History of Flight
The Jabiru UL-450, registration G-TUBB, departed Kemble for a flight to the Methyr Tydfil area and return. After approximately one hour, the engine began to run rough at 2,800 feet. The pilot selected carburettor heat, but the engine stopped shortly after. Attempts to restart using the starter motor failed, indicating a seized engine. A forced landing was conducted on a hill 2 nm south of Brynmawr. During landing, the landing gear detached, and the right wing and flaps sustained minor damage. The pilot, wearing a lap and diagonal harness, exited the aircraft uninjured.
Engine Description
The aircraft was powered by a Jabiru 2200A four-cylinder, four-stroke horizontally opposed air-cooled engine, producing 80 hp at 3,000 RPM. The propeller was directly driven by the crankshaft. Cooling was provided by two ram air ducts directing air over the cylinders and cylinder head fins. The engine had 281.05 hours of operation.
Engine Examination
The engine was examined at the AAIB Farnborough. The number three cylinder spark plug was damaged, and upon cylinder head removal, catastrophic failure was evident. The number three piston was completely destroyed, and the connecting rod was bent. The head of the number three exhaust valve was missing and later recovered from the oil sump; its stem was stuck in the valve guide. The cylinder and head showed extensive discolouration on the lower quarter, indicating several hours of 'blowing'. Discolouration consistent with overheating was also present around the rockers. The cylinder head fins were distorted, and head bolts interfered with the bolt holes, suggesting previous overheating allowed bolt interference. The remaining cylinders showed varying degrees of 'blowing'. The engine design uses a recessed fit without a gasket; correct head bolt torque is essential for gas sealing. The oil pump contained piston fragments, indicating failure occurred while the engine was running. The exhaust valve stem had carbon deposits increasing its diameter by approximately 0.0065 inch. The right-hand ram air duct had been modified with an orifice and hose feeding a cockpit heating system. The available modification to improve rear cylinder cooling had not been applied. Two previous Jabiru 2200 engine failures due to sticking exhaust valves were noted.
Discussion
The engine failure was caused by the number three exhaust valve sticking in the valve guide due to carbon build-up from overheating. The piston struck the valve, detaching its head, leading to piston destruction and connecting rod bending, ultimately seizing the engine. The overheating was exacerbated by the modified ram air duct that diverted cooling air to the cabin heater. Overheating also caused cylinder head distortion, compromising the gas seal. Routine compression checks every 50 hours might have detected the head distortion and carbon accumulation.