Background
The Air Accidents Investigation Branch (AAIB) issued an addendum to its initial bulletin concerning an accident involving a Brooker KH QAC Quickie Q2, registration G-BMZG, at Withbybush Airfield, Haverfordwest on 18 May 2001 at 1845 hrs.
Initial Findings
The original AAIB Bulletin No 2/2002 indicated that carburetor icing was the probable cause of the power loss reported by the pilot. This conclusion was based on the evidence available at the time of initial publication.
Further Examination
After the initial bulletin, the AAIB received a copy of an examination report on the Revmaster engine from G-BMZG. The engine could not be run due to damage sustained in the accident. A compression check of the cylinders showed readings of 73/80, 76/80, and 62/80 for cylinders No. 1, 2, and 3 respectively. Cylinder No. 4 would not hold any compression.
Findings on Cylinder No. 4
The lack of compression in cylinder No. 4 was attributed to a small negative tappet clearance of -0.002 inches, causing the exhaust valve to remain slightly open when it should have been firmly seated. Additionally, the top spark plug from cylinder No. 4 was found loose in its seating.
Causes of the Negative Tappet Clearance
The negative tappet clearance appeared to result from fretting between the alloy cylinder head and the steel cylinder barrel. Examination of the attachment studs on the cylinder No. 3-4 head revealed that two of the eight nuts were slack and two were not properly tight. This condition was considered the cause of the fretting.
Likely Pre-Flight Condition
It is likely that these defects had developed before the accident flight and would have resulted in rough running and a reduction in rate of climb at take-off.