On August 13, 1983, a Bell 206L-1 (registration N1081K) was involved in an aviation accident near Bettles, AK. Investigators recorded the probable cause as: The flow switch was rendered inoperative by debris (dirt and zinc chromate paint chips) that passed through the filter screen openings (0.020 inch) and accumulated in the flow switch, where the clearance between the plunger and casing was only 0.012 inch,… This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced an engine flameout and subsequent hard landing after the low fuel warning light illuminated. Investigation revealed debris in the fuel filters and a flow switch rendered inoperative, preventing fuel transfer from the main cell.
Flight Events
According to the pilot, the low fuel warning light illuminated when the fuel gauge indicated 200 pounds of fuel. The pilot reported seeing the airport lights and decided to continue the flight. Upon arrival, the pilot terminated the approach to a 30- to 50-foot hover with no forward speed. As forward cyclic was applied to hover toward the refueling area, the engine flamed out. The helicopter then landed hard, and the main rotor blades struck the tail boom.
Investigation Findings
An examination revealed that enough fuel remained in the main cell for approximately 20 minutes of flight. Disassembly of the fuel system showed debris, including dirt and zinc chromate paint chips, in the filters. The filter screen had openings of 0.020 inch. Particles were also found in the flow switches. However, the clearance between the flow switch plunger and the casing wall was only 0.012 inch. Evidence indicated that this rendered a flow switch inoperative, preventing fuel from transferring from the main cell to the rear cell where it could be used by the engine.
Probable cause
The flow switch was rendered inoperative by debris (dirt and zinc chromate paint chips) that passed through the filter screen openings (0.020 inch) and accumulated in the flow switch, where the clearance between the plunger and casing was only 0.012 inch, preventing fuel transfer from the main cell to the rear cell.
What happened in the 13 Aug 1983 Bell 206L-1 accident near Bettles, AK?
A helicopter experienced an engine flameout and subsequent hard landing after the low fuel warning light illuminated. Investigation revealed debris in the fuel filters and a flow switch rendered inoperative, preventing fuel transfer from the main cell.
What aircraft was involved and where did it happen?
The accident on 13 Aug 1983 involved a Bell 206L-1, registration N1081K, at Bettles, AK.
What was the probable cause of the accident?
The flow switch was rendered inoperative by debris (dirt and zinc chromate paint chips) that passed through the filter screen openings (0.020 inch) and accumulated in the flow switch, where the clearance between the plunger and casing was only 0.012 inch, preventing fuel transfer from the main cell to the rear cell.
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