What happened
On June 15, 2021, an experimental Vans RV-6A, registration N74MS, was involved in a fatal accident in Buckingham, Pennsylvania. The flight began at Doylestown Airport (DYL), with the pilot performing several legs to Sky Manor Airport (N40) and South Jersey Regional Airport (VAY) before attempting to return to Doylestown.
At approximately 10:20, the pilot notified air traffic via the common traffic advisory frequency that an emergency existed and that a straight-in landing on runway 5 was being attempted. Shortly after, the pilot reported an in-flight fire. Witnesses observed the engine sounding rough and sputtering, followed by a loud "pop" and the appearance of trailing smoke and flames. The aircraft was seen banking and losing altitude rapidly before striking trees and impacting the ground. The pilot sustained 1 fatality.
The investigation
Investigators examined the wreckage and found the aircraft had struck a tree approximately 50 feet above ground level and a tree limb at 36 feet before coming to rest in a field. The engine sustained extensive damage, including a crack in the crankcase above the No. 1 cylinder and a large hole above the No. 4 cylinder. The No. 4 connecting rod had separated from its cap.
Technical analysis of the engine revealed:
- The oil pressure line fittings were blocked with aluminum and silicon oxides, indicating a thermal decomposition event.
- The oil suction screen contained metal debris, and the oil sump contained fragments of the No. 4 connecting rod assembly.
- The oil quick drain valve was leaking despite being closed; the valve was discolored, and its O-rings had melted.
- The left-side exhaust pipe was a slip-fit design and lacked a required exhaust pipe clamp.
- Evidence showed soot on the oil sump downstream of the No. 2 cylinder, where the exhaust pipe had separated from the collector.
Laboratory examination of the connecting rod assembly and bearing fragments showed thermal distress and fatigue fractures.
Findings
- An exhaust system leak occurred due to the absence of a clamp on the left-side exhaust pipe.
- This leak impinged on the oil quick drain, causing the O-rings to melt and allowing oil to escape the sump.
- The escaping oil ignited, resulting in an in-flight fire.
- The fire and subsequent loss of oil lubrication led to the engine failure and the pilot's loss of control.