History of Flight
On June 14, 2009, about 1817 UTC, a Boeing 737-400, registration TC-TLA, operating as Tailwind Airlines flight OHY036 on a wet lease to Onur Airlines, experienced an uncommanded pitch-up event at 20 feet radio altitude during the landing flare on runway 34 at Diyarbakir Airport in southeast Turkey. The flight crew reported that the aircraft rapidly moved to an extreme nose-up position of approximately 30 degrees without command. The crew executed a go-around, controlling the pitch with significant column force, full nose-down stabilizer trim, and thrust. A second approach and uneventful landing were made.
Injuries to Persons
Both crew members sustained and reported injuries. No other injuries were reported among passengers or cabin crew.
Damage to Aircraft
None.
Aircraft Information
The incident aircraft was a Boeing 737-400 equipped with CFM56 engines. The elevator control system uses two hydraulically powered elevators with manual reversion. The system includes two power control units (PCUs) powered by separate hydraulic systems. The incident aircraft was equipped with Parker Aerospace P/N 65-44761-21 elevator PCUs. After the incident, an inspection of the elevator PCUs found the left side PCU control input arm assembly jammed in a position commanding retraction. A piece of foreign object debris (FOD) was found positioned in the gap between the control input arm assembly and its control stop, preventing the arm from returning to neutral. A second piece of FOD was found in the tail cone area. The operator removed the FOD and performed a functional test, which demonstrated normal operation.
Flight Recorder Data
Flight data recorder (FDR) information indicated that on approach, the airplane was on a heading of about 140 degrees at an indicated airspeed of approximately 145 knots, configured for landing with autothrottles on, autopilot off, and flaps set to 30. At a radio altitude of about 20 feet, with the pilot holding back pressure for flare, an uncommanded displacement of both elevators occurred, increasing the pitch attitude from about four degrees to about forty degrees within approximately 14 seconds. The crew trimmed stabilizer to full nose-down and attempted to move columns forward. They then turned off hydraulic power to the flight controls, causing elevators to deflect to neutral, resulting in a rapid pitch change to about negative five degrees. After restoring hydraulic power, pitch-up tendencies continued. The crew controlled the airplane with full nose-down stabilizer, thrust, roll, and significant effort by both crew members.
Tests and Research
Boeing's Material and Process Technology group identified the two FOD items as having the same dimensions and metallurgical composition as rollers installed in a DAS10-26B1-502 bearing, which is used in the elevator upper output torque tube crank assembly. Energy Dispersive X-Ray Spectroscopy showed both parts are made of 52100 low alloy steel. The left elevator PCU was examined and a reenactment demonstrated that a trapped bearing roller could offset the control input arm in a downward direction, actuating the PCU to raise the elevator, consistent with FDR data. Post-incident examination of the elevator system components in the tail cone showed the left elevator upper torque tube output crank bearing appeared new, in good condition, and intact. Both PCUs and associated components were found intact and properly connected.