What happened
On 25 September 2016, a Reims Cessna F406 Caravan II, registration G-FIND, was conducting a series of training flights to perform proficiency checks on two company pilots. During the fourth flight of the day, which involved a simulated engine failure and a transit from Coventry to East Midlands Airport, the aircraft was in a steady cruise with the autopilot engaged.
As the aircraft approached its destination, it suddenly entered a significant nose-down pitch. The handling pilot attempted to correct the attitude by applying back pressure and pressing the autopilot disconnect button on his control wheel, but the aircraft continued to pitch down. The crew noted that no aural warning accompanied the pitch change. To regain control, the pilot reached across to operate the disconnect button on the left control wheel and switched the autopilot master to the off position.
At this stage, the aircraft had reached a 1/10 degree nose-down attitude with increasing airspeed, making the aircraft difficult to manage. The commander observed that the pitch trim indicator showed an abnormal forward position. After manually resetting the trim wheel to the takeoff setting, the pilot was able to maintain stable flight. The remainder of the flight was completed manually without further issues, and there were no injuries to the two crew members or the passenger on board.
The investigation
Investigators performed a detailed inspection of the pitch trim system components. They established that the trim setting during the event was nearly at the maximum nose-down position. The investigation also looked into a potential autopilot malfunction, specifically whether a spurious nose-down input occurred or if the safety system—designed to disengage the autopilot if the pitch exceeded 21 degrees—had failed. While flight tests suggested that the control forces required to counter such a trim setting could be managed, the possibility of an electric trim malfunction could not be ruled out.
Findings
- The aircraft's pitch trim was found to be in an almost fully nose-down configuration during the incident.
- An electric trim malfunction was considered a possible cause, though not definitively proven.
- The investigation examined the failure of the safety system intended to disconnect the autopilot during extreme pitch angles.
Safety action
Following the incident, the Air Data Computer and Autoflight Computer were replaced in G-FIND. The original units were sent to the manufacturer for fault diagnosis. Additionally, the operator had previously implemented a modification to the aircraft's autopilot system—consisting of a prominent disconnect switch and warning light—which had been used successfully by the crew to regain control during this event.