What happened
On 27 February 2005, a Silence Twister, registration G-TWST, was conducting a local test flight near Wycombe Air Park. During the flight, the engine began running roughly and power decreased. The pilot attempted several corrective actions, including applying carburettor heat and switching fuel tanks, but the engine performance did not improve. The pilot decided to perform a precautionary landing at Aylesbury (Thame) Airfield.
After landing on a recently ploughed grass runway, the pilot conducted engine checks. While the engine initially ran normally during ground checks, the pilot subsequently attempted a short field takeoff. During this climb, the engine power failed again. Due to insufficient altitude to turn back into the wind, the pilot performed a circuit and landed downwind. Because the landing gear was not retracted, the aircraft nosed over upon impact. The impact caused the landing gear to collapse and the engine to detach from the fuselage. The pilot sustained serious injuries, specifically fractured vertebrae.
The investigation
The investigation focused on the cause of the engine power loss. The pilot, who was also the aircraft's builder, examined the fuel system in his workshop. While the carburettor showed no faults, the investigation uncovered resin particles within the fuel tanks, filters, and the inlet side of the electric fuel pump.
Further examination of the fuel tank construction revealed that the manufacturing process for the glass reinforced plastic (GRP) tanks was inadequate. Specifically, a wax releasing agent used during production had not been sufficiently removed. This allowed resin used to bond the internal ribs to adhere to the wax and subsequently enter the fuel lines.
Findings
- The engine power loss was caused by a fuel flow restriction resulting from resin particles within the fuel system.
- The presence of resin in the fuel lines was due to an inadequate cleaning process of the wax releasing agent during the fuel tank manufacturing process.
- The engine may have functioned during the initial ground run because the fuel demand was low, but the restriction became critical during the high-demand takeoff phase.
- The risk of such contamination was also increased by the recent switching of fuel tanks just prior to the takeoff attempt.