The private pilot reported that, before departure, he performed an engine run-up with carburetor heat applied, and no anomalies were noted.
He departed for the personal local flight, and when the Globe GC1B reached about 100′ above ground level, the engine power decreased from 2,400 to 1,600 rpm, so he executed a forced landing to a field near Lake Elmo, Minnesota.
A post-accident examination of the airplane and engine revealed that the throttle body separated from the air intake manifold due to overload likely associated with impact. The fuel nozzle and primary venturi were missing from the carburetor and were not located.
Although the engine could likely have started without these components installed, it is unlikely that it could have produced much more than idle power.
Sliding marks on the sides of the throttle body revealed evidence of contact with the legs of the primary venturi. The contact marks had areas free of black deposits whereas areas adjacent to the marks were covered with deposits, indicating that a primary venturi had been installed until recently.
The deposits on either side of the marks were not disturbed, indicating that the primary venturi did not rotate out of position. That means the primary venturi either fractured in service or was separated and lost from the throttle body after the carburetor was disassembled during the initial post-accident examination.
The FAA had previously issued an airworthiness directive (AD), which required that the carburetor be inspected at each annual, 100-hour, or progressive inspection to determine if the primary venturi was loose or missing.
According to the maintenance logbooks, the last inspection conducted in accordance with the AD occurred about 1.5 months and one flight hour before the accident.
Although the weather conditions at the time of the accident were conducive to the formation of carburetor icing at cruise power, it is not likely that carburetor ice caused the venturi or fuel nozzle to break because the pilot had used carburetor heat during the run-up and the engine was operating at takeoff power.
The accident is consistent with a loss of engine power due to the carburetor’s primary venturi, fuel nozzle, or both separating after takeoff. The reason for the separation could not be determined.
Probable cause: A loss of engine power due to the carburetor’s primary venturi, fuel nozzle, or both separating after takeoff.
NTSB Identification: CEN16LA061
This December 2015 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
