Cessna 421C accident near Mcgregor, Texas, December 8, 2023

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

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, and the pilot 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 initiated a turn back toward the departure airport; however, the airplane could not maintain altitude as he expected. The pilot declared an emergency and, after determining that the airplane would not make it to the airport or a closer one, he performed a forced, gear-up landing into a flat, harvested corn field. The airplane came to rest upright, sustaining substantial damage to the lower fuselage. After the accident, the right engine was placed in a test cell; 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 at ground impact; 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 had fractured in shear overload into multiple pieces. Although mechanical interference between the pilot spool and one (or more) of the parts that rotate about it could impart shear loads on the spring pin, the investigation identified no evidence of metal fatigue, binding, or interference from foreign material. Thus, the reason for the failure of the spring pin was not determined. During the accident flight, when the spring pin failed, it precluded the pilot from controlling the propeller pitch to maintain engine rpm. It also allowed the propeller to move to low pitch, which resulted in excessive drag that further degraded the performance characteristics of the airplane.

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.