[AAR-11/05, ¶2.2.2] According to the Pilatus PC-12 AFM, the fuel boost pumps operate automatically if a low fuel pressure state exists—which occurs when fuel system pressure drops below 2 psi—and the pump's switch is set to the AUTO position.
- About 1 hour 13 minutes into the flight (about 1323), both fuel boost pumps began cycling because of a low fuel pressure state. . . . Both fuel boost pumps continued cycling during the next 4 1/2 minutes. No CAWS low fuel pressure cautions were logged during the flight, indicating that the low fuel pressure state was alleviated by the operation of one or both fuel boost pumps.
- About 1 hour 18 minutes into the flight (about 1328), the left fuel boost pump was on continuously, and the right fuel boost pump was off. The continuous operation of the left fuel boost pump indicated that the pump had been commanded to operate by the fuel balancing system to automatically correct a fuel imbalance of at least 70 pounds while the pump also operated to maintain adequate fuel system pressure. The fuel imbalance, which had developed between the left and the right fuel tanks (with the left tank containing more fuel than the right tank), would have been displayed by a two-bar differential on the fuel quantity indicator. This imbalance was likely created because the right fuel boost pump had delivered more fuel to the engine than the left fuel boost pump had delivered (likely as a result of a restricted flow of fuel from the left wing tank) during the time that both fuel boost pumps were simultaneously cycling. During the next 3 minutes, the operation of the left fuel boost pump had likely reduced the left-wing-heavy fuel imbalance to within about 15 pounds.
When this mishap occurred the PC-12 AFM stated that the maximum fuel imbalance for the PC-12 was 178 pounds with a maximum three-bar differential displayed on the fuel quantity indicator.
- About 1331, the right fuel boost pump resumed cycling. The operation of the right fuel boost pump indicated that (1) the fuel pressure output of the left-side fuel system had degraded to less than 2 psi, even with the left fuel boost pump on continuously; (2) the required fuel system pressure could no longer be maintained through the operation of the left fuel boost pump; and (3) the right fuel boost pump was needed to maintain fuel system pressure to the engine.
- About 2 hours 17 minutes into the flight (about 1427), the CAWS annunciated a right fuel low (R FUEL LOW) caution; the Pilatus PC-12 AFM stated that this caution was logged when the amount of usable fuel in a tank (in this case, the right wing tank) was 133 pounds or less. The expected fuel load at this point in the flight, assuming 1,268 pounds of fuel in each tank (1,252 pounds of which was usable fuel) at the beginning of the flight and equal fuel consumption from each tank during the flight, would have been 812 pounds of usable fuel per tank. However, the fuel load at the time of the R FUEL LOW caution was estimated to be 1,368 pounds (1,352 pounds of which was usable fuel) in the left tank and 149 pounds (133 pounds of which was usable fuel) in the right tank, which corresponded to a left-wing-heavy imbalance of 1,219 pounds. This imbalance would have been displayed to the pilot on the fuel quantity indicator as a 26-bar differential.
- Five seconds later, the right fuel boost pump turned on continuously and remained that way until the end of the flight. (The left fuel boost pump had been on continuously since about 1426 and remained that way until the end of the flight.) At the time of the last CAWS entries for the flight (about 1433), the fuel imbalance would have been displayed to the pilot as a 27-bar differential. (Each side of the fuel quantity indicator contains a total of 28 bars.)
- For two of the three flight legs on the day of the accident, the pilot allowed the airplane to depart with a takeoff weight that was over the PC-12 maximum takeoff weight.
- The maximum allowable fuel imbalance between the left and the right fuel tanks was estimated to have been exceeded sometime between 1331 and 1335. The PC-12 AFM stated that, when this imbalance occurred, the pilot should land the airplane as soon as practical. The pilot did not divert to another airport at that time, even though three suitable airports along the airplane's route of flight—BOI, TWF, and LLJ—were available to the pilot.
