What happened
Two distinct events occurred involving a Bell 429 GlobalRanger, registration G-WLTS, during the performance of Power Assurance Checks (PAC) at Melksham Airbase.
On 15 June 2018, while the pilot was conducting a PAC, the aircraft experienced a sudden and violent rotation. During the check, the pilot had their feet off the pedals to facilitate writing performance data on a kneeboard. The aircraft underwent two and a half complete rotations in approximately ten seconds. The pilot, disoriented by the movement and initially suspecting a landing gear collapse, eventually shut down both engines and applied the rotor brake to stop the rotation. No injuries were reported, and there was no damage to the aircraft.
On 2 January 2019, a second event occurred during a similar ground procedure. In this instance, the pilot kept their feet on the pedals. The pilot noted significant feedback forces and an uncommanded movement of the pedals toward full left deflection. Unlike the first incident, the pilot was able to contain the movement through foot pressure and eventually cleared the condition by using the trim release button.
The investigation
The AAIB examined flight data from both incidents. For the 2018 event, data showed that the automatic flight control system (AFCS) transitioned to an 'in-flight' state because total torque exceeded 30%. The investigation focused on why the pedals moved to maximum deflection.
Testing revealed that the aircraft's automatic pedal trim logic could induce a runaway. If the pedals are moved away from their detent position and then reversed before the system establishes a new detent, the trim actuator continues to drive in the initial direction. If the pedals are restricted from moving, the artificial feel spring 'winds up,' creating high pressure. When the restriction is removed, the spring drives the pedals to full deflection.
Additionally, the investigation looked into the quality of the cockpit voice recorder (CVR) data. It was discovered that audio clipping was occurring on the crew channels. This was traced back to a 201s minor modification involving the installation of a TETRA communications system, which replaced the original audio controller. The new controller's output levels were higher than the CVR's input requirements, causing signal distortion that had gone undetected during annual inspections.
Findings
- The primary cause of the 2018 rotation was a yaw trim runaway triggered by the AFCS logic during a power assurance check.
- The pilot's posture during the first event (feet off the pedals) prevented them from immediately countering the uncommanded pedal movement.
- The automatic pedal trim function remains operational on the ground if force trim is engaged, a detail not previously highlighted in the manuals.
- The 2019 event was successfully contained because the pilot maintained physical control of the pedals.
- The CVR audio distortion was caused by an interface mismatch between a newly installed audio controller and the existing flight recorder system.
Safety action
- The manufacturer published a revised Rotorcraft Flight Manual (RFM) providing procedures to respond to a yaw trim runaway and to reduce the risk of recurrence.
- Transport Canada issued an Emergency Airworthiness Directive to mandate these manual updates.
- The flight recorder manufacturer updated its inspection reporting to include signal waveform plots, allowing for easier identification of audio clipping.