Plane & Pilot
Tuesday, October 4, 2011

Fuel Imbalance

Wing heaviness may be a minor annoyance or a genuine threat to safety

LEFT-WING HEAVY. A fuel imbalance resulting from icing within the fuel system turned extreme during a March 2009 Pilatus PC-12 accident in Montana.
Most pilots know what it feels like when an airplane is wing heavy because there's more fuel on one side than the other. Corrective action may have included switching the fuel selector from right to left or left to right, adding lateral trim, adjusting your control inputs, or reconfiguring a complex fuel system to balance tank quantities. Many pilots view this aspect of fuel-system management as being so routine that there's nothing that can get you into trouble. A recently completed investigation by the NTSB of an accident in which the pilot and all 13 passengers were killed tells a different story.

The accident occurred on March 22, 2009, at about 2:32 p.m., local time. The single-engine turboprop Pilatus PC-12/45 was diverting to Bert Mooney Airport (BTM), Butte, Mont., when it crashed about 2,100 feet west of runway 33 at BTM. The airplane was owned by a leasing company, and was operating as a personal flight under Part 91. The passengers consisted of three families en route to Bozeman, Mont., for a skiing vacation. Two of the passengers were daughters of a principal of the leasing company. The flight departed Oroville Municipal Airport (OVE), Oroville, Calif., on an IFR flight plan with a destination of Gallatin Field (BZN) in Bozeman. The airplane was operating in visual conditions at the time of the accident.

The day before the accident, the pilot had the airplane fueled with 222 gallons of Jet-A fuel at the municipal airport in Redlands, Calif. (REI), where the airplane was based. The fueling-station manager told investigators that the pilot didn't ask for a fuel system icing inhibitor (FSII) to be added with the fuel. The Safety Board noted that all jet fuels contain trace amounts of water, and an FSII lowers the freezing point to -40º C, to prevent the water from turning into ice crystals, which can block a fuel line or filter. The Pilatus PC-12 Airplane Flight Manual says, "anti-icing additive must be used for all flight operations in ambient temperatures below 0 C." According to the NTSB, most PC-12 flights would require the use of a fuel additive, since on a standard day, 0 degrees C occurs at 7,500 feet.

On the morning of the accident, the pilot flew the PC-12 to the Vacaville, Calif., airport (VCB). Surveillance video showed the pilot fueling the airplane at VCB. The video showed no evidence that the pilot sampled fuel from either of the underwing fuel tank drains or the fuel filter drain to check for water, ice crystals or other contamination. The fuel wasn't premixed with an FSII, and the fuel pump didn't contain provisions for injecting an FSII during fueling.

The airplane departed VCB with nine passengers (four adults and five children), although the flight plan indicated only four passengers. After arrival at Oroville, Calif., four more passengers (two adults and two children) boarded, resulting in a total of 13 passengers (six adults and seven children, who ranged in age from one to nine years). The flight plan indicated eight passengers and the pilot would be on board at that point. The accident airplane was configured with two pilot and eight passenger seats. Because each flight on the day of the accident was a single-pilot operation, one seat in the cockpit could be used by a passenger.


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