What happened
On 6 September 2007, a Reims Cessna F406 Caravan II, registration G-FIND, was conducting a routine asymmetric engine training flight near Coventry. The flight was part of a crew proficiency check, with the left engine set to zero thrust to simulate a feathered condition.
While flying a left-hand orbit at 1,000 feet, the pilot flying experienced an uncommanded right roll. Initially attributed to turbulence, the situation escalated as the pilot found it increasingly difficult to maintain control using the control wheel. The pilot reported needing excessive physical force to achieve even limited aileron deflection. As the aircraft descended towards final approach, the crew experienced a sudden lurch, and the pitch trim began to run away to a full nose-up position. The rudder pedals also appeared to be jammed in a neutral position. Despite these severe difficulties, the crew managed to land the aircraft safely on Runway 05.
The investigation
Following the incident, investigators examined the aircraft's control systems and autopilot components. While the primary flight control cables were found to be unobstructed, testing revealed that the autopilot servos could produce extremely high forces when engaged.
Investigators also noted that the autopilot mode indicator was malfunctioning, remaining in a dim mode regardless of ambient light levels, which prevented the crew from easily identifying the system's status. Furthermore, the investigation highlighted that the autopilot engagement switch is located in a cluster of 12 switches and requires very light pressure to activate, making it susceptible to accidental engagement by the pilot's knee or other cockpit movements.
Findings
- The incident was likely caused by the inadvertent engagement of the autopilot during the flight.
- The pilot's ability to monitor the autopilot status was compromised by a failed mode indicator light.
- The physical location of the autopilot activation switch makes it vulnerable to accidental activation during asymmetric flight maneuvers.
- The crew experienced significant control resistance due to the autopilot servos and yaw damper being active.
Safety action
- The operator has contracted a design organisation to develop a modification for the autopilot system. This will include a prominent new disconnect switch and a warning light positioned within the pilot's normal scan to allow for easy isolation of the servos and trim actuator in the event of accidental engagement.