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On November 7, 1996, at about 2015 eastern standard time, a McDonnell Douglas MD-11, N814DE, owned and operated by Delta Airlines, Inc., as Flight 37, experienced a horizontal stabilizer trim failure while en route to Covington, Kentucky. The aircraft diverted to Wright-Patterson Air Force Base in Dayton, Ohio, and landed without further incident. There were no reported injuries among the 13 crewmembers and 196 passengers who were deplaned normally. However, two passengers were taken to an area hospital for observation. The flight originated from Gatwick Airport, London, United Kingdom, at 0510 EST, with an intended destination of the Greater Cincinnati International Airport, Covington, Kentucky. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan had been filed. The flight was conducted under the provisions of 14 CFR Part 121 as an international-scheduled-passenger flight.
While en route, the flight crew experienced a failure the pitch trim control system. The flight crew was unable to change the pitch trim by the yoke mounted trim switches or the center console mounted horizontal stabilizer control handles. The flight crew stated that when the slats were extended on descent, they were unable to change stabilizer trim, and all hydraulic systems checked OK. When the autopilots were engaged a "STAB OUT OF TRIM" message appeared. Disengaging the autopilots caused the message to disappear, but the configuration showed 1.0-degree aircraft nose up, and aft pressure was needed on the control yoke to hold altitude. The flight crew declared an emergency, and the aircraft was diverted to Wright-Patterson Air Force Base near Dayton, Ohio where it landed uneventfully.
The McDonnell Douglas MD-11, S/N 48623, Fuselage Number 605, Aircraft 814, was manufactured and certificated on September 8, 1996. Federal Aviation Administration records indicated that the aircraft was registered to Delta Airlines on September 26, 1996, and maintenance records indicated that the aircraft had accumulated 540 hours and 70 cycles, at the time of the incident.
EXAMINATION OF THE HORIZONTAL STABILIZER
Shortly after landing Delta maintenance personnel stated that they began trouble shooting the reason for the horizontal stabilizer lockout. They stated that they initially thought that the horizontal stabilizer gearbox was lacking oil, which caused the shear pin to fail, locking the stabilizer in place as designed. During removal of the gearbox it was noted that the jackscrews, both left and right, were lacking lubrication. The gearbox and both jackscrews were sent to Delta Airlines in Atlanta, Georgia for further examination, and the aircraft was repaired and returned to service on November 11, 1996.
On November 12, 1996, the horizontal stabilizer pitch trim system was examined. Present at the examination were representatives from the National Transportation Safety Board, Federal Aviation Administration, Delta Airlines and McDonnell Douglas.
The system included: two (2) hydraulic motors, one (1) gearbox, one (1) drive unit, two (2) drive chains, and two (2) screw actuator assemblies. The examination revealed that the fuse pin had sheared in the output drive unit of the horizontal stabilizer drive gearbox. The sheared fuse pin locked the horizontal stabilizer in its last set position. In addition, there was no visible grease film on the screw actuator assemblies as called for in Douglas Parts Service manual DPS 3.17-51, which stated in part, "Apply heavy coat of DPM 5850 to threads of screw. Apply Lubricant to fitting on top of sprocket until lubricant extrudes between sprocket and screw." In addition, the Lubrication Chart on page 40 of DPS 3.17-51 depicted a brush symbol to indicate brush application of the required lubricant to the surface of the screw threads.
Douglas Aircraft Engineering agreed that the lack of adequate lubrication on the thread of the screw actuator assemblies, lead to a "chattering" on the nuts of the screw actuator assemblies which lead to the failure of the fuse pin. (See attached Photographs).
A review of Assembly Orders at Douglas Aircraft (DAC) for fuselage 605 revealed that lubrication requirements per DPS 3.17-51 were specified at two stages of the assembly.
1. After initial installation in the aft fuselage barrel assembly. 2. After wash, prior to delivery to the customer.
The assembly orders were found stamped by DAC production personnel and DAC Inspection personnel indicating completion and acceptance of the lubrication. However, the investigation revealed that no lubrication had been applied to the screw actuator nuts.
Operators of fuselages 599 through 607, were notified of the suspected nonconformance. The following is a summary of the DAC Product Support findings:
Fuselages 599 and 601 were found properly lubricated.
Fuselages 602, 604, 606, and 607, were inspected for lubrication, and found to be inadequately lubricated.
Fuselages 608, 609, 610, and 611, while still in assembly were inspected and rejected, their horizontal stabilizer trim systems were subsequently lubricated properly per DPS 3.17-51.
All uninstalled screw actuators at DAC were returned to the manufacturer for proper lubrication was accomplished.
As a result of this investigation, Douglas Aircraft Corporation (DAC) has disciplined the Production and Quality Assurance employees involved in this matter.
DAC Production has initiated an awareness flowdown program for Production employees.
DAC Product Verification (Quality Assurance) has developed a plan to elevate the skill level of DAC Inspectors by providing extensive training.
DAC Quality Assurance - Regulatory Affairs will conduct quarterly evaluations to validate the corrective action plan provided to the FAA.
In addition, Delta Airlines has added to their aircraft acceptance inspection program, a visual inspection of the horizontal stabilizer assembly, to ensure adequate lubrication is present.