WINSTON, MT, USA
N47AH
Bell 47-G3B
Post accident inspection 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 was still locked to the seat, allowing the bellows to screw down onto the tube, thereby restricting the fuel delivery orifice. The seat and lock nut remained securely bolted to the float. The last recorded carburetor repair was on 7/22/93, with 817 hours since that repair. Records did not reflect the nature of that repair. According to the carburetor manufacturer's engineering drawings, the bellows is bonded to the threaded tube with a permanent type plastic resin.
On October 18, 1997, approximately 2130 mountain daylight time, a Bell 47G-3B, N47AH, being operated and flown by a private pilot, sustained substantial damage during a hard landing following a forced landing near Winston, Montana. The pilot and passenger were uninjured. Visual meteorological bright night conditions existed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and was returning from Townsend, Montana, en route back to Helena. The pilot reported to an FAA inspector that while climbing through 4,800 feet above ground, the helicopter yawed momentarily and then the engine abruptly ceased developing power. The pilot transitioned into an emergency autorotation and was unable to regain power during the descent. Just prior to landing, the pilot maneuvered to the right to avoid trees and touched down hard on downward sloping terrain. The left skid buckled and the main rotor struck the tail boom, separating the tail rotor and its associated drive and gear box. Post accident inspection revealed a loose bellows housing on the MA-6AA carburetor's automatic mixture control , marked with P/N 293-549. The bellows housing was found loose on the threaded tube, which was still locked to the seat, allowing the bellows to screw down onto the tube, thereby restricting the fuel delivery orifice. The seat and lock nut remained securely bolted to the float bowl ,however one of the screws was not properly safetied with the tab washer. The last recorded carburetor repair was on 7/22/93, with 817 hours since that repair. Records did not reflect the nature of that repair. According to the carburetor manufacturer's engineering drawings, the bellows is bonded to the threaded tube with a permanent type plastic resin. A check of FAA service difficulty reports found one instance of a failure of the bonding material.
Fuel starvation due to fuel flow restriction from a failed carburetor automatic mixture control device. Factors include night light conditions for the forced landing.
Source: NTSB Aviation Accident Database
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