N6642MStinson 108-3 · 1948

On July 9, 2019, about 1320 Pacific daylight time, a Stinson 108-3 airplane, N6642M. was substantially damaged when it was involved in an accident near Klamath Falls, Oregon. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that he and a passenger were relocating the airplane to Alaska. The airplane departed from Red Bluff Municipal Airport (RBL), Red Bluff, California, and landed at Crater Lake-Klamath Falls Airport (LMT), Klamath Falls, Oregon, for a refueling stop. The accident occurred after the takeoff from LMT. The pilot stated that the takeoff was normal but that the engine lost partial power when the airplane was at an altitude of above 1,000 ft above ground level. The pilot initiated a forced landing to a nearby field, where the left wing contacted the ground.

The pilot stated that he had experienced previous problems with the carburetor. He stated that, after the carburetor underwent an overhaul, the engine ran better but that it would occasionally not get up to full engine speed, so the airplane would not have top end power. To work around the issue, the pilot used a procedure of slowly advancing the throttle to the first three-quarters of travel and then applying the remaining throttle rapidly to attain full power. The pilot stated that this issue did not occur during the first leg of the trip.

The mechanic who worked on the airplane before the accident flight stated that the owner, who was not the accident pilot, reported the maintenance problem. The mechanic also stated that he checked the timing, valve lash, induction system, and fuel line to the airframe fuel filter and found no problems, so he determined that the carburetor was the issue. The mechanic temporarily replaced the carburetor with another one and detected a noticeable increase in engine power, which confirmed that the maintenance problem was due to the original carburetor. The carburetor was then provided to the facility that had previously performed an overhaul, and repairs were made. Afterward, the carburetor was reinstalled on the airplane.

Postaccident 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 found to be within acceptable limits. Valve clearance on the left-side cylinders (Nos. 1, 3, and 5) were found to be greater than 0.20 inch. According to the manufacturer’s engine service manual, the valve clearance is 0.04 inch.

A compression test was conducted on each cylinder. Each cylinder held 80 pounds per square inch (psi). Cylinder No. 1 held 62 psi and leaked air past the exhaust valve. Cylinder No. 2 held 48 psi and leaked air past the piston rings and from the exhaust valve. Cylinder No. 3 held 73 psi and leaked air past the piston rings. Cylinder No. 4 held 56 psi and leaked air past the piston rings. Cylinder No. 5 held 58 psi and leaked air out of the intake valve. Cylinder No. 6 held 33 psi and leaked air out of the intake valve and past the piston rings. No other anomalies were noted that would have precluded normal operation.