On October 19, 1997, a Bell 47-G3B (registration N47AH) was involved in an aviation accident near Winston, MT. Investigators recorded the probable cause as: Fuel starvation due to fuel flow restriction from a failed carburetor automatic mixture control device. Factors include night light conditions for the forced landing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
Post-accident inspection of a carburetor found a loose bellows housing on the automatic mixture control, restricting the fuel delivery orifice. The bellows was supposed to be bonded with permanent plastic resin.
Inspection Findings
A post-accident inspection of the carburetor revealed a loose bellows housing on the MA-6AA carburetor's automatic mixture control. The bellows housing was found loose on the threaded tube, which remained locked to its seat, allowing the bellows to screw down onto the tube and restrict the fuel delivery orifice.
The seat and lock nut were securely bolted to the float. The last recorded carburetor repair occurred on July 22, 1993, with 817 hours of operation since that repair. Records did not specify the nature of the repair performed.
According to the carburetor manufacturer's engineering drawings, the bellows is bonded to the threaded tube using a permanent type plastic resin. The loose condition of the bellows housing contradicts the intended design specification.