NTSB Factual Report

On May 16, 2017, about 2030 eastern daylight time, a Stinson 108-2, N9366K, was substantially damaged during a forced landing near West Creek, New Jersey. The private pilot received minor injuries. The airplane was privately owned and operated. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a visual flight rules flight plan was filed for the cross-country flight. The flight originated from Eagles Nest Airport (31E), West Creek, New Jersey, around 2020, and was destined for Frederick Municipal Airport (FDK), Frederick, Maryland.

According to the pilot, he departed earlier that morning from FDK, flew to Sanford, Maine, and was returning to FDK, with several scheduled fuel stops. Throughout the day, he landed at seven airports, and reported no anomalies with the airplane. Before he departed 31E for the final leg of the flight back to FDK, he topped the airplane off with 26 gallons of fuel. After departure, the pilot flew toward the coast of New Jersey, and about 2,000 ft mean sea level, the engine began to "shake." He immediately turned the airplane back toward 31E, and soon after the engine lost all power. Smoke filled the cockpit and the pilot noticed an "orange glow" under the floor boards near the firewall. The pilot initiated an emergency descent, turned the fuel selector to the off position, and noted that the "orange glow" stopped. The pilot attempted to return to 31E, however, the airplane struck trees and terrain about 1 mile from to the approach end of the runway.

Examination of the wreckage by a Federal Aviation Administration Inspector revealed that it came to rest in a near vertical attitude. Both wings exhibited leading edge crush damage and the empennage was bent toward the right. The engine remained attached to the airframe. Examination of the engine revealed a breach in the top section of the crankcase.

According to FAA records, the airplane was manufactured in April 1947. It was equipped with a Franklin 6A4-165-B3, a 165-hp, engine. The most recent 100-hr inspection was performed on September 16, 2016, at a Tachometer time of 1039.6. According to the engine maintenance logs, the valve clearances were checked on the engine when the Nos. 3 and 6 cylinders were replaced for low compression on June 19, 2015, at a tachometer time of 945.3 hours. In addition, the Nos. 2 and 5 cylinders were replaced for low compression on September 16, 2016, at a tachometer time of 1039.6 hours. The maintenance records did not note another valve clearance check or have a recorded overhaul since the engine was rebuilt in 1970 and no information about engine operation from June 24, 1971 through June 19, 2015. Furthermore, the manufacturer recommended valve clearance check was every 200 hours at a minimum and overhaul was at 600 to 700 hours. At the time of the accident the airplane had accumulated 1125.2 hours on the tachometer.

An examination of the engine revealed that the No. 4 cylinder was fracture separated from the crankcase. The piston, sections of the cylinder, and sections of the crankcase were missing. The crankshaft and remaining cylinders were intact, and the engine rotated smoothly when the propeller was turned by hand. The exhaust pushrod shroud and exhaust pushrod of the No. 4 cylinder were both bent. The No. 4 exhaust valve stem was fractured about halfway along its length. The lower half of the stem and face were missing. The No. 4 exhaust valve guide clearance was excessive, the guide and tip area of the valve contained carbon deposits. The cylinder walls contained circular gouges consistent with the exhaust valve face diameter.

The remaining section of the No. 4 exhaust valve stem was sent to the NTSB Materials Laboratory for examination. The examination revealed that the valve stem exhibited reverse bending fatigue crack propagation. In addition, high levels of lead (Pb) and bromine (Br) were present on the exhaust valve. These elements were consistent with constituents in combustion compounds derived from the tetraethyl lead additives added to avgas aviation fuel. More information can be found in the Materials Laboratory report in the docket for this case.

According to the Sky Ranch Engineering Manual, "high temperatures in the exhaust valve guide oxidizes oil and forms carbon deposits on the valve guide and these deposits" can result in a stuck valve condition. Furthermore, the "most frequent reason for elevated valve temperatures is valve leakage. All of the combustion gas must pass around the valve face as it goes out the exhaust port. A valve that is not contacting the seat properly cannot conduct as much heat into the cylinder head as a valve with good seating. Elevated valve stem temperatures may then" result in a stuck valve. Then, because of "high temperatures and combustion deposits on the exhaust valve stem, this area of the guide gets bigger. This increases the clearance between the guide and the stem and allows combustion products and heat to travel up the valve stem. These combustion products create lead deposits and acids which increase the corrosive environment."

NTSB Probable Cause

A total loss of engine power due to the No. 4 cylinder’s exhaust valve getting stuck and the subsequent fracturing of the valve face stem.