WPR12TA323 · Aero Commander 500 S

The flight mission was to provide airborne coordination for an aerial fire suppression effort. The pilot was a company employee, and the other crewmember, who was not functioning as a pilot, was an employee of the Bureau of Land Management (BLM).

Although the pilot was required by the company to remain present for the duration of the airplane fueling before the flight, he did not do so. A design peculiarity of the fuel quantity system prevented in-cockpit determination of the actual fuel quantity once the fuel level rose above a value that was 21 gallons below the actual maximum capacity. Only a visual inspection of the tank would enable determination of the actual fuel quantity above that level or whether the tank was completely full. The pilot reported that he began the flight with full fuel, but there was no evidence that he visually checked the fuel level. Therefore, it is possible that the flight began with as much as 21 gallons less than the pilot believed. After takeoff, the pilot radioed to his dispatch center that he had 4 hours 30 minutes of fuel on board. Company rules and the exclusive-use contract with the BLM required the pilot to comply with Federal Aviation Administration regulations that precluded departure without a 30-minute fuel reserve.

The fire was located about 15 minutes (still air) flight time from the base airport, but the return flight would be subject to a headwind of about 10 knots. Exclusive of the 30-minute fuel reserve requirement for the beginning of the flight, there were no mission-specific or other factors that dictated the pilot's turnback time. About 4 hours 15 minutes after departure, with the airplane fuel quantity gauge indicating 0 gallons of fuel, the pilot turned back to base. Shortly thereafter, both engines lost power due to fuel exhaustion, and the pilot conducted a forced off-airport landing.

The company's and BLM's guidance differed in several aspects regarding the flight planning, briefing, and communication requirements. Due in part to those differences, the two crewmembers did not conduct any preflight briefing, or any in-flight communications, regarding fuel status. Neither of the two was required to, and they did not, establish a minimum fuel value which, when reached, would require them to depart the operating area and return to base.

The fuel quantity system had been calibrated about 14 months before the accident, and postaccident examination of the airplane did not reveal any mechanical problems that would have resulted in premature fuel exhaustion. The pilot reported that he relied on his time and fuel consumption calculations instead of the cockpit fuel quantity gauge to determine the fuel remaining and his turnback time. His calculations were based on starting the flight with full fuel tanks, and a nominal flight-planning fuel consumption rate, neither of which could be verified after the accident. Postaccident calculated fuel consumption rates for the flight were in the same range as both recent historical data from the accident airplane and the manufacturer's published values.

Although the pilot could have used the fuel gauge in addition to his calculations to determine the remaining fuel, he chose not to. The more conservative risk management approach would have been to turn back to base when the fuel gauge reached the minimum value necessary to complete the flight safely. In addition, neither of the crewmembers explicitly determined and communicated a minimum fuel quantity value that mandated a return to base, and none of their guidance explicitly recommended or required them to do so.

Verbatim NTSB analysis and probable cause from the NTSB dataset