Accident Details
On September 7, 2014, at 1336 central daylight time, a Cirrus SR22 airplane, registered N22WX, performed a forced landing near Langola Township, Minnesota. The commercial-rated pilot and three passengers were not injured. The airplane sustained substantial damage. The airplane was registered to N22WX LLC and operated by a private individual under 14 CFR Part 91 as a personal flight. The flight originated from Mankato Regional Airport at 1257 and was en route to Breezy Point Airport.
Engine Failure and Forced Landing
According to the pilot, during cruise flight at approximately 4,500 feet mean sea level, airspeed began to decay and engine exhaust gas temperatures decreased. Attempts to restore power were unsuccessful, and the engine stopped producing power about one minute later. The pilot performed a forced landing to a rural road. During the landing roll, the pilot maneuvered to avoid an oncoming car and collided with a utility pole guy wire. Substantial damage occurred to the left wing.
Postaccident Inspection
A postaccident inspection by an FAA inspector and representatives from Cirrus Aircraft and Continental Motors found that the throttle arm rotated freely on the engine's throttle body. The throttle body was removed, a new factory throttle body was installed, and a successful engine run was accomplished. The original throttle body was sent to the NTSB for examination.
Throttle Arm Examination
Laboratory examination of the throttle control lever showed free play relative to the shaft and nut. Breakaway tightening torque measured 21 inch-pounds, breakaway removal torque 15 inch-pounds, and average running torque 19 inch-pounds. Continental Motors' Service Bulletin CSB08-0C specifies nut tightening between 100 and 120 inch-pounds. The nut and lever were removed; the lever's knurl teeth crests exhibited flat plateaus with adhesive wear scars. The side of the lever normally secured by the nut showed circumferential marks consistent with contact with the nut flange. The throttle shaft's tapered knurl teeth also had flat plateaus with adhesive wear and debris along flanks. Both components were stainless steel, but tooth count, knurl pitch, and chamfer angle differed between lever and shaft mating surfaces. According to Continental Motors representatives, precise matching of arm and shaft knurls is not crucial; the knurls act as a friction surface when torqued to specifications. Testing with dissimilar surfaces within tolerances indicated significant force would be needed to compromise set torque. A review of the FAA's Service Difficulty Report database found no similar circumstances.
Maintenance History
Logbook entries showed the engine was a factory rebuilt IO-550-N7B, rebuilt on March 20, 2014, and installed on April 10, 2014. The pilot reported a history of engine problems at idle, leading to fuel pump replacement on August 13, 2014, at an engine total time of 62.5 hours. Two days before the accident, the engine idled higher than normal. The accident occurred at 73.1 hours total engine time. An FAA inspector interviewed mechanics at the repair station that performed throttle body installation; no discrepancies in their procedures were found.