WPR23LA175

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.

On May 3, 2023, about 0745 mountain daylight time, a Cessna P337H, N62PC, was substantially damaged during a forced landing near Laurel, Montana. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal cross-country flight. According to the pilot, this was his first flight in the airplane since he purchased it the previous year. He had planned a cross-country flight from Bozeman Yellowstone International Airport (BZN), Bozeman, Montana, to Beatrice Municipal Airport (BIE), Beatrice, Nebraska. The pilot departed about 0615 from BZN and made a normal ascent to an en route cruise altitude of 11,500 ft mean sea level (msl) and leaned both engines’ fuel mixture to about 14 gallons per hour (gph). He maneuvered the airplane at various altitudes to avoid clouds. Once clear of the clouds, he ascended back to his original en route cruise altitude of 11,500 ft msl. The pilot noted that the performance of the cruise-climb power setting was lower than expected. Within 5 minutes, the oil pressure indicator for the rear engine began to show a decrease and the oil temperature gauge indicated an increase. The pilot opened the rear engine cowl flaps, but there was no change in oil pressure or oil temperature. As the oil pressure continued to decrease with a corresponding oil temperature increase, the pilot decided to shut down the rear engine and make a precautionary landing at Laurel Municipal Airport (6S8), Laurel, Montana. The pilot reported he was about 40 miles from 6S8 at 8,500 ft msl when he shut down the rear engine and feathered the propeller. He increased the power to the front engine to about 31 to 32 inches of manifold pressure, 2,500 rpm, and increased the fuel flow to about 18 gph; however, there was not enough power to maintain altitude. About 1/4 mile from 6S8, at an altitude of about 5 to 10 ft above ground level (agl), the airplane stalled and impacted an open field. During the accident sequence the left wing impacted a steel fence post, and the airplane spun about 180° where it became wedged into an irrigation ditch. Postaccident examination at the accident site revealed that the entire airplane came to rest upright with the fuselage wedged perpendicular to an irrigation ditch. The airframe, both wings, and both tailbooms were substantially damaged. The left side of the airframe from the front engine to the rear of the airplane was coated with oil. The rear engine remained attached to the airframe and the propeller assembly remained attached to the engine. The forward engine remained attached to the airframe; however, the propeller blades and hub separated from the engine and were located adjacent to left side of the front engine. Examination of the airframe revealed that an exhaust gas temperature (EGT) gauge was not installed on the airplane’s instrument panel, but that the probes and wiring harness were present. Examination of both engines revealed that the rear engine had ahole in the oil sump. The crankshaft was not able to be rotated due to impact damage, and the engine-to-magneto timing could not be established. The spark plugs were removed and the cylinders were borescoped, showing damage to the internal cylinder barrels as well eroded piston faces consistent with detonation. Visual examination of the forward engine showed no obvious holes in the engine case. The spark plugs were removed and exhibited grey colored signatures; the cylinders were borescoped showing similar detonation signatures to the rear engine. The forward engine was manually rotated; however, the engine-to-magneto timing was not established. The FAA’s Pilot’s Book of Aeronautical Knowledge states in part, “An overly lean mixture causes detonation, which may result in rough engine operation, overheating, and/or a loss of power.” It continues: Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness, or loss of power. Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings. Common operational causes of detonation are: - Use of a lower fuel grade than that specified by the aircraft manufacturer - Operation of the engine with extremely high manifold pressures in conjunction with low rpm - Operation of the engine at high power settings with an excessively lean mixture