NTSB Factual Report

On September 7, 2014, at 1336 central daylight time, a Cirrus SR22 airplane, N22WX, conducted a forced landing near Langola Township, Minnesota. The commercial-rated pilot and three passengers were not injured. The airplane was substantially damaged. The airplane was registered to N22WX LLC and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The flight originated from the Mankato Regional Airport, Mankato, Minnesota, at 1257 and was en route to the Breezy Point Airport, Breezy Point, Minnesota.

According to the pilot, during cruise flight, about 4,500 feet mean sea level, the airplane's airspeed began to decay and the engine's exhaust gas temperatures began to decrease. The pilot attempted to restore power to the engine without success. About one minute later the engine stopped producing power. The pilot performed a forced landing to a rural road. During the landing roll the pilot maneuvered the airplane to miss an oncoming car and collided with a utility pole guy wire. Substantial damage was sustained to the airplane's left wing.

A postaccident inspection of the airplane by an inspector from the Federal Aviation Administration (FAA) and representatives from Cirrus Aircraft and Continental Motors discovered that the throttle arm rotated freely on the engine's throttle body. The throttle body was removed from the airplane and a new factory throttle body was installed on the engine and a successful engine run was accomplished. The original throttle body was shipped to the National Transportation Safety Board laboratories for an examination.

The throttle control lever exhibited free play relative to the shaft and nut when wiggled by hand. Using a manual torque wrench, the breakaway tightening torque was 21 inch-pounds force (in-lbf), the breakaway removal torque was 15 in-lbf, and the average running torque was 19 in-lbf. According to the Continental Motors' Critical Service Bulletin CSB08-0C, the nut is to be tightened between 100 in-lbf to 120 in-lbf. The nut and the throttle control lever were removed from the assembly. The crests of the throttle control lever's knurl teeth exhibited flat plateaus with adhesive wear scars on the surface of the plateaus. The side of the control lever that the nut is normally secured against displayed circumferential marks consistent with contact with the periphery of the nut flange on its surface. The crests of the throttle body valve shaft tapered knurl teeth exhibited flat plateaus with adhesive wear scars on the surface of the plateaus and accumulated wear debris along the teeth flanks. Both the throttle control arm and throttle shaft were constructed of stainless steel. The number of teeth, knurl pitch, and chamfer angle differed between the level and the shaft mating surfaces.

According to representatives from Continental Motors, precise matching of the arm and shaft knurls is not crucial to proper operation. Instead, the knurls act as a friction surface and when the nut is torqued to factory specifications. For testing purposes, the company used mating surfaces that were as dissimilar as possible but still within tolerances, and found it would take significant force on the arm to compromise the set torque. A review of the FAA's Service Difficulty Report database did not find any similar circumstances to the accident airplane.

A review of logbook entries found that the engine was a factory rebuilt IO-550-N7B. The rebuild was completed on March 20, 2014, and was installed on N22WX on April 10, 2014. The pilot reported that the engine had a history of problems when the engine was at idle. As a result, the engine's fuel pump was replaced on August 13, 2014, at an engine total time of 62.5 hours. The engine performed normally at idle, but two days prior to the accident it idled higher than normal. The accident occurred at an engine total time of 73.1 hours.

An inspector from the FAA interviewed mechanics at the repair station that performed the installation of the throttle body. They were also knowledgeable of the manufacturer's instructions and no discrepancies in their procedures were found.

NTSB Probable Cause

Maintenance personnel’s failure to apply sufficient torque on the throttle control lever nut, which resulted in a loss of throttle control and subsequent loss of engine power.