N750AZZenith Aircraft STOL CH 750

On July 24, 2016, about 0915 eastern daylight time, an experimental amateur-built Zenith STOL CH 750, N750AZ, was substantially damaged during a forced landing following a loss of engine power during the initial climb after takeoff from Greene County Airport (WAY), Waynesburg, Pennsylvania. The flight instructor and a student pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local instructional flight conducted under the provisions of 14 Code of Federal Regulations Part 91.The purpose of the flight was for the student pilot and flight instructor to practice touch-and-go landings. After about 0.8 hours of touch-and-go landings, they added 8 gallons of fuel. During the ensuing climb, about 1/2-mile beyond the runway at 300 feet, the engine began to vibrate and lose power. The instructor took control of the airplane and decided to land straight ahead. He noted that there was no oil pressure, normal oil temperature, and decreasing engine rpm. He elected to perform a forced landing on the midfield of the Green County Fairgrounds.

During the landing roll, the airplane impacted tractor tires and the landing gear folded back. Examination of the wreckage by a Federal Aviation Administration (FAA) inspector revealed damage to the wing spar and wing struts. Delaminating of the composite propeller, consistent with impact damage, was also noted.

The airplane was equipped with a Continental O-200-A EXP, 100-horsepower engine, which was examined by an FAA inspector. The accessory section, and oil pump was removed for inspection, with no noted anomalies; about 5 quarts of oil was drained from the oil sump. The oil filter was opened and free of debris. The oil pressure sending unit was removed and tested, no malfunction was observed. Thumb compression was obtained on all cylinders. The #2 cylinder had lower compression than the other cylinders. Engine powertrain continuity was established and no anomalies that would have precluded normal operation were observed. Fuel drained from the wing tanks were free of debris or contamination.

The closest weather reporting facility was the about 15 miles north of the accident site. At 1035, the weather conditions reported at Washington County Airport (AFJ) included temperature 29 degrees C; dewpoint 23 degrees C.

According to a statement provided by the flight instructor, the carburetor heat was not used during takeoff, "as recommended in the pilot manual," and "carburetor heat was applied at the first sign of vibration and power reduction." After applying carburetor heat and noting the loss of RPM, the instructor turned the carburetor heat off to get as much power from the engine as possible to extend their glide range.

An FAA carburetor icing probability chart indicated the temperature and dew point conditions were conducive to the formation of serious icing at glide power, and icing at glide and cruise power.

According to the FAA Pilot's Handbook of Aeronautical Knowledge, carburetor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the carburetor's venturi, which can cause a sharp temperature decrease in the carburetor. If water vapor in the air condenses when the carburetor temperature is at or below freezing, ice may form on the internal surfaces of the carburetor, including the throttle valve. This then restricts the flow of the fuel/air mixture and reduces engine power. Generally, the first indication of carburetor icing in an airplane with a fixed-pitch propeller is a decrease in engine rpm, which may be followed by engine roughness. Under certain conditions, carburetor ice can build unnoticed until power is added.

The handbook further described that carburetor heat is an anti-icing system that preheats the air before it reaches the carburetor, and is intended to keep the fuel/air mixture above the freezing temperature to prevent the formation of carburetor ice. Carburetor heat can be used to melt ice that has already formed in the carburetor if the accumulation is not too great, but using carburetor heat as a preventative measure is the better option.