Accident Overview
On January 4, 2017, at approximately 1430 Pacific standard time, an experimental amateur-built Schosanski Cozy MK IV R airplane, registration N4518S, collided with terrain following a partial loss of engine power at Napa County Airport (APC) in Napa, California. The commercial pilot, who was also the airplane's builder, was not injured. The airplane sustained substantial damage to the left wing. The flight was operated under Title 14 Code of Federal Regulations Part 91 as a personal flight, in visual meteorological conditions with no flight plan filed. The local flight had departed Napa about 1400 PST.
Pilot and Aircraft Information
The 72-year-old pilot held a commercial pilot certificate with single-engine land and instrument ratings. His most recent third-class medical certificate was issued on January 3, 2017, with the limitation "must have available glasses for near vision." He reported 814 total flight hours, with 331 hours in the accident aircraft make and model.
The airplane was manufactured in 1991 and powered by an automotive Mazda 13B rotary engine modified by the owner. The pilot had removed the turbo system, derating the engine to 150 hp. The most recent annual inspection was completed on July 29, 2016, at 205 flight hours by the pilot, who held a repairman's certificate for this airplane. At the time of the accident, the engine had 223 total hours and 17 hours since the last inspection. An industry expert noted that approximately 400 Cozy MK IV airplanes are in service, but only about 6 are powered by Mazda 13B rotary engines.
Accident Sequence
The pilot reported practicing touch-and-go maneuvers on runway 24. After multiple traffic pattern circuits, he noticed he would land farther down the runway than desired and initiated a go-around. As he advanced the throttle, the engine initially responded normally but then rolled back to an unknown lower power setting. The pilot decided to land on the remaining runway, but the airplane overran the departure end and came to rest in a marsh area. In a postaccident interview, the pilot stated the engine may have experienced a partial power loss rather than a total loss.
Engine and Fuel System Examination
The airplane had two inboard fuel tanks, each holding 26 gallons, with a high-pressure fuel pump per tank. Fuel flowed from the selected tank through a line to the fuel injectors. Right and left fuel lines joined at a fuel filter, then ran to primary and secondary fuel rails, which returned fuel to the gascolator and selected tank. Overfilled tanks would expel excess through vent lines. The engine used fuel injectors with pulse-activated electronic valves, programmable to adjust fuel delivery. The ignition controller managed fuel injector settings based on rpm and three butterfly valves.
A postaccident examination was conducted by the pilot under FAA oversight. Both high-pressure fuel pumps functioned normally. The fuel filter was clean. Fuel injectors functioned normally when tested via the engine control unit (ECU) in diagnostic mode simulating 3,000 rpm. Spark plugs were partially corroded from saltwater exposure. Coil/igniter and ignition harness resistance tests, per the Haynes Mazda Automotive Repair manual, were within specifications. The ECU ran in diagnostic mode with spark plugs removed, and each coil fired, confirming continuity from ECU to spark plugs through igniters.
The ECU was designed to receive input from the crank angle sensor, manifold pressure, and air temperature sensor, then send impulses to coils and signals to fuel injectors. A timing test to verify pulse intervals could not be performed because the ECU was built by an individual no longer providing that service. The pilot verified the crank angle sensor was properly aligned and in normal condition.
Two apex seals per rotor corner were installed to seal rotor edges. According to a Mazda rotary engine conversion guide, this engine is susceptible to premature apex seal failures, avoidable with sufficient lubrication. The engine lubricated apex seals with a two-stroke sump delivering oil directly to the seals; it functioned normally when tested. The top seal from one rotor side was not recovered after the accident. The pilot presumed that a damaged or missing apex seal caused the power loss, as its absence would significantly reduce engine power.