Cessna P337H accident near Laurel, Montana, May 3, 2023

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

The pilot was on a cross-country flight when he maneuvered the airplane to avoid clouds. The maneuvering included a descent followed by an ascent back to the previous altitude. He noted that the airplane’s performance at the cruise-climb power setting was lower than expected and five minutes later the oil pressure indicator for the rear engine began to decrease, with a corresponding increase in indicated oil temperature. The pilot diverted to a nearby airport and shut down the rear engine; however, with the rear engine shut down and the forward engine at full power, there was not enough power to maintain altitude. The airplane stalled about 10 ft above the ground and the left wing sustained substantial damage when it impacted a steel post. The airplane rotated 180° and sustained further substantial damage to the airframe after it came to rest in an irrigation ditch. Postaccident examination of the rear engine revealed that the Nos. 4 and 5 cylinders had signatures consistent with detonation in the form of erosion on the edges of the piston faces. A postaccident examination of the front engine revealed that it also exhibited erosion signatures of the piston faces and internal damage to the cylinders that likely resulted in a degradation of power. Also, the investigation could not determine the reason for the detonation, causes include improper ignition timing, high inlet air temperature, engine overheating, carbon build-up or oil in the combustion chamber, an issue with the fuel octane, or an overly lean fuel-to-air mixture. Although the magneto-to-engine timing could not be determined postaccident, there were no postaccident anomalies noted with either the magnetos or the ignition harnesses. Without an EGT gauge, the effect on the engine temperatures as a result of the pilot’s leaning procedures is unknown.

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.