On August 7, 2016, about 1709 central daylight time, an experimental amateur-built Vans RV-4, N924WZ airplane, was substantially damaged during a forced landing while on approach to Miles Field Airport (3KY9), Shelbyville, Kentucky. The private pilot owner was seriously injured. No flight plan was filed for the personal flight that originated at the Capital City Airport (FFT), Frankfort, Kentucky. Visual meteorological conditions prevailed for the local flight conducted under the provisions of 14 Code of Federal Regulations Part 91.According to a Federal Aviation Administration (FAA) inspector, the pilot completed a series of touch-and-go landings at 3KY9, flew to FFT to refuel, and was returning to 3KY9 at the time of the accident.
In a statement to the FAA, the pilot reported that he observed a deer while on the final approach leg of the traffic pattern and he elected to execute a low approach. He stated the engine was "running fine" until he applied full power on climbout; then the engine only produced partial power. Because the airplane was still able to climb, the pilot elected to fly the traffic pattern and make another approach. After turning onto the base leg of the traffic pattern, he added carburetor heat and 10 degrees of flaps. After he reduced engine power on the final approach leg of the traffic pattern, the engine began "running bad" and "a little rough." The airplane began sinking too fast, the pilot added full power and turned the carburetor heat off. The engine continued to lose power and the airplane kept descending until it impacted terrain about 200 feet from the runway.
The airplane's wings sustained leading edge damage, the cockpit was heavily damaged and folded open, and there was significant damage to the firewall.
The airplane was equipped with a Lycoming O-320, 150-horsepower engine. Initial examination of the engine by an FAA inspector revealed that both magnetos produced spark at their respective distributor blocks and the spark plugs appeared to be new and undamaged. The fuel selector was in the "ON" position. The pilot's total flight experience could not be reconciled, but at the time of his most recent application for an FAA medical certificate on August 13, 2015, he reported 170 total hours of flight experience. The closest weather reporting facility was the about 11 miles east of the accident site. At 1653, an aviation routine weather report at FFT was reporting in part: temperature 86° F; dewpoint 57° C.
An FAA carburetor icing probability chart indicated the temperature and dew point conditions were conducive to the formation of serious icing at glide 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.