What happened
On 11 December 2002, a Grob G115E Tutor, registration G-BYUG, was conducting a training flight for the Royal Air Force University of Glasgow Air Squadron. While climbing through 2,800 feet, the pilot noticed a vibration and unusual noise from the engine following a full throttle application. Shortly after, a loud bang occurred, the engine began emitting smoke, and the propeller ceased rotation.
Following a mayday call, the instructor decided to perform a forced landing to avoid built-up areas and the nearby Loch Lomond. During the descent, the aircraft's glide performance was lower than anticipated, necessitating a change in the landing site to a smaller field. The aircraft touched down approximately one third into the field, but the subsequent braking attempt resulted in a skid. The aircraft struck a small ridge, passed through a hedge, and eventually came to rest in a barbed wire fence. The two crew members sustained minor injuries.
The investigation
The AAIB examined the Lycoming AEIO-360-B1F engine to determine the cause of the power loss. The investigation found that the No 4 connecting rod had detached at its big end. This detachment caused the rod to become trapped between the camshaft and crankshaft, which subsequently pulled the piston pin from the piston. The force of this movement was sufficient to punch a hole through the top of the crankcase.
Metallurgical analysis of the engine components revealed that the primary failure originated at the big end of the No 4 connecting rod. Investigators determined that either a bolt had failed or its associated nut had become detached. The remaining big end bolt failed due to a bending fatigue mechanism, and it was noted that the retaining nut on this second bolt was only engaged by a single thread.
Findings
- The engine failure was initiated by the failure of a big end bolt or the detachment of its retaining nut on the No 4 connecting rod.
- The secondary bolt failed due to high stress bending fatigue.
- The detachment of the connecting rod led to the piston pin being pulled from the piston and the creation of a hole in the crankcase.
- The engine had completed 892.4 hours of operation since being new.