FK9 ELA SW N611SP accident near Canton, Georgia

Probable cause

A partial loss of engine power during cruise flight due to an overly lean fuel-air mixture, which resulted from an incorrectly installed float chamber vent line and led to a subsequent off-airport landing.

NTSB analysis

The pilot reported that, while in cruise flight, the engine experienced a partial power loss. The airplane was unable to maintain altitude, so the pilot chose to perform an off-airport landing to an open field. During the landing roll, the airplane encountered a berm, became airborne, and then landed hard.

Examination of the engine revealed that the carburetor float chamber vent lines had been incorrectly routed to the air filter. The engine manufacturer's installation manual cautioned that the float chamber vent lines must not be routed into the slipstream or down the firewall because "pressure differences between the intake pressure in the carburetor chambers may lead to engine malfunction due to incorrect fuel supply." Therefore, it is likely that the incorrectly installed vent line resulted in back pressure to the float bowl that exceeded the normal operating range, which would have affected the engine's fuel-air mixture and led to the partial loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA14LA089 for the full record.