What happened
On 18 April 2025, a replica Percival Mew Gull, registration G-HEKL, was conducting a flight near Great Massingham Airfield, Norfolk. The flight was intended to be a one-hour session to continue the running-in process of a de Havilland Gipsy Queen I engine following a recent top-end rebuild.
After approximately 30 minutes of flight, the engine began to run unevenly, and the pilot noted a drop in the temperature of the No 3 cylinder head. While attempting to return to the home airfield at Crowland, the engine experienced an uncontained failure. A loud bang and smoke were observed, and oil sprayed across the windscreen. The force of the failure caused part of the engine cowling to detach and fold back. Due to the altered handling characteristics caused by the open cowl and the loss of visibility, the pilot was unable to reach the airfield and instead performed a forced landing in a field approximately 1 mile north-west of Great Massingham. The aircraft sustained significant damage and tipped onto its nose upon stopping, though the pilot escaped unharmed.
The investigation
Investigators examined the engine and the wreckage to determine the sequence of failure. The engine strip revealed a large hole in the right side of the crankcase, with the No 3 piston and connecting rod missing. Examination of the main bearings showed that No 5 had suffered severe overheating and a loss of its inner white metal surface. Additionally, the bearing cap bolts for bearings 3, 4, and 5 were found to be only finger-tight.
Debris from the failure, specifically a piece of the No 3 big end bearing shell, had migrated into the magneto and distributor gears, causing the distributor gear drive shaft to shear and resulting in total engine stoppage. The investigation also looked into the cylinder surfaces, noting white metal deposits and evidence of glazing on several cylinders.
Findings
- The engine failure was triggered by oil starvation, which led to the failure of two main bearings and the No 3 cylinder connecting rod end cap.
- The specific cause of the oil starvation could not be definitively established, though it is possible that high oil temperatures during the running-in process allowed low-viscosity oil to escape through loose bearing caps.
- The uncontained failure of the No 3 cylinder component caused the engine to stop by damaging the magneto and shearing the distributor drive shaft.
- The pilot's decision to return to the home airfield, while considering alternative landing sites, left limited time to manage the sudden handling changes and reduced visibility caused by the engine failure and detached cowling.