What happened
The pilot departed the airport with approximately 75 gallons of fuel on board, flying roughly 19 miles toward a designated spray location. About 45 minutes into the flight, the pilot observed that the fuel quantity gauge was decreasing at an accelerated rate and decided to return to the airport.
When the aircraft was eight miles from the airport, it began losing power. The pilot subsequently initiated a forced landing in an open field. During the landing roll, the aircraft struck a lava bed, resulting in substantial damage.
The investigation
An FAA inspector examined the aircraft and found that there was no fuel remaining in the tanks, though no evidence of a pre-existing fuel leak was discovered. A post-accident inspection of the carburetor revealed that the mixture control back-suction bleed tube within the carburetor bowl assembly was plugged. According to the carburetor's certificate holder, such a blockage causes a rich mixture condition. The aircraft's normal fuel consumption rate is between 32 and 35 gallons per hour.