What happened
On 16 August 2023, a BN2T Islander, registration G-BSWR, was conducting emergency services operations at approximately 1,500 ft under Visual Flight Rules (VFR). During the flight, the pilot experienced intense, violent vibrations affecting both the aircraft and the rudder pedals. The pilot noted that while yaw control became difficult, the aircraft remained controllable. The intensity of the shaking increased as the airspeed was reduced. An observer on board noted that the top of the vertical stabiliser appeared to be oscillating side-to-side by an estimated 30 to 40 cm. Due to concerns regarding the structural integrity of the aircraft, the pilot declared an emergency and diverted to a nearby private airstrip in Tandragee, County Armagh. The aircraft landed without further incident, though the vibration remained severe during the landing phase.
The investigation
Following the landing, a physical examination of the aircraft revealed that the rudder trim tab had become disconnected from its control rod. Under normal operating conditions, these components are secured with a bolt, nut, and split pin; however, none of these fastening elements were found at the scene. The investigation noted that the bearing in the control rod eye-end was free to rotate and otherwise appeared acceptable for flight. The inspection also identified minor damage to the rudder skin and the attachment structure of the trim tab. The aircraft manufacturer subsequently reviewed the damage and approved a bespoke repair scheme, which was implemented before the aircraft was moved to a maintenance facility in Aldergrove. The manufacturer stated they had no record of similar occurrences and did not believe the loss of the bolt would lead to a catastrophic failure.
Findings
- The primary cause of the vibration was the separation of the rudder trim tab from its control rod.
- The investigation could not determine a definitive cause for the loss of the hardware because the bolt, nut, and split pin were not recovered.
- Two possible scenarios were considered: either the bolt suffered a structural failure in flight, or the failure of a split pin allowed the nut to loosen and migrate.
- The aircraft operator noted that the bolt had been last disturbed in May 2023, and a subsequent independent check at that time had confirmed the presence of a split pin.