On July 17, 2018, about 1345 central daylight time, an Aero Commander S2R airplane, N5553X, was substantially damaged when it was involved in an accident near Vinton, Iowa. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot stated he departed about 1254 from Vinton Veterans Memorial Airpark (VTI), Vinton, Iowa, with a load of fungicide. He sprayed two fields and had emptied all the fungicide in the airplane’s hopper. He reported that the takeoff, flight to and from the fields, and flight while spraying the fields were uneventful. He then headed back to VTI for a straight-in approach for runway 9. At 1/2 mile out on short final, he slowed the airplane for landing, and when the airplane crossed the end of the runway, he reduced the throttle back to the normal landing position. After a couple of seconds, the pilot realized the airplane was not slowing down. He then reduced the throttle back more, and the engine rpms did not drop. At that point, the airplane was almost halfway down the runway, and the pilot decided to go around. The pilot stated that he advanced the throttle, the airspeed increased, and he proceeded to go around. The pilot flew the pattern back around and was almost abeam the landing threshold when he had to increase the throttle to maintain engine rpms. He stated that when he turned to final, he reduced power “just a little bit” and the engine lost “most” of its power. The pilot stated he pushed the throttle full forward and moved it back and forward again with no response from the engine. The pilot chose to conduct a forced landing in a sandy cornfield to the right of the approach end of runway 9, and the airplane sustained substantial damage to the fuselage and empennage.

Examination of the throttle control system revealed that teeth on the carburetor lever stop assembly and throttle control lever were worn out and no longer interconnected (see Figure 1); the lever stop assembly teeth were noted to be severely worn. When the throttle control lever was actuated, the worn-down teeth of the mating parts would glide over each other with no respective response from the carburetor lever stop assembly. The castellated hex retaining nut was found properly cotter pinned. Upon removal of the cotter pin, the nut could not be tightened to the next accessible castellation. The lever stop assembly roll pin was tight and had to be removed with a drift punch. Both levers fit tight on the throttle shaft. A logbook review indicated that a carburetor overhaul and installation was completed 135 hours before the accident. The associated parts were removed and sent to the National Transportation Safety Board Materials Laboratory for further examination, which revealed that the lever stop assembly had a composition consistent with aluminum alloy, while the composition of the throttle control lever was carbon steel. The hardness of the steel throttle control lever averaged 52 HRA and the hardness of the aluminum lever stop averaged 34 HRA; these data were typical for the respective part compositions and microstructures.

Figure 1: Diagram of Throttle Control Lever and Lever Stop from Accident Airplane with Inset Photo of Worn Teeth

The carburetor was removed for examination and bench testing. The carburetor passed the bench test, and no anomalies were found that would have precluded normal operation.