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At 1810 Eastern Standard Time on 30 August 2004, a Fairchild Industries Inc. SA226-T Merlin III aircraft, registered VH-SSL, departed Bankstown, NSW on a charter flight to Glen Innes, NSW with the pilot and seven passengers on board.

The pilot reported that he manually flew the aircraft in instrument meteorological conditions during the climb to flight level (FL)160. On levelling at the cruise level, he noticed that the aircraft was flying in a slightly right-wing low attitude. The pilot said that he applied left rudder trim to level the wings and engaged the autopilot. About 2 ½ minutes later, the autopilot suddenly disengaged without warning. The aircraft then rolled rapidly to the right and entered a steep spiral descent. A review of air traffic control radar data indicated that about 50 seconds later, the aircraft levelled at 5,200 ft. After the pilot regained control of the aircraft, he reported that he noticed that the right fuel tank gauge reading was 350 kg (437.5 L) greater than the left fuel tank gauge reading, and that the aircraft was ‘very heavy on the right hand side’. The pilot then climbed the aircraft to FL130 and diverted to Tamworth, NSW without further incident. There were no reported injuries to any of the aircraft’s occupants.

The aircraft’s fuel system included a cross-flow valve that allowed pilots to balance the fuel between the aircraft’s fuel tanks if needed. The Merlin III Aircraft Flight Manual contained the aircraft operating checklists. The BEFORE STARTING ENGINES and DESCENT checklists required that the fuel system cross-flow valve switch be closed. The pilot reported that during the diversion to Tamworth he used the cross-flow valve to reduce the fuel imbalance. A subsequent engineering examination revealed no defects in the aircraft’s fuel tanks, fuel vent systems, the cross-flow system, and the cross-flow valve.

The pilot reported that he conducted the last flight in the aircraft a few days before the occurrence flight. The fuel remaining on board the aircraft after that flight was 500 L, and the right fuel tank contained about 150 L less than the left fuel tank. The pilot believed that he might have used the fuel cross-flow valve during that flight. When the aircraft was refuelled before the occurrence flight, 700 L of fuel was added to the right tank and 550 L to the left tank to give a total fuel load of 1,750 L. The pilot reported that after the refuelling ‘the gauges were pretty well reading the same’.

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