What happened
On 20 November 1998, a private flight involving a MW6 (Modified), registration G-MYCU, experienced significant engine issues shortly after departing from Radlett Airstrip in Hertfordshire. Approximately five minutes into the flight, the pilot noted an increase in engine speed, which subsequently stabilised. In response to this fluctuation, the pilot decided to return to Radlett for a landing.
During the subsequent approach, the engine speed dropped to approximately 5,000 RPM. Shortly thereafter, the engine seized. The pilot was forced to land the aircraft in a field located short of the runway. The field was occupied by sheep at the time of the incident; during the landing roll, the aircraft struck and killed one sheep. The impact caused minor damage to the aircraft's lift strut and right-hand landing gear, but the pilot sustained no injuries.
The investigation
Following the incident, an investigation was conducted using the pilot's accident report and AAIB enquiries. Investigators examined the engine to determine the cause of the power loss and eventual seizure. The inspection focused on the connection between the carburettor and the inlet manifold.
Findings
Technical examination revealed that the carburettor was not securely fastened. On this specific engine model, the carburettor is attached to the inlet manifold via a short rubber hose. The investigation established that the loose connection allowed air to enter the manifold, which resulted in a weakened fuel mixture. This lean-running condition led to increased cylinder temperatures, evidenced by metallic 'pickup' and the eventual seizure of the piston within the rear cylinder bore.