Flight Details
The flight departed with approximately 20 gallons of fuel in each tank and offloaded skydivers at 10,500 feet. It then descended to return with the throttle at idle, the fuel/air ratio leaned, and carburetor heat applied.
Engine Failure and Forced Landing
At 2,000 feet, the pilot began to level off and realized engine trouble, initiating emergency procedures for an engine failure at altitude. The pilot checked the fuel selector valve, which was in the same position as for takeoff, "both on." He then cycled the selector to left, right, and both positions with negative results, and also checked the ignition and all fuses. Unable to restart the engine, the pilot performed a forced landing in a field. Before touchdown, he turned off the fuel selector valve; after touchdown, the airplane nosed over.
Post-Accident Examination
Approximately 45 minutes after the accident, a mechanic examined the fuel strainer and found it empty. About 24 hours later, an FAA inspector examined the airplane. The flexible hose from the fuel strainer to the carburetor contained a slight amount of fuel (approximately a capful). No discrepancies were noted with the fuel strainer, ignition, or vent systems. After the FAA released the airplane for recovery, the carburetor was removed; when the throttle was activated, no fuel spray was observed from the accelerator pump discharge nozzle. The fuel flow rate through the fuel selector valve was lower with the selector positioned to the right tank than with it positioned to left or both. Disassembly of the fuel selector valve revealed no evidence of debris or wear; no determination was made as to the reason for the low flow rate with the selector in the "right" position. Historical uneven fuel consumption, with greater consumption from the left tank, had been noted while operating the airplane in skydiving operations involving mainly climbs and descents. After repairs, the engine was placed back on the airframe and operated without discrepancies.
Fuel System Testing
Post-accident testing using an exemplar carburetor, the accident fuel strainer, and the flexible hose from the strainer to the carburetor was performed with the assembly inverted. The test revealed that the carburetor leaks fluid to a certain level but fluid remains in the bowl. Additionally, with the assembly inverted and the carburetor bowl empty but fluid in the fuel strainer and flexible line, fluid does not flow into the carburetor bowl.
Maintenance History
The fuel selector valve was last repaired on March 18, 1977, during which an o-ring on the fuel selector shaft was replaced.