Amateur-built ZENITH STOL CH750 N328SK accident near Napa, California

Probable cause

The pilot's use of fuel below the required octane rating, which caused engine detonation and damage and subsequently led to oil exhaustion and a total loss of engine power in flight. Contributing to the accident was the pilot's inadequate service/maintenance of the airplane.

NTSB analysis

The pilot/owner was conducting a flight in the experimental amateur-built airplane with a pilot-rated passenger, who was a potential buyer. The pilot reported that they flew in the local area for about 20 minutes, and then smoke entered the cockpit. The pilot declared an emergency and headed toward the departure airport. Within 1-2 minutes, the engine lost all oil pressure and then lost total power.

The pilot initially attempted to glide to the airport but, because it was too far away, decided to land on a road atop a nearby levee. The airplane touched down normally, but due to wind, the left wingtip struck the ground; the airplane spun, veered off the levee, and sustained substantial damage to the wing and fuselage.

The engine exhibited some impact-related damage but did not exhibit any external evidence of nonimpact-related damage. The engine was devoid of lubricating oil, and significant oil residue was observed on the muffler and the lower fuselage. Postaccident examination indicated that the source of the oil was via the seal for the engine case drive penetration of the starter ring gear, located just above the muffler. The oil exited via this seal and produced smoke when it contacted the hot muffler.

The engine control unit (ECU) was removed and shipped to the engine manufacturer's facility, where it was installed on a testbed engine. That engine operated normally with the accident ECU installed.

The engine was disassembled and examined in detail. All four pistons displayed evidence of significant erosion damage consistent with detonation, with at least one piston eroded to the point that it could no longer seal against the cylinder wall. A common cause of detonation is using a lower octane fuel than specified by the manufacturer. The damage was consistent with the use of fuel with octane rating(s) significantly below that required for the engine. Although the pilot did not specify the octane rating of the fuel that he used, it is likely that he used fuel below the required octane rating.

At least one piston was eroded to the point that it allowed cylinder combustion gasses to enter and pressurize the engine crankcase. This crankcase pressurization forced the engine oil past the starter ring gear drive seal and out of the engine, which resulted in the exhaustion of the engine oil and a subsequent complete loss of engine power. The pilot-provided maintenance records did not include any entries that documented maintenance activities, including oil changes. Had the pilot conducted regular inspections and maintenance, it is possible that the detonation-caused internal engine damage would have been detected before the accident flight.

Text from the NTSB. Open the NTSB docket for WPR18TA223 for the full record.