What happened
On December 8, 2023, a Cessna 421C, registration N146TJ, was engaged in a positioning flight from Waco, Texas, to Boerne, Texas. While climbing through approximately 11,500 feet, the right engine speed suddenly increased from 2,200 rpm to 4,500 rpm. The pilot attempted to use the propeller condition lever to manage the change, but it had no effect. To reduce the engine speed, the pilot moved the right engine mixture control toward the cutoff position, which brought the speed down to approximately 2,000 rpm.
As the pilot attempted to return to Waco, the aircraft's airspeed began to decay, requiring a descent rate of 1,000 to 2,000 feet per minute to maintain sufficient airspeed. The pilot declared an emergency and diverted toward McGregor Executive Airport. However, the right engine began producing massive vibrations when various control settings were attempted. Approximately 4 miles from the airport, the pilot determined the aircraft could not reach the runway and executed a forced landing into a flat, harvested corn field. The aircraft landed with the gear retracted and sustained substantial damage to the lower fuselage; the pilot was not injured.
The investigation
Post-accident testing of the right engine in a test cell showed the engine started without issue. An examination of the right propeller blades and hub assembly indicated the blades were at or near the low pitch stop upon ground impact, meaning the propeller was not feathered. No mechanical anomalies were found in the propeller hub that would have prevented normal operation.
Investigators examined the right propeller governor and found that the steel spring pin holding the pilot spool in place had broken into multiple pieces. Metallurgical analysis determined the pin had failed due to shear overload. While the failure of this pin allows high-pressure oil to drive the propeller to a low pitch—causing an RPM runaway and excessive drag—investigators found no evidence of metal fatigue, binding, or foreign material interference to explain why the pin experienced such high shear loads.