Stinson 108 accident near Newport, Pennsylvania, September 17, 2020

The NTSB's narrative final · quoted from the NTSB record

The flight instructor stated that he and the pilot fueled the airplane, departed, and flew for about 1.5 hours when, while cruising at 3,500 ft, the engine “started to sputter.” He checked the magnetos, adjusted the mixture, switched fuel tanks, and pumped the primer and throttle. When he pumped the primer, the engine would “run for about 5 seconds then quit,” and the propeller continued to windmill. Unable to restore power to the engine, he elected to execute a forced landing to a field. During the landing rollout, the airplane impacted a berm resulting in substantial damage to the fuselage. Postaccident examination of the engine revealed that although the cockpit mixture knob was set to full rich, the carburetor mixture control cable was disconnected at the carburetor and the carburetor mixture lever on the carburetor was in idle cutoff position. The hardware that would have attached these components was not located. Given this information, it is likely that hardware holding the mixture control cable to the mixture control arm loosened and departed during flight, allowing the disconnected mixture control arm to migrate to the cut-off position, resulting in a total loss of engine power. Since both the accelerator pump and the primer system bypass the carburetor and delivers some fuel directly to the engine even when the mixture control is in the “cutoff” position, this would explain why the instructor reported momentary bursts of power when he pumped the primer or throttle.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.