Tailwind un-commanded pitch-up spurs NTSB recommendations

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A June 2008 un-commanded pitch-up of a Boeing 737-400 operated by Tailwind Airlines has resulted in five recommendations by the US National Transportation Safety Board to the FAA focusing on elevator system redesign for 737-300/400/500 models.

The aircraft, registration number TC-TLA, experienced the pitch-up 20ft above the ground during approach to Diyarbakir Airport in Turkey.

NTSB states the flightcrew performed a go-around and controlled the aircraft's pitch with significant column force, full nose-down stabilizer trim and thrust.

During the second approach the crew controlled the aircraft and landed by using forceful control column inputs to maintain pitch control, and sustained injuries during the go-around.

NTSB states it determined the incident was caused by foreign object debris (FOD), a metal roller element from an elevator bearing, that jammed the power control unit (PCU) on the left elevator.

"During its investigation of this incident, the NTSB identified safety issues relating to the protection of the elevator PCU input arm assembly, design of the 737 elevator control system, guidance and training for the 737 flight crews on a jammed elevator control system and upset recovery training," the board states.

Its five recommendations focus on those concerns. NTSB has asked FAA to require Boeing to develop a method to protect the elevator power control unit from FOD, and mandate that operators modify their aircraft with that method of protection.

NTSB also believes a re-design of the -300-500 series elevator system to prevent a single point jam from restricting the movement of the system is necessary, and that operators should be required to implement the new design.

The board points out that no override mechanism existed on the incident aircraft, and while the pilots exerted enough force on the control columns to override the jam, NTSB is "concerned that other jam scenarios may exist in which pilot inputs would not be enough to successfully control the airplane".

The board also believes Boeing needs to develop recovery strategies - checklists or memory items - for 737 models lacking a mechanical override for a jammed elevator in the event of a full control deflection of the elevator system.

The Tailwind flight crew did not have time to reference the 737 flight crew operations manual (FCOM) or the quick reference handbook, NTSB explains, and its review revealed no checklists or procedures from un-commanded elevator deflection or a jammed elevator control system in the FCOM.

Additionally, NTSB states flight data recorder from the aircraft indicated once the flight crew re-established minimum control over the pitching tendency, they turned off the hydraulic power to the flight controls.

"By turning off the hydraulic power during the go-around manoeuvre, the flight crew adversely affected the airplane's controllability," says NTSB. As a result, its final recommendation is within the recovery strategies it believed Boeing should develop, the consequences of removing all hydraulic power to aircraft as a response to any un-commanded control surface should be clarified.