What happened
On 1 September 2001, a Jodel D1t aircraft, registration G-BHFF, was conducting a visual flight rules (VFR) mission from White Waltham Airfield toward a farm airstrip near Colerne. While flying approximately 4nm north-west of Marlborough, the pilot noted that the engine began running roughly. After notifying Lynham ATC, the engine vibration intensified to such an extreme degree that the pilot was forced to shut down the engine and issue a MAYDAY call.
Seeking a safe landing site, the pilot initially considered an area of open ground but found that the subsequent terrain consisted of standing crops. While attempting to maneuver toward an alternative field, the pilot determined that the necessary flight path would be unsafe and instead opted for a different nearby area. During the touchdown, the aircraft's left wing was low and the plane drifted to the right, leading to the collapse of the left landing gear. Both occupants of the aircraft escaped without injury.
The investigation
Following the incident, investigators examined the Continental Motors Corp A65-8F piston engine. The inspection revealed that the valve head of an exhaust valve had detached, which subsequently damaged the piston and the cylinder. Records indicated that approximately 72 operating hours prior to the accident, the cylinders had been removed for a 'top end' overhaul due to low compression, a process that included lapping the valves. At the time of the accident, the engine had reached 1,080 total hours. However, the specific broken valve was not available for metallurgical analysis.
Findings
- The primary cause of the engine failure was the detachment of an exhaust valve head.
- This mechanical failure caused significant internal damage to the cylinder and piston.
- The collapse of the left landing gear was a result of the aircraft's lateral drift and low wing position during the forced landing.