Figure: Available airports about the time of the diversion to Butte, from AAR-11/05, figure 7.
This was the proverbial case of a pilot operating a perfectly good airplane outside the bounds of the flight manual, making a few critical mistakes on the way to the scene of the accident, and failing to keep any kind of situational awareness.
Why do some pilots routinely ignore flight manual limitations? Perhaps it is insidious, they get away with small transgressions which lead to bigger ones. When an emergency occurs, a sense of urgency is displaced because "I've gotten away with this before." The NTSB report speculates that the pilot may have gotten away without adding fuel icing inhibitor many times in the past and quite often managed to fly the airplane outside fuel balance limits.
So let's read as given that the pilot got himself into a serious bit of trouble. But the airplane was still flyable for nearly an hour after it first exceeded fuel balance limits. In that time the pilot overflew several viable divert locations. Perhaps he never fully understood the severity of his predicament and thought his plan to overfly several viable divert locations in favor of one more conducive to passenger needs was perfectly sound.
What follows are quotes from the sources listed below, as well as my comments in blue.
- Date: March 22, 2009
- Time: 1432 MDT
- Type: Pilatus PC-12/45
- Operator: Eagle Cap Leasing of Enterprise, Oregon
- Registration: N128CM
- Fatalities: 1 of 1 crew, 13 of 13 passengers
- Aircraft Fate: Destroyed
- Phase: Approach
- Departure Airport: Oroville Municipal Airport, Oroville, California (KOVE)
- Destination Airport: Bert Mooney Airport, Butte, Montana (KBTM)
- On March 22, 2009, about 1432 mountain daylight time, a Pilatus PC-12/45, N128CM, was diverting to Bert Mooney Airport (BTM), Butte, Montana, when it crashed about 2,100 feet west of runway 33 at BTM. The pilot and the 13 airplane passengers were fatally injured, and the airplane was substantially damaged by impact forces and a postcrash fire. The airplane was owned by Eagle Cap Leasing of Enterprise, Oregon, and was operating as a personal flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The flight departed Oroville Municipal Airport (OVE), Oroville, California, on an instrument flight rules (IFR) flight plan with a destination of Gallatin Field (BZN), Bozeman, Montana. Visual meteorological conditions (VMC) prevailed at the time of the accident.
- On March 21, 2009 (the day before the accident), the pilot had the airplane fueled with 222 gallons of Jet A fuel at Redlands Municipal Airport (REI), Redlands, California, where the airplane was based. During a postaccident interview, the fueling station manager stated that the pilot did not request that a fuel system icing inhibitor (FSII) be added. (All jet fuels contain trace amounts of water, and a FSII lowers the freezing point of water to -40° 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 (AFM), section 2, Limitations, dated March 30, 2001, stated that an "anti-icing additive [FSII] must be used for all flight operations in ambient [outside air] temperatures below 0° C." On a standard day, the temperature is 0° C at 7,500 feet, so most PC-12 flights would require the use of a FSII.
- On March 22, 2009, the pilot departed REI for Nut Tree Airport (VCB), Vacaville, California, about 0742 PDT and arrived at VCB about 0930 PDT. Data downloaded from the airplane's central advisory and warning system (CAWS) showed that, for the flight from REI to VCB, the left and right fuel boost pumps began cycling (that is, turning on for about 10 seconds and then off for about 1 second) about 1 hour 30 minutes into the flight. About 15 minutes later, the left fuel boost pump was on continuously, and the right fuel boost pump was off. The left fuel boost pump remained on continuously for the rest of the flight.
In the "auto" mode the fuel pumps would cycle on whenever the fuel pressure dropped below 2 psi, a possible indication of fuel system icing.
- Fuel records indicated that the airplane had been refueled with about 128 gallons of Jet A fuel after the airplane's arrival at VCB. During a postaccident interview, the VCB airport manager stated that he saw the airplane at the airport's self-service fueling island but did not observe the airplane being refueled. The fuel dispensed at the VCB self-service fueling island was not premixed with a FSII, and the fuel pump did not contain provisions for injecting a FSII during fueling. VCB personnel found no evidence to suggest that the pilot had used any other method to add a FSII to the fuel either before or during the fueling. [The NTSB obtained surveillance video from VCB that showed the accident airplane's arrival at the self-service fueling island. The airplane was visible on the video for about 36 minutes. During that time, the pilot exited the airplane and serviced it with fuel, the pilot and passengers boarded the airplane, and the airplane departed. The video showed no evidence that the pilot had sampled fuel from either of the underwing fuel tank drains or the fuel filter drain.]
The PC-12 AFM, §8, states: "Before the first flight of the day and after each refueling, use a clean container and drain at least one sample of fuel from each tank drain valve to determine if contaminants are present (and that the airplane has been fueled with the proper fuel)." Had this been done at this point, evidence of fuel icing could have been detected.
