In the Pilatus, fuel pumps are turned on or off, manually or automatically, as needed to feed more fuel from the fuller tank. (There is no provision for pumping fuel directly from one tank to the other.) The electronic fuel-quantity indicators display two bands of bars, like the signal-strength bars on a cell phone, except that each band contains 28 bars representing the 1,370-pound fuel capacity of each wing. The POH recommended controlling the pumps manually if a discrepancy of more than three bars appeared, and landing if the discrepancy could not be resolved.
The nonvolatile memory of the airplane’s central advisory and warning system retained a record of fuel levels and pump operation during the accident flight, and made clear to investigators that fuel had ceased to flow from the left tank about 80 minutes after takeoff. At the same time, fuel returned from the engine was going to both tanks. As a result, the left tank became gradually fuller as the right tank emptied.
Belatedly, the pilot decided to divert to Butte, which was only slightly closer than his destination of Bozeman. By the time he got there the left tank was completely full of inaccessible fuel and the right nearly empty. The aileron trim was at the right-wing-down stop. Because of weather and altitude limitations, he had to descend steeply, and he used full left rudder trim to maintain a right slip to kill altitude. He was still too high to land, and it was during the go-around that he lost control of the airplane and crashed in a cemetery beside the airport. All aboard died.
By that time, nearly an hour had passed since the fuel imbalance had reached the point at which the POH advised landing immediately. Why the pilot chose to continue, passing many usable airports, some of them quite close to his track, only he knew. The NTSB listed three causes of the accident in chronological order: the failure to get ice inhibitor with the fuel, the decision to press on rather than land en route and, finally, the loss of control.
Unlike an airplane that is mistrimmed, an airplane that is unbalanced, either laterally or longitudinally, needs to keep speed up in order to preserve control authority. It must also be maneuvered gingerly, to minimize inertial forces. Statically, the Pilatus can supposedly maintain level flight at 90 knots with a 1,300-pound fuel imbalance, but with full left rudder trim (which tends to push the left wing down) and in dynamic maneuvering, considerably more control authority is needed.
Ironically, a check pilot who had flown with the accident pilot described him as possessing a “very high level” of competency and “superb” professional judgment. Even Homer sometimes nods.
This article is based on NTSB reports of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.
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