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The aircraft had been assessed as having 280 L of fuel on board prior to departure from Canberra, based upon the delivery pilot's calculation. ATSB calculations, using the operator's own fuel planning figures, indicated that the amount of fuel consumed during the 1.8 hour delivery flight from Tamworth would likely have equated to a fuel remaining figure of approximately 265 L. In addition, no allowance was able to be determined for the amount of fuel consumed during the engine ground runs at Tamworth, between the time of the last recorded refuel and the delivery flight. Accordingly, the actual fuel tank contents on departure from Tamworth for the delivery flight could not be accurately determined and would probably have been an amount less than the maximum useable.

While planning for the accident flight utilising a computer-based application, the pilot used an incorrect figure (465 L) with respect to total versus useable fuel tank contents (454 L). That led the pilot to include 11 L of fuel, which was unavailable for engine consumption. In addition, he omitted to plan an allowance for fuel consumption during the approximately 12 minute Sydney Harbour scenic flight. Using the operator's fuel consumption planning figure, this may have involved consumption of up to 20 L of additional fuel.

The pilot's use of the computer-based flight planning application included calculations for aircraft weight and balance. His utilisation of the full fuel planning figure in these calculations, instead of the assessed fuel load figure of 280 L, may have led to an erroneous perception that he would be overweight following embarkation of the five passengers at Bankstown, if he added fuel prior to takeoff from either Bankstown or Canberra. Had the pilot used the assessed fuel tank content figure, he would have realised that he could have added fuel and remained within aircraft weight limitations.

The manufacturer's fuel usage figures are determined using a recommended technique for leaning of the fuel mixture supplied to the engines. During the accident flight, the pilot deviated from the recommended technique. That deviation would likely have increased the engine fuel consumption.

Technical investigation of the right engine indicated that it was capable of normal operation. Examination of the left engine indicated that it should still have supplied at least partial power in flight. The failure mode of the exhaust valve rocker stud and the bending of the inlet valve pushrod were examined in consultation with a representative of the engine manufacturer and engine overhaul specialists. Although an exact reason for the failure could not be determined, it is possible that the damage may have occurred following the loosening of one or both of the rocker pivot retaining nuts. As the nut(s) loosened during engine operation, the exhaust valve rocker would have lifted, preventing the exhaust valve from opening. During the valve overlap stage of the engine operating cycle, prior to the start of the induction stroke, the ignited and expanding combustion gasses may have been unable to escape through a now closed exhaust valve. That scenario would have greatly increased internal cylinder pressure which could have prevented the inlet valve opening and, consequently, resulted in the bending of the inlet valve push rod as it tried to move the valve. The damage to the inlet valve pushrod was also considered to have been consistent with the rotation of the engine with a "hydraulic lock" situation existing in the cylinder. Such a condition may occur in the cylinder if the engine was overprimed prior to an engine start.

It is likely that the engine failures occurred as a result of fuel exhaustion.

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