- The airplane departed VCB about 1020 PDT with nine passengers (four adults and five children) and the pilot on board, although the flight plan indicated that four passengers and the pilot would be on board during that flight leg. The airplane arrived at OVE about 1033 PDT. CAWS data showed no fuel boost pump activity during the 13-minute flight. At OVE, four passengers (two adults and two children) boarded the airplane, resulting in a total of 13 passengers (six adults and seven children, who ranged in age from 1 to 9 years). The flight plan indicated that eight passengers and the pilot would be on board the airplane during the final flight leg. The accident airplane was configured with two pilot seats 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.
- At 1359:28, the pilot contacted the Salt Lake ARTCC sector 6 radar controller. At that time, the airplane was operating at flight level 250, as assigned by air traffic control (ATC). Radar data showed that, at 1402:52, the pilot changed the airplane's route of flight and turned to the left toward BTM without ATC clearance. At 1403:25, the pilot contacted the controller to request a change in destination to BTM but did not provide a reason for the requested divert.
- At 1424:38, the pilot requested a lower altitude, and the controller cleared the airplane to descend and maintain 12,200 feet, which was the minimum IFR altitude at that point in the approach. The pilot acknowledged this clearance. Radar data showed that the airplane descended below 12,200 feet at 1426:49 and continued descending. (The pilot was required to comply with the previous ATC-issued altitude restriction until he had the airport in sight and could provide his own terrain clearance.) CAWS data showed that, about 1427 (2 hours 17 minutes into the flight), the system provided a caution to the pilot indicating that a low fuel condition existed in the right fuel tank.
- At 1428:43, the pilot reported that he had the airport in sight and canceled his IFR clearance. Radar data about that time showed that the airplane was 8 miles southwest of BTM at an altitude of 11,100 feet. At 1428:49, the controller acknowledged the pilot's IFR clearance cancellation, instructed him to squawk visual flight rules (VFR) transponder code 1200, and advised him that no known or observed traffic existed between the airplane and the airport. The pilot did not acknowledge this transmission, and no further communications occurred between the pilot and ATC.
- Radar data showed that the airplane continued to squawk the previously assigned discrete transponder code as the airplane continued toward the airport. BTM has no ATC tower, but an employee at a BTM fixed-base operator (FBO), who was monitoring the common traffic advisory frequency when the accident airplane was approaching the airport, heard the pilot indicate that the airplane would be landing on runway 33. The last recorded radar target, at 1430:25, showed that the airplane was at an altitude of 9,100 feet (3,550 feet above ground level) and about 1.8 miles southwest of the runway 33 threshold.
- The airplane impacted the ground about 2,100 feet west of the runway 33 centerline. The first emergency call reporting the accident to police was received at 1432:26. Witnesses to the accident reported that the airplane was approaching runway 33 at a higher altitude than other airplanes that land at the airport. The witnesses also reported that the airplane had entered a steep left turn at an estimated altitude of 300 feet agl and that the nose of the airplane then pitched down suddenly.
[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, ¶22.214.171.124] 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.
- The low fuel pressure state and the restricted fuel supply from the left tank during the accident flight were the result of an accumulation of ice in the fuel system with an initial concentrated amount of ice at the airframe fuel filter.
- If the pilot had added a fuel system icing inhibitor to the fuel for the flights on the day of the accident, as required, the ice accumulation in the fuel system would have been avoided, and a left-wing heavy fuel imbalance would not have developed.
- About 1 hour 21 minutes into the flight, the fuel supplied to the airplane's engine was being drawn solely from the right fuel tank by the right fuel boost pump, and the left-wing-heavy fuel imbalance continued to increase.
- The left and right fuel tanks were equally receiving fuel through the fuel return lines, but the left-wing-heavy fuel imbalance continued to increase during the flight because fuel was only being drawn from the right fuel tank.
- Although the pilot should have diverted to a nearby airport once the maximum allowable fuel imbalance had been exceeded, the pilot eventually diverted to Bert Mooney Airport likely because he recognized the magnitude of the situation and his attempts to resolve the increasing left-wing-heavy fuel imbalance had been unsuccessful.
- The airplane was controllable in static flight with the left-wing-heavy fuel imbalance that existed at the time of the accident, but the pilot lost control of the airplane with the dynamic maneuvers during the final moments of the flight.
- The pilot underestimated the seriousness of the initial fuel imbalance warnings because he had not experienced any adverse outcomes from ignoring similar previous warnings.
[AAR-11/05, ¶3.2] The National Transportation Safety Board determines that the probable cause of this accident was (1) the pilot's failure to ensure that a fuel system icing inhibitor was added to the fuel before the flights on the day of the accident; (2) his failure to take appropriate remedial actions after a low fuel pressure state (resulting from icing within the fuel system) and a lateral fuel imbalance developed, including diverting to a suitable airport before the fuel imbalance became extreme; and (3) a loss of control while the pilot was maneuvering the left-wing-heavy airplane near the approach end of the runway.
NTSB Aircraft Accident Report, AAR-11/05, Loss of Control While Maneuvering, Pilatus PC-12/45, N128CM, Butte, Montana, March 22, 2009
Pilatus Pilot's Operating Handbook and FOCA Approved Airplane Flight Manual (also FAA approved for U.S. registered aircraft in accordance with FAR 21.29), PC-12 Series, revised 1 September 1984.