Primary finding
Probable cause
A partial loss of engine power due to failure of the No. 1 cylinder intake valve head.
Investigator assessment
Analysis narrative
The pilot reported that while en route he noticed a loss of engine power and the oil pressure decreasing. He performed a forced landing to a road. During the landing rollout the right wingtip struck a sign, which resulted in substantial damage. Postaccident examination of the engine revealed the No. 1 cylinder was fractured from the engine and remained partially attached by exhaust tubes and the intake manifold. The intake valve stem was fractured about an inch from the interior base of the valve. The piston and intake valve were destroyed by the internal forces in the engine and the fragments were not located.
Source record
Factual narrative
On July 12, 2023, about 1624 Alaska daylight time, a Stinson 108 airplane, N97719, was substantially damaged when it was involved in an accident near Ninilchik, Alaska. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated he departed Homer Airport (HOM) destined for Anchorage, Alaska. A few miles northwest of the Ninilchik Airport (NIN) he noticed a loss of engine power and the oil pressure decreasing. The pilot determined he was too far from the NIN airport to return so he elected to perform a forced landing to a road. During the landing roll the right wingtip struck a traffic sign, which resulted in substantial damage to the right wing. Postaccident examination of the engine revealed the No. 1 cylinder was fractured from the engine and remained partially attached by exhaust tubes and the intake manifold. The intake valve head and stem was fractured about an inch from the interior base of the valve. The piston and intake valve were destroyed by the internal forces in the engine and the fragments were not located. The pilot reported the engine had a total time of 626 hours at the time of the accident. The last annual inspection was completed 12 months before the accident at an engine total time of 556.4 hours. The engine logbook showed the manufacturer’s recommended time between overhaul is 1,200 hours.