Cessna 172 N64460 accident near Fullerton, California

Probable cause

Partial loss of engine power during takeoff due to an age-degraded carburetor heat valve seal, which detached and partially blocked the carburetor air inlet. Contributing to the accident was an inadequate maintenance inspection.

NTSB analysis

Shortly after takeoff during a practice go-around, the engine lost partial power. With limited time to troubleshoot, the flight instructor maneuvered the airplane for an off-airport landing, and the airplane sustained substantial damage after impacting a street.

Postaccident examination revealed that the aft section of the carburetor heat butterfly valve seal had detached and partially blocked the air inlet of the carburetor venturi. The obstruction reduced the amount of air available for combustion, resulting in a rich fuel-to-air mixture, and partial loss of power.

The valve seal was composed of a fiber-reinforced rubberized material that degraded and became brittle over the 45-year life of the airplane. The mechanic who performed the last inspection stated that he confirmed the operation and condition of the butterfly valve by observing it through both the airbox assembly forward inlet and the heated air inlet on the rear right side of the unit. He stated that the aft section of the seal is hard to view due to the way it folds behind the butterfly valve and that the only way to clearly see it is through the heated air inlet, which was small.

Text from the NTSB. Open the NTSB docket for WPR19LA176 for the full record.