Figure: Available airports about the time of the diversion to Butte, from AAR-11/05, figure 7.

Eddie Sez:

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.


Accident Report


Narrative

[AAR-11/05, ¶1.1]


Analysis

[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.

[AAR-11/05, ¶2.2.3]

[AAR-11/05, ¶2.2.6]

[AAR-11/05, ¶1.6.5.3] 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).

[AAR-11/05, ¶1.6.5.4]

[AAR-11/05, ¶1.16.4]


Probable Cause

[AAR-11/05, ¶3.1]

[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.


See Also:


References

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.