Tuesday, October 4, 2011
Wing heaviness may be a minor annoyance or a genuine threat to safety
The airplane impacted the ground about 2,100 feet west of the runway 33 centerline. Witnesses reported the airplane was at a higher altitude than other airplanes that land at the airport. They 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.
The Pilatus Flight Manual instructed pilots to monitor the fuel quantity gauges during normal operation (with the fuel-boost pump switches in the AUTO position) to verify that the fuel was balanced laterally. The AFM stated that the maximum fuel imbalance for the PC-12 was 26.4 gallons or about 178 pounds. If the fuel-balancing system were unable to automatically maintain fuel balance, the pilot could manually operate the fuel pumps.
The investigation concluded that 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 get worse.
Investigators believe that the pilot most likely recognized that the fuel imbalance wasn't being corrected and, as a result, repositioned the left fuel-boost pump switch from ON to AUTO and the right fuel-boost pump switch from AUTO to ON to observe the effect that the changed switch state would have on the fuel boost pumps' operation. Similar switch-repositioning activities occurred later in the flight, which were attributed to the pilot's attempts to rebalance the lateral fuel load through manual activation of the fuel-boost pumps. Data indicated that the left side wound up with 1,219 pounds more fuel than was on the right side.
To compensate for the fuel imbalance, the pilot was likely using right aileron trim to maintain level flight. It's likely that the pilot had engaged the yaw damper, independent of the autopilot, to counter any yawing and rolling oscillations that the airplane was experiencing.
During examination of the wreckage, the NTSB found the aileron-trim actuator fully extended. This position correlated to full Right Wing Down aileron trim, which would counter a left-roll tendency. The rudder-trim actuator was found fully extended, which correlated to full Nose Left rudder trim. The rudder-trim function of the yaw-damper system had likely trimmed the rudder automatically in the Nose Left direction. The pitch trim was oriented Nose Down, which would have increased the control wheel forces required for the pilot to maintain control of the airplane.
The NTSB determined 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.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.
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