What happened
On 18 May 2001, a Quickie Q2, registration G-BMZG, was conducting a private local flight departing from Withbybush Airfield, Haverlam, Haverfordwest. The takeoff from Runway 27 proceeded without initial incident. However, upon reaching an altitude of approximately 200 feet, the engine began to operate roughly. In response, the pilot attempted to rectify the issue by applying carburettor heat and verifying that the fuel mixture was rich and the magnetos were operational.
Finding no suitable landing sites for an immediate forced landing, the pilot decided to return to the airfield via a left turn to land on Runway 03. After notifying the control tower of these intentions, the aircraft crashed on the airfield just before reaching the runway. The impact caused the wings to detach from the fuselage, and the aircraft was damaged beyond repair. The two crew members sustained serious injuries during the accident.
The investigation
Investigators examined the Revmaster R2100D engine to determine the cause of the power loss. While initial assessments suggested carburettor icing might be responsible, a subsequent technical examination of the engine provided a different explanation. Although the engine could not be run due to the severity of the crash damage, a compression test was performed on the cylinders.
Results showed that while cylinders 1, 2, and 3 maintained relatively high compression, cylinder No. 4 failed to hold any compression. This failure was attributed to a negative tappet clearance of -0.002 inches, which prevented the exhaust valve from seating properly. Additionally, the top spark plug in the fourth cylinder was found to be loose.
Findings
- The investigation identified that the lack of compression in the fourth cylinder was caused by an exhaust valve that remained partially open.
- This mechanical defect originated from fretting between the steel cylinder barrel and the alloy cylinder head.
- The fretting was caused by improper assembly, as two of the eight attachment nuts were found to be slack and two were insufficiently tightened.
- Mechanical defects in the engine assembly likely caused the engine to run roughly and reduced the aircraft's climb performance prior to the accident.