What happened
On 13 June 2002, a Jabiru UL-45 0, registration G-TUBB, was conducting a private flight from Kemble toward Merthyr Tydfil. Approximately one hour into the flight, while cruising at 2,800 feet, the pilot noted the engine began running roughly. Despite applying carburettor heat, the engine subsequently stopped. An attempt to restart the engine using the starter motor failed, as the engine and propeller were completely seized.
The pilot performed a forced landing on a plateau near Brynmawr, South Wales. The terrain featured steep slopes on three sides of the landing site. During the landing, the aircraft's landing gear detached, and the right wing and flaps sustained damage. The pilot, wearing a proper harness, escaped the aircraft with no injuries.
The investigation
The AAIB examined the Jabiru 2200A engine, which had completed 281.05 hours since manufacture. The investigation revealed that the number three cylinder had suffered a catastrophic failure. The piston was destroyed and the connecting rod was bent. The head of the number three exhaust valve had detached and was found in the oil sump, while the remaining valve stem was stuck in its guide.
Evidence of significant overheating was found on the cylinder head and fins. The investigation also identified that the right-hand ram air duct had been modified to divert cooling air to a cockpit heating system via an orifice and hose. Furthermore, the investigation noted that the cylinder head bolts had interfered with the bolt holes due to thermal distortion, which likely compromised the gas seal between the head and the cylinder.
Findings
- The engine failure was caused by the number three exhaust valve becoming stuck in its guide due to excessive carbon buildup.
- This valve failure led to the piston striking the valve, resulting in the destruction of the piston and the eventual seizure of the engine.
- The modification of the ram air duct, which diverted cooling air for cockpit heating, exacerbated the overheating of the number three cylinder.
- Overheating caused distortion of the cylinder head, which compromised the integrity of the gas seal.
- There was ambiguity regarding the required frequency of compression checks in the engine manuals, and the lack of such checks may have prevented the early detection of the overheating and valve issues.