Tipping Point

The outside air was cold enough to cause water dissolved in the Pilatus PC-12/45’s Jet-A fuel to form ice crystals that accumulated on the main fuel filter and built up sufficiently on components in the left wing tanks to block the flow of fuel from the tanks. The airplane became increasingly left-wing-heavy as excess fuel returned by the engine was added to the fuel trapped in the left tanks while the right tanks continued feeding the engine.

The pilot pressed ahead toward the planned destination until he apparently realized that his efforts to balance the fuel and to correct the low-fuel-pressure condition were not working. He diverted the flight to an alternate airport but lost control of the airplane while maneuvering to land. The PC-12 crashed near the runway, killing all 14 people aboard.

In its final report on the March 22, 2009, accident, the U.S. National Transportation Safety Board (NTSB) said that the probable causes were “the pilot’s failure to ensure that a fuel system icing inhibitor was added to the fuel [and] his failure to take appropriate remedial actions after a low-fuel-pressure state (resulting from icing within the fuel system) and a later fuel imbalance developed.”

The accident airplane was operated by a company owned by three partners. Media reports said that the passengers on the accident flight were family members en route to a resort near Bozeman, Montana, U.S., for a week of skiing.

The company’s contract pilot, 65, held an airline transport pilot license and had 8,840 flight hours, including 1,760 hours in PC-12s. He had retired from the U.S. Air Force as a transport and instructor pilot in 1972, and had flown for several airlines and as a PC-12 pilot for an air ambulance operator before being hired by the company in 2002.

A former chief pilot for the air ambulance operator told investigators that the pilot was “extremely knowledgeable” about the airplane. A PC-12 training center instructor said that the pilot had demonstrated “superb” judgment and a “very high level” of competence.

PC-12 Fuel System

A brief description of the airplane’s fuel system might aid in understanding the circumstances that led to the accident.

The system holds a maximum of 2,704 lb (1,227 kg) of usable fuel in a main tank and a collector tank in each wing. Ejector pumps transfer fuel from the wing tanks to the engine. By design, more fuel is supplied to the engine than is necessary for combustion, and excess fuel is sent back to the tanks via return lines.

Electric boost pumps are activated automatically or manually to correct a low-fuel-pressure condition, typically caused by filter blockage or the failure of an ejector pump or the engine-driven fuel pump, or to balance the fuel in the wing tanks.

Fuel quantity is indicated by two vertical arcs, each comprising 28 “bars,” or liquid crystal display segments. Each bar represents about 48 lb (22 kg) of fuel. The PC-12 airplane flight manual (AFM) states that if an imbalance of three bars is indicated and cannot be corrected, the pilot should “land as soon as practical.”

Of particular note is that the AFM requires an icing inhibitor to be blended with the fuel for all operations in ambient temperatures below freezing.¹

“On a standard day, the temperature is 0 degrees C [32 degrees F] at 7,500 ft, so most PC-12 flights would require the use of an [icing inhibitor],” the report said. “All jet fuels contain trace amounts of water, and a fuel system icing inhibitor lowers the freezing point of water to minus 46 degrees C [minus 51 degrees F] to prevent the water from turning into ice crystals, which can block a fuel line or filter.”

The report noted, however, that refueling records for the accident airplane indicated that the pilot did not always ensure that an icing inhibitor was added. He did not request an icing inhibitor when the airplane was refueled at its home base in Redlands, California, the day before the accident.

Too Many People

The accident flight from Redlands to Bozeman comprised three legs, with stops in Vacaville, California, and Oroville, California, to pick up passengers.

The pilot departed from Redlands at 0742 local time. The average outside temperature recorded by the engine trend-monitoring system was minus 24 degrees C (minus 11 degrees F) at the cruise altitudes, Flight Level (FL) 260 (approximately 26,000 ft) and FL 220.

After arriving in Vacaville at 0930, the pilot used the airport’s self-service facility to refuel the PC-12. Investigators found no evidence that an icing inhibitor was blended with the fuel.

The instrument flight rules (IFR) flight plan filed by the pilot for the next leg of the trip listed five occupants. However, there were 10 people aboard when the airplane departed from Vaca­ville at 1020. The average outside air temperature at the cruise altitude, 6,000 ft, was minus 4 degrees C (25 degrees F).

Four more passengers boarded the airplane at Oroville. Although the pilot’s IFR flight plan listed nine people, seven adults and seven children ranging in age from 1 to 9 were aboard the 10-seat airplane when it departed for the flight to Bozeman at 1210 local time.

“At least four of the seven children on board the airplane were not restrained or were improperly restrained,” the report said. “After the accident, one of the owners of the airplane (who organized the flights) stated that the airplane had carried the same number of adult and child passengers on previous flights.”

The limitations section of the PC-12 AFM specifies that the maximum number of passengers is nine. The report noted, however, that the U.S. Federal Aviation Administration (FAA) allows a child under age 2 to be held on an adult’s lap. The FAA also permits two people to occupy one seat in a noncommercial airplane if their total weight does not exceed 170 lb (77 kg) and they can be secured properly by the seat belt.

