History of the flight
The Piper Aircraft Corporation PA-31-350 Navajo Chieftain, registered VH-UBC, departed Albury on a charter flight with a pilot and six passengers on board. About 5 minutes into the flight, as the aircraft climbed through approximately 5,000 ft, the pilot reported that the right fuel flow light illuminated. The pilot moved the right engine mixture control lever to full rich and advised the Albury Tower controller that he was returning to Albury. A short time later, the right engine started surging. The pilot reported that he changed the right fuel selector from the inboard to the outboard tank selection, although he was aware that there was only a small quantity of fuel in that tank. The engine continued to surge and he reselected the inboard tank. The pilot reported that he did not shut down the engine and feather the propeller because he thought the engine was producing some power.
The pilot reported that approximately a minute after the onset of the right engine problem, the left fuel flow light illuminated and the left engine also started surging. He advised the controller that he was diverting to Holbrook. The pilot found a break in the clouds and descended the aircraft, maintaining visual contact with the ground. On levelling out after the descent through cloud, he reported that the engines operated smoothly, but at reduced power. He reported that he maintained blue line speed for a short time, before power reduced to a level which would not allow altitude to be maintained. During the descent, the pilot opened the crossfeed valve and checked that all fuel pumps were on, mixture controls were rich and the inboard tanks selected. Unable to restore power, the pilot decided to make an emergency landing in an open field below the aircraft. Before landing, the pilot extended the flaps and the landing gear and instructed the passengers to prepare for an emergency landing.
The aircraft contacted the ground with its right wingtip and slewed for approximately 93 metres while rotating almost 180 degrees to the right. The aircraft was substantially damaged, but there was no fire. Neither the pilot nor the passengers sustained any injuries. The pilot reported that before exiting the aircraft he switched off the fuel pumps, magnetos and master switches.
Weather
The weather at Albury and the surrounding area was overcast with low cloud and fog patches. Rain and isolated thunderstorms were forecast for the area. Moderate icing was forecast above 10,000 feet.
Pilot qualification
The pilot held a Commercial Pilot Licence (Aeroplane) with a Multi-engine Command Instrument Rating and was appropriately endorsed on the aircraft type. He held a valid Class 1 medical. On 4 August 2003 he satisfactorily completed a type proficiency check on the Chieftain aircraft. Although not a training and checking requirement for charter flights, the operator additionally stipulated this check for its pilots.
Aircraft
The aircraft was owned by a locally based company that used it to transport its employees between numerous production facilities and was operated by an Albury based charter and training operator on their behalf.
The aircraft was maintained in accordance with the maintenance requirements applicable at that time and had a valid Maintenance Release. It had flown approximately 50 hours since the last maintenance release (periodic) inspection. There were no outstanding maintenance issues at the time of the accident.
Fuel status
The aircraft had flown during the previous day, returning to Albury late that afternoon. The pilot who flew the aircraft that day reported that the aircraft and its systems operated normally. He refuelled the aircraft for the next day's flying, filling only the inboard tanks. No fuel was added to the outboard tanks and the pilot estimated that about 25 to 35 litres remained in each outboard tank.
The rostered pilot reported that, on the morning of the occurrence, he carried out a preflight check during which he visually inspected the fuel tank contents. The pilot reported that he found both inboard tanks full, but could not see any fuel in the outboard tanks. In accordance with the operator's practice, the pilot started and warmed the engines so that the flight could proceed without delay when the passengers arrived.
The `Before starting engine' and `Before take-off' checklist procedures required the pilot to check that the fuel selector valves were selected to INBOARD tanks. Both the manufacturer's and the operator's `Before taxiing' checklist procedures then required the pilot to check the fuel selector at each detented position. The operator reported that pilots were encouraged to check the operation of the fuel selector valves in all detented positions during the engine warm up run. The pilot reported that in order to conserve the fuel in the inboard tanks for the trip, he preferred to warm the engines using the fuel from the outboard tanks.
The departure had been delayed due to fog at the destination. The flight commenced approximately two hours later than planned, when the fog cleared. The pilot reported that he carried out another engine warm up with the passengers aboard and had checked the INBOARD tanks selection before take-off.
Fuel system
The aircraft fuel system consisted of four fuel cells, two in each wing, and had a total capacity of 734 litres, of which 690 litres were useable. It was also fitted with two long-range nacelle tanks that were not used on this flight. The inboard tanks each had a capacity of 212 litres and each of the outboard tanks 155 litres. The tanks were a bladder type and were located between the main and the rear wing spars. The filler caps were located at the outboard end of each tank. The slenderness of the tanks and the wing dihedral resulted in the fuel accumulating at the inboard end of each tank. That meant that unless the tank was full, or nearly full, it was not possible to assess the quantity of fuel remaining in the tank by visual inspection or by dipping through the filler cap opening.
The left and the right wing fuel systems were independent. Two fuel selector valves, one for each wing's fuel system, allowed the pilot to select either OUTBOARD, OFF or INBOARD positions. When a tank was selected, the fuel was fed to the respective engine system. A crossfeed line with a crossfeed valve was the only interconnection between the two wing fuel systems. The crossfeed valve is normally closed.
Wreckage Examination and Component Testing
The aircraft sustained substantial damage during the emergency landing. The right wing was damaged, the landing gear was torn off and both propellers damaged. Damage to the blades of both propellers was almost identical and was consistent with them rotating at impact. Calculations based on an aircraft speed of about 110 kts, as reported by the pilot, and propeller blade slash marks at the initial point of impact, indicated that both propellers were rotating at approximately 1,830 RPM at impact.
When visually inspected through the filler caps shortly after the accident, both inboard tanks were full, but no fuel could be seen in the outboard tanks. The investigation determined that there were 210 to 211 litres of fuel in each inboard tank. The right outboard tank contained approximately 25 litres and the left outboard tank approximately 1 litre of fuel.
Detailed examination of the tanks and the fuel system found no evidence of flow restriction or the presence of any foreign material inside the system. Fuel samples taken from the aircraft were tested and found to comply with the respective fuel specification. The fuel was of the correct type and grade for the aircraft.
Both engines were removed from the aircraft. When tested, they operated normally in accordance with the manufacturer's test schedule. There was no evidence of any defect that would have accounted for the reported malfunction.
The fuel system and its components were tested in situ, and found to operate normally. The selector valves and the pumps were removed and tested. Operation of all but the left high-pressure fuel pump was normal.
The left high-pressure fuel pump failed to deliver the required pressure and fuel flow and was found to leak at the rate of about 1/4 litre per minute. The test facility specialist reported that the possibility of the fault resulting from damage during the accident could not be excluded.
There was no evidence of any other abnormality of the individual fuel system components and controls.
Fuel consumption
Calculated fuel consumption, based on the manufacturer's Take Off and Climb performance charts, determined that from the time the aircraft commenced take-off to its emergency landing, approximately 32 litres of fuel would have been consumed. In addition, 12 litres of fuel was estimated to have been used during the engine warm ups and taxiing.