What happened
On 10 June 2005, an Avid Speedwing Mk4, registration G-BUFV, was engaged in a private flight near Toomebridge. Following an initial successful 45-minute flight, the pilot elected to perform a second flight without shutting down the engine. Shortly after departing, at an altitude of approximately 300 ft, the pilot heard a metallic ringing sound and noticed a change in the engine note accompanied by a loss of power. Although the engine continued to respond to throttle inputs, it could not maintain altitude.
After attempting to reset the ignition and fuel pumps, the pilot identified a potential landing site. However, the presence of overhead electricity cables and trees made a land-based approach unfeasible. The pilot instead opted to ditch the aircraft in Lough Neable. The aircraft struck the water roughly 150-200 meters from the shore, over a sandbank where the water depth was approximately 4 ft. Both the pilot and the passenger were uninjured and were able to exit the aircraft through the perspex roof before being rescued by emergency services.
The investigation
An examination of the BMW R100 engine was conducted by the AAIB. During the strip-down of the engine, investigators discovered that the cylinder head nuts on the left cylinder were tightened to a level below the manufacturer's required torque specification of 26-29 lbf ft. This lack of proper tension resulted in a hole being found in both the head gasket and the outer cylinder casing, where hot gases had burned through the components.
Findings
- The engine power loss was caused by insufficient torque applied to the cylinder head nuts.
- The improper tightening allowed hot gases to escape, damaging the gasket and the cylinder casing.
- The pilot's decision to ditch in the lake was necessitated by the risk of colliding with trees and power lines during a land-based forced landing.