- The pilot began to divert to BTM about 30 minutes after the maximum allowable fuel imbalance was estimated to have been exceeded. At that time, LLJ was the closest airport to the airplane's position. Once the airplane's route of flight changed, DLN became the most suitable diversion airport relative to the airplane's position, but the pilot decided to continue to BTM.
- The pilot's decision to divert to BTM may also have been influenced by passenger convenience considerations. Specifically, the passengers could have easily arranged for ground transportation from BTM to their destination; such arrangements would have been more difficult from LLJ or DLN, which were the most suitable airports at the time that the pilot began to divert to BTM.
[AAR-11/05, ¶126.96.36.199] Pilatus tested the PC-12 to ensure compliance with 14 CFR 23.23, "Load Distribution Limits," for normal flight conditions, as documented in Pilatus Engineering Report ER 12-03-80-002 (dated February 1994). This report showed that the PC-12 was tested beyond the AFM's maximum fuel imbalance limit of 178 pounds. Specifically, with the PC-12 loaded at the most critical weight and center of gravity and with the most critical operating condition (landing gear extended, flaps extended to the landing position, and engine power on), both wings-level and turning stall flight tests were performed with a fuel imbalance between 240 and 380 pounds. According to Pilatus, all of these tests were flown successfully, and the pilot did not report any problems performing the maneuvers. Pilatus further indicated that, in terms of aircraft handling, the first indication of fuel asymmetry was the need to increase the amount of aileron trim, which occurred with a fuel imbalance of 130 pounds, or about 10 percent of the total fuel capacity in one tank (displayed as a two- to three-bar differential).
- One condition that was considered in the fuel system hazard assessment was an excessive fuel imbalance between both wing fuel tanks. Regarding this condition, the assessment stated, "the difference in fuel weight will produce a rolling moment on the aircraft. This moment may be counteracted by changing the trim setting and the failure condition may be removed by differential operation of the booster pumps." The assessment further stated, "in the event of a major fuel imbalance which cannot be corrected by operating the booster pumps, the rolling moment may become too large to be counteracted by trimming and it may be necessary to amend the planned mission."
- An addendum to the assessment (which was also included in Pilatus Engineering Report ER 12-28-00-001) further considered the effects of an excessive fuel imbalance between both wing fuel tanks. According to the addendum, if a fuel imbalance between both wing fuel tanks exceeded 25 percent of the full fuel tank load, "the resulting rolling moment cannot be corrected by trimming alone and the control [wheel] must be used." The addendum cautioned that this scenario would increase pilot workload and decrease the airplane's safety margin in the event of a maneuver requiring higher-than-usual levels of piloting skill.
- The position of the aileron and rudder trim actuators, as recovered in the wreckage, indicated full RWD aileron trim and full ANL rudder trim, which is consistent with a forward sideslip to the right and a left-wing-down rolling moment. A forward right sideslip is a maneuver that the pilot could have used to create drag and increase the descent rate without increasing airspeed, even though the maneuver would have increased the wheel force required to maintain control of the airplane. The rudder input associated with a prolonged rudder pedal input would also result in full ANL rudder trim; the autotrim function would automatically operate the rudder trim system to offload the pedal forces applied by the pilot.
- Pilatus calculated that a sideslip with the accident conditions and airspeeds close to the PC-12 stall speed (93 knots) would require 22° (55 percent) of the 40° of available aileron (a left aileron input of 8° and a right aileron input of -14°) with full aileron trim (15°), which would result in a 20-pound control wheel input. These calculations assumed that enough rudder was available to balance the yawing moment (resulting from sideslip) and that the rudder had little effect on the rolling moment.
- In addition, Pilatus found that, with the sideslip maneuver, static conditions, and straight and level flight, an airplane with a fuel imbalance of about 1,300 pounds would have been controllable to an airspeed of about 90 knots with about one-half of the available aileron. Pilatus' calculations did not consider the airplane dynamics (for example, yaw and roll rate) associated with landing or the possibility that the pilot was performing a go-around maneuver at low speed at the time of the maximum fuel imbalance.