Amateur-built RV-12 N28BS accident near Novato, California

Probable cause

A partial loss of engine power during initial climb for undetermined reasons.

NTSB analysis

The accident flight was the airplane’s first flight since receiving maintenance that included a condition inspection and the rebuilding of both carburetors. The pilot reported that, following an uneventful engine run-up, he initiated takeoff. During the takeoff roll, the airplane seemed a little “lethargic” and, when the airplane was about 100 ft above ground level, the engine began to run rough and lost partial power. He initiated a 180° turn to the departing runway; however, the airplane was “too high and too fast” to land on runway, and he made a left turn to a nearby road. The airplane landed hard, and the nosewheel landing gear collapsed. Review of the downloaded data from the airplane’s avionics showed that, during the takeoff sequence, the engine rpm decreased from 5,000 rpm to about 3,200 rpm, along with a corresponding decrease in the exhaust gas temperatures for the Nos. 1 and 3 cylinders. At the time of the rpm decrease, the fuel pressure and fuel flow remained within factory specifications.

Postaccident examination of the recovered airframe and engine revealed no evidence of a preexisting mechanical anomaly that would have precluded normal operation. The mixture control was found about one turn out from full rich, and a carburetor float bowl vent line was found disconnected from the carburetor for the Nos. 1 and 3 cylinders. A functional test conducted using a similar airplane and engine equipped with a similar fuel mixture system with the float vent line disconnected could not replicate the partial loss of engine power. The test airplane’s engine ran normally, and no change in engine performance was observed with manipulation of the mixture knob from full rich to lean.

Text from the NTSB. Open the NTSB docket for WPR24LA023 for the full record.