There was no evidence that the pilot performed weight-and-balance computations for any leg of the trip. Investigators estimated that the airplane was within center-of-gravity limits for all three legs but was over the maximum takeoff weight by 432 lb (196 kg) on departure from Vacaville and by 572 lb (259 kg) on departure from Oroville.

The flight plan showed an estimated time en route of 2.5 hours with 3.5 hours of fuel aboard for the leg from Oroville to Bozeman. Shortly after departure, the pilot was cleared by air traffic control (ATC) to navigate directly to Bozeman.

The average outside air temperature at the cruise altitude, FL 250, was minus 40 degrees C. Data recorded by the PC-12’s central advisory and warning system (CAWS) showed that the boost pump in the left collector tank operated nearly continuously beginning about an hour after departure, a sign that ice was restricting the flow of fuel from the left tanks, causing a fuel imbalance and contributing to the low-fuel-pressure condition.

The boost pump in the right collector tank also operated intermittently in response to the low-fuel-pressure condition caused by the partial blockage of the filter and by the decreasing flow of fuel from the left tanks.

At 1335, or about an hour and 15 minutes after departure, the fuel quantity indicator showed a three-bar differential. “About 1 hour 21 minutes into the flight, the fuel supplied to the airplane’s engine was being drawn solely from the right fuel tanks by the right fuel boost pump, and the left-wing-heavy fuel imbalance continued to increase,” the report said.

Despite the low-fuel-pressure condition, however, the engine operated normally throughout the flight.

Self-Induced Pressure

The pilot had told a training center instructor that he never felt company pressure to fly in unsafe conditions. NTSB concluded that his decision to continue toward Bozeman rather than to land at one of several suitable alternate airports that were available as the fuel imbalance worsened likely was influenced by self-induced pressure to avoid inconvenience to his passengers.

However, about two hours into the flight, the pilot likely “recognized the magnitude of the situation” and requested clearance from ATC to divert to Butte, Montana. He did not provide a reason for the request, and ATC did not question it.

Investigators were unable to determine why the pilot chose to land at Butte, rather than at a closer alternate airport. Moreover, at this point, the distances to Bozeman and to Butte were similar, and the weather conditions were nearly identical, with 10 mi (16 km) visibility, a broken ceiling at about 6,000 ft and winds from the northwest at 8 kt.

Soon after ATC approved the request to divert to Butte, the pilot asked for clearance to descend. The controller issued the altimeter setting for Butte and cleared the pilot to descend at his discretion to 14,000 ft. The report noted that the minimum IFR altitude for the area was 13,100 ft.²

About 10 minutes later, at 1422, the pilot was cleared to descend to 13,000 ft and to report when he had the airport in sight. The pilot acknowledged the instruction, requested a lower altitude and was cleared to descend to 12,200 ft, the minimum IFR altitude.

‘Extreme’ Imbalance

Shortly thereafter, however, the airplane descended below the assigned altitude. At this point, the fuel quantity indicator showed a 22-bar differential, which was characterized by the report as an “extreme” imbalance.

At 1427, the controller advised the pilot that the airport was 12 nm (22 km) ahead and asked if he had the field in sight. The pilot responded, “Yeah, as soon as we get past one more cloud.”

Recorded ATC radar data showed that the PC-12 was at 11,100 ft and 8 nm (15 km) southwest of the airport when the pilot reported that he had the field in sight and canceled his IFR clearance.

The Butte airport is an uncontrolled field located in a valley at 5,550 ft in mountainous terrain. The pilot reported on the common traffic advisory frequency that he intended to land on Runway 33. “The last recorded radar target, at 1430:25, showed that the airplane was at an altitude of 9,100 ft (3,550 ft above ground level) and about 1.8 nm [3.3 km] southwest of the Runway 33 threshold,” the report said.

A witness said that as the airplane neared the runway, it appeared to be much too high to land. The witness said that the airplane then flew northwest, away from the runway, entered a steep left turn about 300 ft above the ground, pitched nose-down and descended rapidly.

The PC-12 crashed and burned in a cemetery about 2,100 ft (640 m) west of the approach end of the runway. CAWS data indicated that the left wing tanks were filled to capacity and that the right wings tanks contained only 66 lb (30 kg) of fuel on impact.

Investigators were unable to identify the source of the restriction to the flow of fuel from the left tanks. “Ice accumulation in the fuel system … could degrade the performance of many fuel system components, including the fuel boost pumps and valves,” the report said.

“If the pilot had added [an ice 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.”

The accident investigation generated several recommendations (see p. 11). Among them was that the FAA and the European Aviation Safety Agency should raise pilot awareness about manufacturers’ requirements to blend an icing inhibitor with jet fuel and about the potential consequences of noncompliance. 

This article is based on NTSB Accident Report NTSB/AAR-11/05, “Loss of Control While Maneuvering; Pilatus PC-12/45, N128CM; Butte, Montana; March 22, 2009.” The report is available at ntsb.gov/investigations/AccidentReports/Pages/aviation.aspx.

Notes

1. The most common fuel system icing inhibitor is diethylene glycol monomethyl ether, known by its trade name, Prist.
2. The FAA defines minimum IFR altitude as “1,000 ft [or 2,000 ft in mountainous areas] above the highest obstacle within a horizontal distance of 4 nm [7 km] from the course to be flown.”