Accident Details
On May 16, 2017, at about 2030 eastern daylight time, a privately owned Stinson 108-2, registration N9366K, was substantially damaged during a forced landing near West Creek, New Jersey. The private pilot sustained minor injuries. The personal flight was operated under 14 Code of Federal Regulations Part 91 and conducted in visual meteorological conditions with a filed visual flight rules flight plan.
The flight originated from Eagles Nest Airport (31E), West Creek, New Jersey, around 2020, with Frederick Municipal Airport (FDK), Frederick, Maryland, as its destination. Earlier that morning, the pilot departed from FDK, flew to Sanford, Maine, and was returning to FDK after multiple fuel stops. He reported no anomalies until the final leg.
After departing 31E, the pilot climbed toward the New Jersey coast. At about 2,000 ft mean sea level, the engine began to shake. He immediately turned back toward 31E, but soon the engine lost all power. Smoke filled the cockpit, and the pilot observed an orange glow under the floorboards near the firewall. He initiated an emergency descent, turned the fuel selector off, and noted that the glow stopped. Attempting to return to the airport, the airplane struck trees and terrain about 1 mile from the approach end of the runway.
Aircraft and Engine Information
According to FAA records, the airplane was manufactured in April 1947. It was equipped with a Franklin 6A4-165-B3, 165-hp engine. The most recent 100-hour inspection occurred on September 16, 2016, at a tachometer time of 1039.6 hours. Engine maintenance logs indicated valve clearances were checked when cylinders No. 3 and 6 were replaced for low compression on June 19, 2015 (tachometer 945.3 hours), and cylinders No. 2 and 5 were replaced for low compression on September 16, 2016 (tachometer 1039.6 hours). No further valve clearance checks or overhauls were recorded since the engine was rebuilt in 1970. Information from June 24, 1971 through June 19, 2015 was absent. The manufacturer recommended valve clearance checks every 200 hours and overhaul at 600–700 hours. At the accident time, the tachometer read 1125.2 hours.
Engine Examination
A Federal Aviation Administration Inspector examined the wreckage, which came to rest in a near vertical attitude. Both wings exhibited leading edge crush damage; the empennage was bent rightward. The engine remained attached. Inspection revealed a breach in the top section of the crankcase.
Detailed examination showed the No. 4 cylinder had fracture-separated from the crankcase. The piston, sections of the cylinder, and crankcase sections were missing. The crankshaft and remaining cylinders were intact, and the engine rotated smoothly when the propeller was hand-turned. The exhaust pushrod shroud and pushrod of the No. 4 cylinder were bent. The No. 4 exhaust valve stem was fractured about midway. The lower half of the stem and face were missing. The No. 4 exhaust valve guide clearance was excessive, with carbon deposits on the guide and tip area. Circular gouges on the cylinder walls matched the exhaust valve face diameter.
The remaining section of the No. 4 exhaust valve stem was sent to the NTSB Materials Laboratory. Examination revealed reverse bending fatigue crack propagation. High levels of lead (Pb) and bromine (Br) were present on the exhaust valve, consistent with combustion compounds from tetraethyl lead additives in avgas.
According to the Sky Ranch Engineering Manual, high temperatures in the exhaust valve guide can oxidize oil, forming carbon deposits that may lead to a stuck valve. The most frequent reason for elevated valve temperatures is valve leakage; a valve not seating properly cannot conduct heat into the cylinder head. Elevated valve stem temperatures can then cause a stuck valve, and high temperatures and combustion deposits increase guide clearance, allowing combustion products and heat to travel up the stem, creating lead deposits and acids.