Engine Valve Failure Leads to Forced Landing of Jabiru Aircraft

Casualties unknown • Upper Dean, 10 nm north of Bedford, Bedfordshire, GB

A Jabiru UL-D aircraft was forced to land in a ploughed field near Bedford after an engine vibration led to a total power loss.

What happened

On 12 September 2007, a Jabiru UL-D, registration G-JAAB, was conducting a private flight from Rochester to Manchester Barton. Approximately two hours into the flight, the pilot experienced a sudden onset of engine vibration. While reducing power helped mitigate the vibration, the aircraft was unable to maintain its altitude. The pilot declared a MAYDAY and, after determining that a nearby grass strip was too short for a safe landing, opted to perform a forced landing in a large, ploughed field near Upper Dean, Bedfordshire.

During the approach, the engine stopped completely. Although the aircraft maintained a normal attitude during touchdown, the nosewheel dug into the soft, tilled ground. This caused the aircraft to veer right and the left wing to strike the ground, resulting in damage to the landing gear and left wing. There were no injuries to the two occupants.

The investigation

An engineering examination of the engine revealed that the No 3 cylinder exhaust valve had failed in fatigue, with the head of the valve breaking away from the stem. Metallurgical analysis of the valve and the cylinder head showed significant mechanical damage caused by the repeated impact of the detached valve head against the piston crown.

Further investigation included the examination of an engine from another aircraft, G-CBIP, which had also experienced a similar failure. Both engines had been configured with 'lean burn' jets as part of the Jabiru 'Engine Economy Tuning' service letter, intended to improve fuel consumption at cruise power. Analysis of the failed valves showed evidence of grain boundary precipitation, a microstructural change indicating that the components had been operating at temperatures exceeding the typical maximum range for the material.

Findings

  • The primary cause of the engine failure was fatigue crack growth in the No 3 cylinder exhaust valve stem.
  • The fatigue cracks likely originated from corrosion pits on the valve stem surface, which acted as stress concentrators.
  • The engine was running at excessively high temperatures, causing the valve steel to undergo microstructural changes that increased its susceptibility to corrosion.
  • The use of 'lean burn' jets, introduced to improve economy, contributed to the overheating of the exhaust valves.

Probable cause

The engine failure was caused by fatigue cracking of the No 3 cylinder exhaust valve, initiated by corrosion pits. This process was accelerated by excessive exhaust gas temperatures, likely resulting from the use of 'lean burn' carburetor jets which caused the engine to run too hot.

Frequently asked questions

What happened in the 2007-09-12 Jabiru UL-D accident near Upper Dean, 10 nm north of Bedford, Bedfordshire, GB?

A Jabiru UL-D aircraft was forced to land in a ploughed field near Bedford after an engine vibration led to a total power loss.

What aircraft was involved and where did it happen?

The accident on 2007-09-12 involved a Jabiru UL-D, registration G-JAAB, at Upper Dean, 10 nm north of Bedford, Bedfordshire, GB.

What was the probable cause of the accident?

The engine failure was caused by fatigue cracking of the No 3 cylinder exhaust valve, initiated by corrosion pits. This process was accelerated by excessive exhaust gas temperatures, likely resulting from the use of 'lean burn' carburetor jets which caused the engine to run too hot.

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