On January 4, 2017, about 1430 Pacific standard time, an experimental amateur-built Schosanski, Cozy MK IV R airplane, N4518S, collided with terrain following a partial loss of engine power at Napa County Airport (APC), Napa, California. The commercial pilot was not injured. The airplane sustained substantial damage to the left wing. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan had been filed. The local flight departed Napa, California, about 1400 PST. The pilot, and builder of the airplane, reported that he was practicing touch-and-go maneuvers on runway 24 at the time of the accident. After multiple tours in the airport traffic pattern the pilot noted he would be landing farther down the runway than he desired and initiated a go-around maneuver. As he advanced the throttle the engine started to respond normally, but then rolled back to an unknown lower power setting. The pilot decided to land on the remaining runway but the airplane over ran the end of the runway and came to rest in a marsh area beyond the departure end of the runway. During a postaccident interview, the pilot stated that the engine may have experienced a partial loss of power instead of a total power loss. The pilot, age 72, held a commercial pilot certificate with ratings for single-engine land and instrument airplane. The pilot's most recent third-class medical certificate was issued on January 3, 2017, which included the limitation, "must have available glasses for near vision." According to the pilot, his flight time included 814 hours of total flight time in all airplanes, of which 331 hours had been accumulated in the airplane make and model. According to FAA records, the airplane was manufactured in 1991. The airplane was powered by an automotive Mazda 13B rotary engine that was modified by the owner for use in the airplane. The pilot removed the turbo system, which derated the engine to 150 hp. Maintenance records showed that the airplane's most recent annual inspection was completed on July 29, 2016 at 205 flight hours by the pilot, who also held a repairman's certificate for this airplane. At the time of the accident, the engine had accumulated a total of 223 hours total time in service and 17 hours since the airplane's most recent inspection. An industry expert reported there are approximately 400 Cozy MK IV airplanes in service; however, only about 6 of them are powered by Mazda 13B rotary engines. Fuel System The airplane was equipped with two inboard fuel tanks that each held 26 gallons and both were equipped with a high pressure fuel pump. During normal operation, fuel would flow from the chosen tank through a line directly to the fuel injectors at the engine. The right and left fuel lines "T" off and run to a fuel filter, which runs to a primary fuel rail and a secondary fuel rail. The fuel rails return fuel back to the gascolator and the selected fuel tank. If a tank is overfilled, the excess will be expelled through the tank's vent lines. The engine uses fuel injectors with pulse activated electronic valves to admit fuel into the engine, which can be programmed to increase/decrease the pulse which determines how long the injectors will stay open and deliver fuel to the engine. The airplane's ignition controller manages the fuel injector settings based on rpm simultaneously with the three butterfly valves. Engine Examination A postaccident examination was completed by the pilot with oversight from a Federal Aviation Administration inspector. Both high pressure fuel pumps were tested and functioned normally. The fuel filter was free of contaminations. The fuel injectors functioned normally when tested using a diagnostic mode on the engine control unit (ECU) that simulates an engine run at 3,000 rpm. The spark plugs were partially corroded from the engine's exposure to salt water. A test of the resistance in the coils/igniters and ignition harness was performed in accordance with the Haynes Mazda Automotive Repair manual and functioned to specification. The ECU was also run in diagnostic mode with the spark plugs removed and each coil fired, which confirmed continuity from the ECU to the spark plugs through the igniters. ECU The ECU was designed to take information from the crank angle sensor, manifold pressure and air temperature sensor from the throttle body and send impulses to the coils and signals to the fuel injectors to moderate fuel flow and ignition power supplied to the spark plugs. A test of the ECU's timing would determine if it was delivering pulses at the appropriate intervals; however, this test could not be accomplished as it was built by an individual who no longer provides this service. The pilot verified that the crank angle sensor was properly aligned and in normal condition. Apex Seals Two park apex seals are installed in each of the three corners of each rotor to seal the edges of the rotor. According to a Mazda rotary engine conversion guide, this engine is susceptible to premature apex seal failures, which can be avoided by sufficient lubrication. This particular engine lubricated the apex seals with a two-stroke sump that delivered oil directly to the apex seals and designed to burn with a minimum of deposits, and functioned normally when tested. The top seals from one of the rotor sides was not recovered after the accident. The pilot presumed the damaged or missing apex seal caused the power loss, as the absence of this seal would significantly reduce engine power.