History of the Flight
The incident occurred during the last flight of a series that began on 20 December 2002 in Manchester. The crew reported for duty at 1855 hrs for a return flight to Tenerife-South. The aircraft, G-XLAD, had just returned to service after engineering work to investigate an airframe vibration. During descent into Tenerife, a burning plastic smell was noticed on the flight deck around FL150. The crew requested a priority landing, and on final approach at about 12 nm in VMC, the commander's Primary Flight Display (PFD) data became intermittent, followed by the first officer's and standby PFDs. The autopilot disengaged without an aural warning. The first officer landed manually. After shutdown, no burning signs were found. An engineer inspected the aircraft on 21 December and found no faults, but noted that both navigation multi-mode receivers had tripped out on the previous approach. The aircraft was cleared for flight, but the company instructed positioning to Las Palmas for a sub-charter to Brussels. On the flight to Las Palmas, a successful ILS approach was conducted. During descent into Brussels, the burning smell briefly recurred. The aircraft then flew to Gatwick with only crew and engineer aboard.
During cruise at FL240, both pilots and the engineer noticed repeated gentle yawing. The commander observed a slip to the right requiring about 2.5 units of right aileron. There was no asymmetric power, and the yaw damper indicator was central. The rudder trim was slightly left. When attempting to apply right rudder, the commander found the pedals immobile, though they moved normally between disturbances. The engineer went to the cabin and observed the aircraft drifting right then kicking left. The deviations continued throughout the descent. At FL080, the crew made a PAN call to Gatwick approach, reporting rudder problems. The commander disconnected autopilot and autothrust. The non-normal checklist for uncommanded yaw or roll requires yaw damper off, but it was left engaged. At 3,000 ft, the commander had to apply 5 units of left aileron. On ILS at 300 ft AGL, the aircraft yawed left, slip indicator displaced three-quarters right, and full right aileron was needed. The rudder pedals were locked despite full force. At 30 ft radio altimeter, the divergences suddenly stopped, but the aircraft yawed and rolled right, landing about 30° off the runway track, touching down on right main gear. The commander used nosewheel steering to control the aircraft. After parking, the rudder pedals became free again.
Engineering Investigation
After landing, the aircraft underwent inspections agreed upon by the operator, maintenance organization, and Boeing. Checks of pedal mechanisms, rudder cable system, and rear mechanical components found no restrictions. Rudder cable tensions and pedal forces were within limits. Both stall management yaw damper units were tested and performed normally. The rudder hydraulic actuator was replaced and later tested in the USA under FAA oversight, behaving normally.
System Description
The Boeing 737-800 rudder control system includes two pairs of rudder pedals connected via cables to an aft quadrant, which connects to the rudder torque tube via a control rod. The rudder feel and centering unit, biased by the electric rudder trim actuator, also connects to the torque tube, which provides mechanical inputs to the rudder hydraulic actuator and standby rudder actuator. The FDR records rudder pedal position, rudder position, and rudder pedal force via a transducer on the control rod.
Recorded Data Analysis
The aircraft had a 30-minute CVR and 25-hour FDR. The AAIB was notified days later, so the CVR was overwritten. FDR data from all sectors, especially the incident flight, was analyzed. The pre-flight rudder control check showed normal rudder angle and pedal forces. The data indicated uncommanded yaw events consistent with the crew's reports.