What happened
On 9 January 2006, a SAAB-Scania SF340B, registration G-LGNJ, was conducting a scheduled passenger flight from Stornoway to Glasgow. During the climb to FL135, the crew activated engine anti-ice as the aircraft entered icing conditions. As ice began to accumulate on the windshield, wing leading edges, and propeller spinners, the crew selected continuous operation of the airframe de-icing boots.
While descending through FL080, the aircraft encountered turbulence and a large piece of ice shed from a propeller, striking the fuselage and causing airframe vibration. Shortly after, the autopilot and yaw damper disconnected without warning, resulting in an abrupt 5-degree nose-down pitch. Upon attempting to take manual control, the crew found that the control column was stuck and could not be moved fore or aft.
Despite the restriction, the crew managed to re-engage the autopilot and yaw damper, which stabilized the aircraft. The commander declared a PAN urgency to Air Traffic Control and requested a descent to land at Glasgow. During the landing roll, the crew noted that the power levers could not be retarded below flight idle, though they were able to use the flight-idle override handle to bring the engines to ground idle and park the aircraft safely.
The investigation
The AAIB examined the flight data recorder, which showed that the autopilot had been actively compensating for increased elevator deflection—likely due to ice on the tailplane—just before the disconnection. The investigation also included a review of the aircraft's maintenance history and an analysis of the autopilot's electronic flight control computer.
Engineers inspected the control surfaces and mechanisms, finding no mechanical obstructions or residues from de-icing fluids. The manufacturer also performed simulator tests to evaluate the impact of ice on the horizontal stabiliser and investigated potential mechanical binding within the autopilot servo gear-train.
Findings
- The autopilot disengaged while the system was actively attempting to compensate for an upward trend in elevator deflection, suggesting ice accretion on the tailplane may have reduced aerodynamic effectiveness.
- The sudden nose-down pitch following the disconnection was likely caused by the autopilot's failure to maintain the trimmed condition once the servos were no longer actively managing the pitch.
- While the cause of the control column's temporary immobility could not be definitively identified, the investigation could not rule out a temporary malfunction of the autopilot clutch or a mechanical issue within the servo mounting.
- There was no evidence of mechanical faults in the power lever flight-idle stop system, and the crew's inability to retard the levers was resolved using the override handle.