History of the Flight
The pilot was flying under Visual Flight Rules at approximately 1,500 ft when the aircraft and rudder pedals began shaking violently. He had been airborne for about 1.5 hours. Yaw control was difficult, but the aircraft remained controllable. The shaking persisted regardless of force applied to the rudder pedals and worsened when speed was reduced. A rear-seat observer looked out of his bubble window and estimated the top of the vertical stabiliser was moving side-to-side approximately 30 to 40 cm. Uncertain about the aircraft's structural integrity, the pilot declared an emergency and diverted to a known private airstrip about 5 nautical miles away. The landing was uneventful, but the aircraft was described as 'severely shaking'.
Aircraft Examination
Visual examination found that the rudder trim tab was not connected to its control rod. The two items are normally joined by a bolt, nut, and split pin, but none of these components were recovered. The bearing in the control rod eye-end was reported to be free to rotate and was considered acceptable for further flight. Minor damage was found on the rudder skin and trim tab attachment structure. The aircraft manufacturer assessed this damage before the aircraft flew to the operator's maintenance facility at Aldergrove, where it was subsequently repaired according to a manufacturer's bespoke repair scheme.
Other Information
The aircraft manufacturer stated they did not believe that loss of the bolt could cause a catastrophic failure, and they were not aware of any previous similar occurrences. The operator reported that the bolt was last installed in May 2023 and an independent check confirmed the split pin was fitted at that time.
Analysis
Two scenarios were considered to explain the missing bolt: the bolt failed in flight and the broken parts were lost; or the split pin was not fitted, failed, or fell out, allowing the nut to work loose. The operator reported the eye-end bearing was free to rotate, so it was unclear where sufficient loads could originate to cause the bolt to fail in flight. The operator said the bolt was last disturbed approximately 54 flying hours before the incident and was subject to an independent visual check with no anomalies.
Conclusion
The aircraft and rudder pedals started shaking violently. The pilot completed a successful landing, and the bolt normally joining the trim tab to its operating rod was missing. The missing components were not recovered, so the investigation could not provide a definitive explanation for the root cause.