What happened
On 23 July 2023, a Fournier RF4D, registration G-AWGN, was performing a scheduled aerobatic display at Stow Maries Great War Aerodrome. During the eighth manoeuvre of the sequence, an upward vertical roll, the aircraft's engine ceased functioning at approximately 850 feet. The pilot attempted to restart the engine by diving to 135 knots, but the propeller failed to windmill effectively, turning only 9/0 degrees before stopping completely.
Following the failure, the pilot executed an abbreviated circuit and a glide approach to the runway. Although the pilot frequently practised glide approaches to the runway threshold, the lack of a windmilling engine necessitated a different strategy. The aircraft landed safely on Runway 20 with no injuries to the pilot and no damage to the airframe. However, after the aircraft came to a halt, the left wingtip smoke generator, which had been emitting orange smoke throughout the flight, developed a visible yellow flame. The airport fire service arrived promptly and extinguished the flame using a powder extinguisher.
The investigation
Investigators examined the engine to determine why the propeller had stopped and failed to restart. The inspection revealed several contributing factors to increased internal friction. While the centre bearing was found to be slightly tighter than necessary due to fretting between the two crankcase halves, the investigation also identified scoring on the pistons. This damage was attributed to concrete dust from a poorly maintained taxiway entering the cylinders via an incorrectly cleaned air filter.
Findings
- The engine failure was caused by increased internal friction resulting from a combination of a tight centre bearing and piston scoring.
- The piston scoring was caused by the ingestion of concrete dust through an improperly maintained air filter.
- The pilot's decision to aim for the runway threshold during the emergency was a result of reverting to familiar practice patterns rather than adapting to the specific requirements of an engine-out landing.
- The flame from the smoke generator was a result of the exothermic nature of the chemical reaction used to produce smoke.