What happened
On 29 August 1999, a Tiger Cub 440, registration G-MMIM, experienced an engine failure while operating near Sarn, near Newtown, Powende.
The investigation
Following an initial report, a detailed examination of the engine was conducted. The engine was configured in an inverted mount, driving a two-bladed tractor propeller, and featured cylinder head temperature sensors located under both spark plugs.
During the teardown, investigators discovered that the rear piston had melted near the exhaust port. The top piston ring was found to be damaged, with approximately 15 mm of the ring missing. This loose fragment of the piston ring had subsequently moved into the head, causing the spark plug to become clogged.
Upon inspecting the carburettors, a tiny piece of gasket-like material, measuring roughly 1 x 5 x 0.5 mm, was recovered from the main jet holder of the rear carburettor. The investigation also noted a slight imbalance between the two carburettors, as the front unit responded 0.5 mm before the rear unit. No issues were identified regarding the fuel pump, fuel lines, or the fuel filter located between the tank and the pump.
Findings
- The engine failure was triggered by the rear spark plug becoming blocked.
- This blockage was preceded by a loss of engine power resulting from the melting of the piston.
- The overheating was caused by a lean fuel mixture, which occurred because the main jet was obstructed by debris found within the float bowl.