American Aviation AA-1B N6242L accident near Brenham, Texas

Probable cause

A partial loss of engine power due to an obstructed induction system from a mud dauber nest.

NTSB analysis

The pilot was conducting a personal flight when the airplane had a partial loss of engine power shortly after takeoff about 500 ft above the departure end of the runway. The pilot reported that the engine appeared to be running at idle power and he was unable to maintain altitude. The pilot completed a forced landing into a nearby grass field, during which the nose landing gear separated from the forward fuselage. The airplane’s left wing leading edge was substantially damaged during the forced landing.

Examination of the engine did not reveal any evidence of a mechanical failure that would prevent normal operation. Visual examination of the engine’s exterior revealed that all four engine cylinders had evidence of mud dauber nests within their respective cooling fins. Additionally, a roughly 1.5-inch diameter mud dauber nest was found in the airbox between the air filter and the throat of the carburetor. Additionally, dirt with the same color as the mud dauber nest was observed within the carburetor throat and on the clean side of the air filter. The recovered mud dauber nest exhibited impressions consistent with the spacing of the wire coil that supported the flexible ducting associated with the carburetor heat system. As such, the mud dauber nest would not be visible during a preflight inspection. The flexible ducting also contained various-sized fragments of mud dauber nest debris. Based on the available evidence, it is likely the mud dauber nest became dislodged shortly after takeoff and partially obstructed the induction airflow into the carburetor, which resulted in a partial loss of engine power.

Text from the NTSB. Open the NTSB docket for CEN25LA054 for the full record.