WPR19LA065

The pilot was practicing takeoffs and landings when the engine became unresponsive to throttle inputs. He performed a forced landing onto a highway, during which the airplane was substantially damaged. Examination of the engine revealed that the carburetor throttle lever lock nut was finger-tight, and its cotter pin was absent, which likely resulted in the throttle lever intermittently disengaging from the serrated throttle shaft when the pilot manipulated the throttle. A review of the maintenance log books and an interview with the mechanic revealed that the throttle cable had been removed and subsequently reinstalled onto the carburetor as part of the most recent annual inspection, which was completed about 1 month before the accident. It is likely that, at this time, the mechanic did not install or inadequately installed the cotter pin through the locking nut and bolt.

On January 15, 2019, about 1050 mountain standard time, a Piper PA28-140 airplane, N1697J, was substantially damaged when it was involved in an accident near Salt Lake City, Utah. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he was practicing touch-and-go landings. While on landing approach, he reduced the throttle control to decrease engine rpm; however, the engine remained at 2,200 rpm. He then pushed the throttle back in and the power increased to 2,500 rpm. He attempted to reduce throttle again, but the engine remained at 2,500 rpm. The pilot initiated a go-around, and, shortly thereafter, engine power decreased from 2,500 to 1,500 rpm. With no usable runway remaining, the pilot performed a forced landing to a road, during which the propeller struck the road and the landing gear sheared off the airplane. The service manual for the Marvel-Schebler Model MA4 series carburetor lists a castellated nut and cotter pin to secure the serrated throttle arm to the throttle shaft of the carburetor (see figure 1). Examination of the engine revealed that the carburetor serrated throttle lever locking nut was finger-tight and the cotter pin was absent. When the throttle lever was retracted and extended, the loosely attached throttle shaft did not move, as shown in figures 2 and 3. Figure 1. Excerpt from the Marvel-Schebler Carburetor Service Manual. A review of the engine maintenance records revealed that, on December 14, 2018, as part of the annual inspection, the mechanic wrote, "Verified control movement and lubricated." In an interview with a Federal Aviation Administration (FAA) representative, the mechanic confirmed that he took the throttle cable off to do this work. Postaccident examination of the carburetor at Precision Air Power, Woods Cross, Utah revealed no anomalies that would have precluded normal operation. Figure 2. Note the serrated throttle lever and idle stop at the idle position. Figure 3. Photo showing the serrated throttle lever at full power setting while the idle stop lever remained at idle.