Aircraft and Flight Details
The aircraft involved was a Jabiru SK, registration G-JABA, powered by one Jabiru 2200A piston engine. It was built in 2000. The test flight was conducted on 21 June 2003 at 1353 UTC, departing from Popham Airfield in Hampshire. On board were the pilot (also the owner/handling pilot) and a passenger, both holding Private Pilot's Licences. The pilot had 440 hours of flying experience, with 98 hours on type. The aircraft was refueled to a weight of 430 kg.
Accident Sequence
After a satisfactory engine power check, the aircraft took off from Runway 08. Due to obstacles, an early left turn was required to follow the alignment of Runway 03. At about 300 feet, a vibration was noted, worsening over 20 to 30 seconds until the pilot believed one cylinder was not firing. Insufficient power remained to maintain height or land safely on Runway 03, so the pilot elected to land straight ahead in a field.
Forced Landing and Damage
The field contained unexpectedly tall crops. As the wheels sank into the crops, the aircraft pitched down, the nose landing gear tore off, and the aircraft continued for about 20 yards before coming to rest inverted. Both wings were damaged, along with the roots of the horizontal stabiliser. The engine cowlings, propeller, and spinner were severely damaged, as were the engine mount and firewall. The starboard wing obstructed the passenger door, but both occupants exited through the pilot's door. Both sustained minor injuries.
Engine Examination
AAIB examination of the engine found no external damage, though one propeller blade was splintered. The propeller turned freely. Spark plugs appeared similar. Boroscope inspection showed no cylinder damage, seizure, or scoring. However, upon removing rocker covers and rotating the propeller, no movement of the inlet valve in the forward left-hand cylinder occurred. The valve clearance adjuster on the inlet valve rocker arm had become slack and migrated, causing a maximum clearance of approximately 0.3 inches. The adjuster consists of a threaded rod with a hemispherical recess, secured by a lock nut. The lock nut was present but slack and free to turn, failing to perform its function. The UK agents reported no known issues with this adjuster design. The previous design had been discontinued.
Probable Cause
The investigation considered that the adjuster became slack due to insufficient torque being applied to the lock nut during the last adjustment. The owner/pilot reported that all cylinders had been removed and reinstalled during the pre-flight check, requiring valve clearance resetting. It is presumed that insufficient torque was applied or the lock nut was not tightened properly.