Plane & Pilot
Tuesday, October 4, 2011

Fuel Imbalance

Wing heaviness may be a minor annoyance or a genuine threat to safety

The airplane departed OVE for BZN on an IFR flight plan that showed three hours 30 minutes of fuel on board with an estimated en route time of two hours 30 minutes. Data recovered by investigators revealed that about one hour and 13 minutes into the flight, the left and right fuel-boost pumps began cycling. The airplane had an automatic system in which the fuel-boost pumps would come on to help balance the fuel in the wing tanks and maintain fuel pressure. Investigators believe that fuel pressure was being affected by the buildup of ice on the fuel system.

While the airplane was at FL250 and being handled by the Salt Lake ARTCC, the pilot changed the airplane's route of flight and turned to the left toward his filed alternate of Butte (BTM) without ATC clearance. Then, the pilot asked for a change to BTM but didn't provide a reason for the request. The controller cleared the airplane direct to BTM, and instructed the pilot to maintain FL250.

Radar data showed that the airplane began a descent from FL250. About 25 seconds later, the pilot contacted the controller to request a lower altitude. The controller issued the altimeter setting for BTM, and cleared the airplane to descend to 14,000 feet MSL which was just above the 13,100-foot minimum IFR altitude for the area.

The pilot acknowledged, but then radioed a second request to change destination to BTM. The controller reminded the pilot that he was cleared to BTM, and the pilot responded by stating that the airplane was descending to 14,000 feet. The controller told the pilot to "advise receipt of Butte, Montana, weather and NOTAMs." The pilot responded, "Wilco," but never did report receipt of BTM weather and NOTAMs. When the controller was relieved from his position as part of a normal rotation schedule, he didn't tell the relief controller that the pilot had not reported receipt of weather and NOTAM information for BTM.

The next controller cleared the flight to 13,000 feet. The controller told the pilot to report when he had the airport in sight. The pilot acknowledged. When the pilot requested a lower altitude, the controller cleared him to 12,200 feet, which was the minimum IFR altitude at that point in the approach. The pilot acknowledged.

Radar data showed that the airplane descended below 12,200 feet and continued descending, even though the pilot was supposed to stay at or above 12,200 until he had the airport in sight and could provide his own terrain clearance. Recovered data showed that 2 hours and 17 minutes into the flight, the airplane's warning system provided a caution to the pilot indicating that a low fuel condition existed in the right fuel tank.

The controller advised the pilot that BTM was at 12 o'clock and 12 miles, and asked the pilot whether the airport was in sight. The pilot responded, "Yeah as soon as we get past one more cloud." Then, the pilot reported that he had the airport in sight and canceled IFR. The controller told the pilot to squawk VFR, and that there was no traffic between the airplane and the airport. The pilot didn't acknowledge this transmission, and there was no further contact. Radar data showed that the pilot didn't switch the transponder to the VFR code of 1200. An employee at an FBO heard the pilot radio on Unicom that the airplane would be landing on runway 33.


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