What happened
On 8 May 2016, a private flight involving a Savannah VG Jabiru, registration G-SAVY, was en route from North Coates Airfield to Otherton Airfield. While cruising at 2,000 ft amsl with a speed of 95 mph, the pilot heard a loud thump from the rear of the aircraft. Immediately following this sound, the control column vibrated violently in pitch, nearly being pulled from the pilot's hand.
To mitigate the instability, the pilot reduced the airspeed to 60 mph, which successfully lessened the elevator vibration. Although the aircraft remained controllable in yaw, roll, and engine power, the pilot opted for an immediate precautionary landing. The aircraft landed successfully in a field approximately 2 nm southwest of Mablethorpe, Lincolnshire, with no injuries to the pilot.
The investigation
An inspection of the aircraft revealed that the electric trim servo actuator arm, which controls the left elevator anti-balance trim tab, had fractured. This breakage allowed the tab to rotate freely, creating the intense pitch oscillations experienced during flight. The arm, constructed from a toughened thermoplastic, showed evidence of a downward bending overload rather than fatigue.
Technical examination determined that the elevator's range of deflection exceeded the aircraft's design limits. Specifically, when the elevator was moved to the full up position, the gearing could cause the anti-balance tab to move over-centre, applying bending loads to the actuator arm. This risk was increased when the pilot applied additional nose-down trim. The investigation noted that the pilot's habit of using full up elevator for aerodynamic braking after landings may have contributed to the stress on the component.
Findings
- The primary cause of the incident was the structural failure of the trim servo actuator arm due to excessive bending loads.
- The aircraft's elevator deflection limits were found to be exceeded, allowing the anti-balance tab to reach an over-centre position.
- The pilot's practice of using full up elevator for aerodynamic braking after landings likely induced the necessary loads to cause the fracture.
- This was the second instance of such a failure on this specific aircraft, though no similar failures were recorded for other UK Savannah aircraft at that time.