What happened
On 21 May 2005, a Europa XS, registration G-RMMT, was conducting a series of test flights near Tollerton Airport to satisfy requirements for a Permit to Fly. The flight was being operated by a pilot and an observer under a specific permit for testing purposes. After completing several successful circuits and a one-hour flight, the aircraft returned to the airfield for a second circuit detail.
During the downwind leg of the second circuit, the pilot reduced power and configured the aircraft for approach. As the aircraft turned onto final, the pilot increased the throttle to adjust the descent rate, but the engine failed to respond to the power increase. Despite the pilot switching the fuel selector to reserve and activating the secondary fuel pump, the engine continued to run but provided no additional thrust. The pilot declared a MAYDAY and performed a successful forced landing in a nearby field. There were no injuries to the crew.
The investigation
AAIB investigators examined the aircraft and the engine, which is a Rotax 914 Turbo. The investigation involved a detailed teardown of the engine by the UK agent for the manufacturer. While investigators considered the possibility of carburettor icing due to the high humidity and temperature conditions, the heating effect of the turbo-charger made this unlikely.
Further examination of the dual ignition system revealed that the engine was fitted with an obsolete stator. This specific component had been the subject of a mandatory Service Bulletin issued by the manufacturer. The investigation found that the insulation on a particular cable within the stator assembly was deteriorating. This degradation, which occurs after extensive engine running at high temperatures, allows the internal conductors to short-circuit, leading to ignition failure under high power settings. The cable in G-RMMT was found to be soft and pliable, a sign of the known deterioration.