Accident Overview
On July 9, 2019, about 1320 Pacific daylight time, a Stinson 108-3 airplane, registration N6642M, was substantially damaged during a forced landing near Klamath Falls, Oregon. The pilot and the sole passenger were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Flight History
The pilot reported that he and a passenger were relocating the airplane to Alaska. The trip originated from Red Bluff Municipal Airport (RBL), Red Bluff, California, with a refueling stop at Crater Lake-Klamath Falls Airport (LMT), Klamath Falls, Oregon. The accident occurred after departure from LMT. According to the pilot, the takeoff was normal, but when the airplane reached an altitude of more than 1,000 ft above ground level, the engine lost partial power. The pilot initiated a forced landing to a nearby field, during which the left wing contacted the ground.
Pilot's Account of Previous Issues
The pilot stated that he had experienced prior problems with the carburetor. After an overhaul, the engine performed better but occasionally failed to reach full engine speed, resulting in a lack of top-end power. To manage this, the pilot adopted a specific throttle procedure: slowly advancing the throttle to the first three-quarters of travel, then rapidly applying the remaining throttle to achieve full power. He noted that this issue did not manifest during the first leg of the journey.
Mechanic's Actions
A mechanic who had worked on the airplane before the accident flight indicated that the owner (not the accident pilot) reported the maintenance problem. The mechanic inspected the timing, valve lash, induction system, and fuel line to the airframe fuel filter, finding no anomalies. Concluding the carburetor was at fault, he temporarily replaced it with another unit, which resulted in a noticeable power increase, confirming the diagnosis. The original carburetor was sent to the facility that had performed the overhaul for repairs and subsequently reinstalled.
Postaccident Examination
Following the accident, an examination of the carburetor revealed no malfunctions that would have precluded normal operation. The engine rocker box covers were removed, and valve clearance on the right-side intake and exhaust valves (Nos. 2, 4, and 6) was within acceptable limits. However, the valve clearance on the left-side cylinders (Nos. 1, 3, and 5) exceeded 0.20 inch, whereas the manufacturer's specification is 0.04 inch.
A compression test yielded the following results: Cylinder No. 1 held 80 psi but leaked past the exhaust valve; Cylinder No. 2 held 48 psi, leaking past the piston rings and exhaust valve; Cylinder No. 3 held 73 psi, leaking past the piston rings; Cylinder No. 4 held 56 psi, leaking past the piston rings; Cylinder No. 5 held 58 psi, leaking from the intake valve; and Cylinder No. 6 held 33 psi, leaking from the intake valve and past the piston rings. No other anomalies that would have precluded normal operation were noted.