What happened
On 22 June 2019, a Snowbird Mk IV aircraft, registration G-MVOJ, was undergoing a ground run and taxi test following recent maintenance and fabric replacement. During the taxi, the pilot noted a rattling noise and increased speed down the runway to identify the source. At approximately 45 kt, the aircraft unexpectedly became airborne.
Upon lift-off, the pilot heard a "ping" and the aircraft immediately entered a continuous right-hand turn. Despite the application of full left roll control, the aircraft could not be leveled. The flight path followed a 270-degree arc to the west, during which the aircraft struck a poplar tree. The subsequent descent led to the aircraft striking the ground and a dry-stone wall, eventually "pole-vaulting" over the boundary. The pilot sustained one serious injury.
The investigation
The AAIB examined the wreckage and the aircraft's maintenance history. The investigation focused on the aircraft's control systems and the structural integrity of the wings. Investigators found that the left wing flying wires had been intentionally twisted by the pilot to increase tension. This resulted in visible kinks and loops in the cables.
While the engine and primary flight controls were functional, the investigation looked into the aerodynamic cause of the asymmetric lift. The examination of the spoiler bungee tension showed that while the right bungee required significantly more force to extend the spoiler than the left, this was not considered the primary cause of the roll. The investigation also noted that the pilot had recently performed the fabric replacement and had manually adjusted the flying wires to maintain tautness.
Findings
- The aircraft entered an uncontrollable right turn because the lift produced by the left wing exceeded the capability of the right spoiler to counteract it.
- The increased lift was likely caused by a slight change in the wing's shape due to overly tight flying wires on the left wing.
- The pilot's method of tightening the wires by introducing twists created a wing wash-in effect, increasing the angle of attack on the outer section of the left wing.
- The unintentional takeoff occurred at a speed that, while appearing low to the pilot, was within the normal range for the aircraft's weight and configuration.