N28BS

On October 26, 2023, about 1330 Pacific daylight time, an experimental amateur-built Vans RV-12, N28BS, was substantially damaged when it was involved in an accident near Novato, California. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot, the accident flight was the first flight since receiving maintenance that included a condition inspection, the rebuilding of both carburetors, and the replacement of the Nos. 1 and 3 cylinders’ ignition leads. He stated that, following an uneventful pretakeoff engine run-up, he initiated takeoff on runway 31, and the airplane seemed a little “lethargic” during the takeoff roll. When the airplane was about 100 ft above ground level, the engine began to run rough and lost partial power. The pilot stated he initiated a 180° turn, but the airplane was “too high and too fast” to land on runway 13, so he made a left turn to a nearby road. The airplane landed hard on the road, and the nosewheel landing gear collapsed.

Postaccident examination of the airplane revealed that the fuselage was structurally damaged.

The airplane was equipped with a Dynon FlightDek D180 display unit that recorded various engine parameters, including rpm, exhaust gas temperature, fuel pressure, and fuel flow. Review of the downloaded data showed that, during the takeoff sequence, engine rpm decreased from 5,000 rpm to about 3,200 rpm, and the exhaust gas temperatures for the Nos. 1 and 3 cylinders decreased from about 1,350° F to about 350 to 500° F. At the time of the decreased rpm, fuel pressure was 2.3125 psi, and fuel flow was 5.625 gallons per hour. According to the Pilot Operating Handbook for the Vans RV-12, fuel pressure limits were from 2.2 to 5.8 or 7.2 psi, depending on what engine-driven fuel pump was installed.

The airplane was equipped with an Aircraft Service in-flight mixture control system that was nonstandard for the Rotax engine. Examination of the recovered airframe revealed that the mixture control knob in the cockpit was found one turn out from full rich. The engine remained attached to the airframe, and all accessories remained attached. The carburetor vent lines extended from the firewall to a “T” connector, then to each carburetor. All fuel lines remained secured to their respective fittings, except the carburetor float chamber vent line was found disconnected from the carburetor for the Nos. 1 and 3 cylinders. The mixture system jet was found free of obstructions.

Rotational continuity was established throughout the engine and valvetrain, and thumb compression was obtained on all four cylinders. Compressed air was applied to the engine fuel inlet lines, and no obstructions were found throughout the fuel system. The fuel cap vent was free of obstructions.

Both carburetors were removed and partially disassembled. Both carburetor slides were free to move up and down, and the floats were intact.

According to the mechanic who rebuilt the carburetors before the accident flight, the mixture control system would enable the pilot to lean the engine to obtain 100-200 rpm with a turn or two of the mixture control knob but that he advocates to use the mixture control only at altitudes higher than 4,000 ft. He stated that, at two turns out, the exhaust gas temperatures will rise but remain within limits. He stated that, if the pilot leans it beyond two turns, the engine will develop roughness but not a loss of engine power.

The mechanic performed an engine runup test on his own airplane, which was equipped with the same engine and mixture system. He reported that there was a negligible effect of running the engine through a simulated 5-minute climb with the carburetor float chamber vent line disconnected from the carburetor for the Nos. 1 and 3 cylinders. He stated that operating the mixture control knob through its full travel had an insignificant effect on operation of the engine, which felt and sounded normal throughout the test.