What happened
While operating in instrument meteorological conditions (IMC), the aircraft experienced a failure of the left remote gyroscope. This malfunction caused the primary horizontal situation indicator (HSI) to exhibit a slow, continuous roll. Because the autopilot was set to heading mode, the aircraft began a steady turn to the right as it attempted to track the drifting heading bug.
The pilot initially noticed the deviation and disengaged the autopilot, following the procedures outlined in the pilot operating handbook (POH) regarding FD flag warnings. During the manual recovery, the aircraft underwent altitude variations, likely due to the combination of turbulence and the difficulty of manual flight in IMC. Although the pilot identified the faulty HSI readings, they chose to re-engage the autopilot and heading mode to manage the high workload. This decision caused the uncommanded right turn to resume, forcing the pilot to constantly adjust the heading bug to maintain course.
The investigation
The investigation focused on the mechanical failure of the left remote gyroscope and the subsequent pilot actions. It was determined that the failure directly caused the erroneous HSI display. Investigators also examined the pilot's decision-making regarding autopilot use and communication with Air Traffic Control (ATC). The investigation noted that while the pilot was heavily focused on managing the malfunction, they did not issue a PAN PAN call to alert controllers to the situation.
Findings
- The failure of the left remote gyroscope led to incorrect readings on the HSI.
- The pilot's decision to re-engage the autopilot in heading mode despite the known instrument error led to sustained control difficulties and increased workload.
- The pilot's failure to declare a PAN PAN reduced the ability of ATC to provide necessary assistance during the incident.
- Using an alternate navigation source, such as the left RMI or right HSI, could have prevented the uncommanded turn.