What happened
On 18 May 2001, a Brooker KH QAC Quickie Q2, registration G-BMZG, was involved in an accident at Withbybush Airfield, Haverfordwest. During the flight, the pilot reported a loss of engine power.
The investigation
Initial evidence suggested that carburetor icing was the likely reason for the power loss reported by the pilot. However, subsequent examination of the Revmaster engine provided a different technical explanation for the reduction in power.
Because the engine sustained significant damage during the accident, it could not be run for testing. Nevertheless, a compression check was performed on the cylinders. While cylinders 1, 2, and 3 showed varying levels of compression, cylinder number 4 failed to hold any compression at all. This failure was attributed to a small negative tappet clearance of -0.002 inches, which prevented the exhaust valve from seating firmly.
Further inspection of the engine revealed that the top spark plug in the fourth cylinder was loose in its seat. The investigation also identified that the negative tappet clearance likely resulted from fretting between the steel cylinder barrel and the alloy cylinder head. This fretting was caused by improper assembly, as two of the eight attachment nuts on the cylinder head for numbers 3 and 4 were found to be slack or insufficiently tightened.
Findings
- The engine suffered from improperly tightened attachment studs on the cylinder head.
- This loose assembly led to fretting between the cylinder barrel and the head.
- The resulting negative tappet clearance in cylinder number 4 caused the exhaust valve to remain partially open.
- These mechanical defects likely caused rough engine operation and a reduced rate of climb during the takeoff phase.