What happened
On 25 November 2011, an Alpi Aviation Pioneer 300, registration G-VIXX, was positioned on the apron at Gloucestershire Airport. Following the completion of standard pre-flight inspections, the pilot initiated the engine start sequence. Upon ignition, the engine immediately surged to a high power setting. This sudden acceleration caused the aircraft to move across a taxiway and onto a verge, where it struck a large shipping container. The impact resulted in substantial damage to the aircraft. The pilot sustained minor injuries but was able to exit the cockpit without assistance.
The investigation
The investigation focused on the mechanics of the throttle control system. The aircraft was powered by a Rotax 912 ULS piston engine. Investigators examined the throttle assembly, which utilized a plunger-type design featuring a separate friction nut for pilot adjustment. It was noted that the butterfly valves within the two carburetors are spring-biased toward the fully open position. Because the throttle control used a 'pull only' design, the investigation looked at how the tension of the bias spring interacted with the friction nut.
Findings
- The engine's butterfly valves are naturally biased toward full power due to the design of the carburetors.
- The throttle control mechanism on G-VIXX relied on a friction nut to maintain position.
- The throttle butterfly moved to the full power position due to the action of the bias spring when the friction nut was loosened.
- The existing throttle design may not be appropriate for engines with this specific spring-biased characteristic.
Safety action
- The Light Aircraft Association (LAA) is currently reviewing design requirements for throttle control mechanisms on aircraft under its responsibility that utilize this engine type.