Amateur-built RV6 N74MS accident near Buckingham, Pennsylvania

Probable cause

An exhaust system leak that resulted in failure of the oil quick drain, a loss of oil lubrication, an in-flight fire, and the pilot’s subsequent loss of airplane control.

NTSB analysis

The pilot was en route to his home airport. While approaching the airport, the pilot transmitted over the common traffic advisory frequency that he had an emergency and then that he would be making a straight-in landing due to an inflight. Witnesses stated that the airplane’s engine sounded rough and was sputtering before a “pop” sound was heard. The airplane appeared to be on fire with trailing smoke. The airplane then banked right and left, and turned toward the airport while losing altitude quickly. The airplane then struck the tops of trees before it impacted the ground. Security camera video footage confirmed the witness observations that there was an inflight fire, showed that the fire appeared to be near the engine, and that blue-gray smoke consistent with an oil-fed fire was trailing from the airplane.

Postaccident examination of the engine revealed extensive damage both internally and externally, including a crack in the crankcase above the No.1 cylinder and a large hole in the crankcase above the No. 4 cylinder. The No. 4 connecting rod was separated from the connecting rod cap. The crankshaft displayed damage and heat signatures on the No. 4 connecting rod journal that were consistent with an oil starvation event, and a portion of the connecting rod bearing was melted to the crankshaft journal. The 90° fitting on the oil pressure line above the electronic ignition system on the end away from the accessory housing and the fitting at the start of the oil line were blocked. The bulk of the obstruction in the oil line appeared to consist primarily of aluminum and silicon oxides, as well as other ash products with no apparent metal chips or fragments. This evidence showed that a thermal decomposition event had occurred with the ash products collecting at the elbow and backing into the flexible oil line. The connecting rod assembly, connecting rod bearing inserts, end cap attachment bolts, and a main bearing fragment all exhibited indications of thermal distress, likely due to a loss of oil lubrication. Freestanding aluminum chips and smeared metal on the main bearing fragment indicated that the main bearings were made from aluminum, which was stripped from the metal backing. The fatigue fractures observed on the bolts and connecting rod were likely secondary to the decomposition of the connecting rod bearing insert and the thermal distress event.

The oil quick drain on the engine oil sump was found to be leaking, even though it was in the closed position. Examination of the oil quick drain revealed that it was discolored and that the O-rings were melted, indicating that the oil quick drain had been exposed to a high heat condition. Further examination of the area around the oil quick drain revealed the presence of soot on the bottom of the oil sump downstream of the No. 2 cylinder where the engine’s left-side exhaust pipe was separated from the exhaust collector. Additional examination of the exhaust system indicated that the left exhaust pipe was a slip fit design and that no exhaust pipe clamp was present. This evidence indicated that the source of the thermal distress event was most likely an exhaust system leak that impinged on the oil quick drain, melting the O-rings and resulting in the oil draining from the engine and subsequently igniting. A review of the operating temperatures associated with the engine indicated that the exhaust gas temperature was hot enough to exceed the upper limit of the temperature specifications of the oil quick drain and was also hot enough to ignite the oil. This ultimately resulted in the in-flight fire and led to the pilot’s subsequent loss of airplane control.

Text from the NTSB. Open the NTSB docket for ERA21FA253 for the full record.