Accident Details
On May 3, 2023, at approximately 0745 mountain daylight time, a Cessna P337H, registration N62PC, sustained substantial damage during a forced landing near Laurel, Montana. The pilot, who was the sole occupant, received minor injuries. The aircraft was being operated under Title 14 Code of Federal Regulations Part 91 as a personal cross-country flight.
Pilot's Account
The pilot reported that it was his first flight in the airplane since purchasing it the previous year. He had planned a cross-country trip from Bozeman Yellowstone International Airport (BZN) to Beatrice Municipal Airport (BIE) in Nebraska. Departing BZN around 0615, he climbed normally to a cruise altitude of 11,500 feet mean sea level (msl) and leaned both engines' fuel mixtures to about 14 gallons per hour (gph). During the flight, he maneuvered at various altitudes to avoid clouds, then returned to 11,500 feet msl. The pilot noticed that the cruise-climb power performance was lower than expected. Within five minutes, the rear engine's oil pressure gauge began decreasing and its oil temperature increased. Opening the cowl flaps did not alter the readings.
As oil pressure continued to drop and oil temperature rose, the pilot decided to shut down the rear engine and make a precautionary landing at Laurel Municipal Airport (6S8). He was about 40 miles from 6S8 at 8,500 feet msl when he shut down the rear engine and feathered its propeller. He increased power to the front engine to about 31-32 inches of manifold pressure and 2,500 rpm, with fuel flow raised to about 18 gph, but the airplane could not maintain altitude. Approximately one-quarter mile from 6S8, at an altitude of 5-10 feet above ground level, the airplane stalled and impacted an open field. During the accident sequence, the left wing struck a steel fence post, and the airplane spun about 180 degrees before becoming wedged in an irrigation ditch.
Postaccident Examination
At the accident site, the airplane came to rest upright with the fuselage wedged perpendicular to an irrigation ditch. Substantial damage was noted to the airframe, both wings, and both tailbooms. Oil coated the left side of the airframe from the front engine to the rear. The rear engine remained attached, with its propeller assembly intact. The forward engine was also attached, but its propeller blades and hub had separated and were found adjacent to the left side of the engine.
Examination of the airframe revealed that an exhaust gas temperature (EGT) gauge was not installed on the instrument panel, although the probes and wiring harness were present. Both engines were examined. The rear engine had a hole in its oil sump. The crankshaft could not be rotated due to impact damage, and engine-to-magneto timing could not be established. Spark plugs were removed and cylinders were borescoped, revealing damage to the internal cylinder barrels and eroded piston faces consistent with detonation.
Visual examination of the forward engine showed no obvious holes in the engine case. Its spark plugs exhibited grey-colored signatures, and borescope inspection revealed detonation signatures similar to those in the rear engine. The forward engine could be manually rotated, but engine-to-magneto timing was not established.
Engine Findings
The FAA’s Pilot’s Book of Aeronautical Knowledge notes that an overly lean mixture can cause detonation, leading to rough engine operation, overheating, or loss of power. Detonation is described as an uncontrolled explosive ignition of the fuel-air mixture, causing excessive temperatures and pressures that can quickly lead to component failure. It is characterized by high cylinder head temperatures and is most likely during high power settings. Common operational causes include using a lower fuel grade, high manifold pressure with low rpm, or high power settings with an excessively lean mixture.