Accident Sequence
After departing and climbing to a cruise altitude of 6,500 feet above mean sea level, the pilot reduced power to descend below a cloud layer. Following the power reduction, the throttle became stuck. Unable to move the throttle, the pilot diverted to an alternate airport. With the engine still running, the airplane touched down on the runway, but the pilot was unable to stop. The airplane exited the departure end of the runway, traveled 75 feet, and came to rest on top of a hill.
Pilot Actions
During the landing, the pilot believed the throttle was in the idle position. After the accident, he realized it was slightly open.
Post-Accident Examination
Examination of the carburetor, which had 12 hours since overhaul, revealed that the throttle shaft could not be rotated by hand. A punch and hammer were required to remove the shaft. The shaft was galled where it rested on the bushings. Both ends of the original shaft (part number CF 13-1521) measured .3105 of an inch in diameter. A new shaft supplied by the overhaul facility measured .3104 of an inch in diameter at both ends.
Under the supervision of the NTSB Investigator, the original throttle bushings were reamed with the original router. Then the new throttle shaft was inserted, and it rotated freely. The old shaft was then inserted but was difficult to rotate. After removing the old throttle shaft, the bushings were reamed using a new router the company had bought after the accident. Again, the old shaft was inserted, and this time it rotated freely. The butterfly valve was then installed, and the throttle shaft continued to rotate without difficulty.