On December 8, 2023, about 1232 central standard time, a Cessna 421C airplane, N146TJ, sustained substantial damage when it was involved in an accident near McGregor, Texas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. The purpose of the flight was to reposition the airplane from Waco, Texas (ACT), to Boerne, Texas (5C1). The airplane had just undergone an inspection and the new owners were taking possession of the airplane. The pilot reviewed the logbooks, conducted a preflight inspection, and then filed an instrument flight rules flight plan to 5C1. The pilot, who had previous experience flying the accident airplane, reported that the takeoff and initial climb were normal. As the airplane climbed through about 11,500 ft, the right engine speed increased suddenly from about 2,200 rpm to about 4,500 rpm. The pilot reported that he thought that the right propeller governor had failed. He pulled back the right propeller condition lever, which resulted in no change, so he pulled the right engine mixture control back to near cutoff, which reduced the engine speed to about 2,000 rpm. The right engine continued to run, and the pilot turned the airplane back toward ACT, expecting that the airplane would be capable of holding altitude or climbing about 200 to 300 feet per minute (fpm). As the pilot continued to assess the situation, he identified that the airplane’s airspeed started to decay and that he had to maintain a descent rate of about 1,000 to 2,000 fpm just to hold sufficient airspeed. The pilot pushed the left engine controls to full throttle, full propeller, and full mixture. About 25 miles from ACT, the pilot declared an emergency with air traffic control and chose to divert toward the McGregor Executive Airport (PWG) airport, which was closer. The pilot stated that the right engine continued to run, but, when he tried various propeller, mixture, and throttle settings, it responded with “massive vibrations.” When the airplane was about 1,500 ft and about 4 miles from PWG, the pilot assessed that the airplane would not make it to the airport. He elected to perform a forced landing into a flat, harvested corn field instead. He landed the airplane with the landing gear retracted, and it traveled about 100 ft before coming to rest upright. The airplane sustained substantial damage to the lower fuselage; the propeller blades from both engines struck the ground. Postaccident testing of the right engine in a test cell revealed that the engine started without issue. Examination of the right propeller blades and hub assembly revealed evidence that the blade angles were on or near the low pitch stop when they struck the ground; the propeller was not feathered. No mechanical anomaly was identified with the right propeller hub that would have precluded normal operation. Examination of the right propeller governor revealed that the steel spring pin that held the pilot spool in place was broken into multiple pieces. (During normal operation, the pilot spool is a nonrotating component that is housed inside the rotating drive gear.) Metallurgical examination revealed that the spring pin was fractured in shear overload. No evidence of metal fatigue, binding, or interference from foreign material was identified to explain the source of the shear loads on the spring pin.  According to the manufacturer, if the pilot spool is not held in place, it can move down the drive gear shaft (as would happen if the spring pin were sheared), which would allow high pressure oil to be directed to the propeller dome, driving the propeller to low pitch and allowing an engine rpm runaway.