NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
The Boeing 737-400 airplane experienced an uncommanded pitch up event at 20 feet radio altitude during landing. The flight crew reacted to the uncommanded pitch-up event by adjusting the stabilizer trim position, attempting to move the elevator control columns forward, disengaging the autothrottle, and executing a go-around. The airplane subsequently landed without incident and both crewmembers sustained minor injuries that were incurred during the go around.
Post-incident inspection of the elevator Power Control Units (PCUs) revealed that the left elevator PCU input control arm assembly was jammed by a piece of Foreign Object Debris (FOD) in a position that offset the control arm in a downward direction. With the control arm deflected in this direction and with hydraulic pressure on, the left PCU moved the elevators to a position that pitched the airplane nose up independent of pilot input. An assessment of information obtained from the FDR regarding the functional characteristics of the airplane’s pitch control system indicates that the elevator control system was fully functional and operated as designed during previous flights and up until the uncommanded pitch event occurred. According to Boeing, a jam in either the left or right PCU input control arm assembly could result in the loss of manual control of both elevator PCUs under hydraulic-powered operation. The flight crew’s immediate actions of exerting constant and excessive force on the control columns and executing a go-around at low altitude following the jam resulted in recovery of airplane pitch control despite high control forces.
Metallurgical analysis revealed that the FOD had the same dimensions and material composition as the metal rollers that are contained in a DAS10-26B1-502 bearing, which is installed in only two locations in the aft elevator control system; the right and left ends of the elevator upper output torque tube crank assembly. Post-incident inspections of the airplane’s elevator system components located within the area of the tail cone also revealed that the left hand elevator upper torque tube output crank bearing/sleeve was completely intact with all rollers present. Examination of the maintenance records revealed that the left elevator upper torque tube output crank bearing/sleeve assembly had been replaced in January 2009 to correct an elevator freeplay discrepancy on the incident airplane. However, based on the available information, it could not be determined whether the metal rollers were left behind following the maintenance work performed in January 2009 or were present before the work was performed.