of 17725 flight hours since new and 32 hours since the last "B" check, which was performed on December 9, 1994. There were no entries in the plane's maintenance records prior to December 11, 1994 pertaining to uncommanded autopilot produced pitch up events. MARKAIR maintenance personnel inspected the plane's autopilot system and initially determined that the problem was caused by a defective mode control panel (MCP) in the digital flight control system. The MCP was replaced on December 19, 1994. An operational test indicated satisfactory operation. During a subsequent flight on December 19, 1994, with the B autopilot engaged, the stabilizer out of trim light illuminated briefly. The flightcrew disengaged the autopilot and the airplane reportedly pitched nose down about 10 degrees. No problems were noted by the flightcrew with the A system autopilot. On December 22, 1994, the B system autopilot flight control computer (FCC) was changed out during the troubleshooting process, but the results proved unsatisfactory. On December 23, 1994, the A system FCC was installed in the B system. On December 29, 1994, the autopilot elevator position sensor was replaced; however, additional electronic testing continued to detect faults in the system. On February 2, 1995, the autopilot B channel elevator actuator (No. 7373 158300-101, SN:2314) was removed and replaced. No further problems have been reported with the B system autopilot. The removed elevator actuator was shipped to E-Systems, the manufacturer, in Salt Lake City, Utah on an exchange basis. According to E-system records, the build date of the actuator was August 30, 1988. The "As Delivered Records" from the Boeing Airplane Company indicate that elevator actuator 2314 was not part of the originally installed equipment. According to MARKAIR personnel, the maintenance records on the airplane from Trans European Airlines did not show a history on the removal and replacement of the elevator actuator. METEOROLOGICAL INFORMATION The flight crew reported that the weather conditions at the time of their departure from runway 32 at the Anchorage International Airport was scattered to broken clouds, with an overcast at 8500 feet. The visibility was good in light rain. The ambient temperature was 45 fahrenheit, and the wind was out of the southwest at about eight knots. Neither pilot recalled the presence of airframe icing. TEST AND RESEARCH The rework record and failure analysis reports from E- Systems revealed that the elevator actuator failed continuity and dielectric testing; the manifold failed dielectric at the transducer, and the friction in the crank assembly was beyond the maximum limits authorized. Mr. Don Clavell, supervisor of product support for E-systems, informed the NTSB IIC that the problems noted with the elevator actuator were typical of those found with prior in service units. (Note. The recorded maintenance activity performed on the elevator actuator by E- Systems is included as an attachment to the investigation file.)
THE FAILURE OF THE B SYSTEM CHANNEL AUTOPILOT ELEVATOR ACTUATOR.
Source: NTSB Aviation Accident Database
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