The Australian Transport Safety Bureau (ATSB) investigated this occurrence in accordance with the Transport Safety Investigation Act 2003, for the sole purpose of improving transport safety. It is not the object of an ATSB investigation to determine blame, provide a means of determining liability, or to assist in court proceedings between parties.
FACTUAL INFORMATION
Sequence of events
On 10 August 2004, the pilot of a Piper Aircraft Corporation PA-31-350 (Chieftain) aircraft, registered VH-MZV, was conducting a visual flight rules charter flight from Darwin to Bathurst Island1 and return. The pilot was the only person onboard for the return to Darwin.
As the aircraft approached Darwin, air traffic control cleared the pilot to make a visual approach to runway 11. Recorded radar data indicated that at 5 NM the pilot turned onto the left base leg of the circuit at an altitude of about 1,000 ft. At about that time the pilot broadcast a Mayday2, indicating that both engines had failed. He landed on tidal mudflats to the west of the airport.
Damage was sustained to the propellers and lower fuselage skins and bulkheads. The pilot was not injured.
The pilot recalled that the flight from Bathurst Island had been normal and that he had leaned the engines during the cruise at 3,500 ft. Turning onto the base leg, he reduced airspeed, extended the first stage of flap, lowered the landing gear and completed the pre-landing checklist. As the aircraft's speed reduced, the pilot moved the throttle controls forward, but neither engine responded. There was no significant yawing or rolling associated with the power loss. The pilot considered that both engines lost power simultaneously without any surging, misfiring, rough running or abnormal vibrations. He checked the position of the fuel tank selectors, confirmed that the mixture controls were positioned to full rich, and that the fuel boost pumps and magnetos were on. When those actions did not restore engine power, the pilot retracted the landing gear and concentrated on landing on the mudflats.
Aircraft examination
In accordance with normal procedures, the Australian Transport Safety Bureau (ATSB) placed a protection order3 on the aircraft to preserve the physical evidence while investigators travelled to Darwin. Staff from the Darwin office of the Civil Aviation Safety Authority attended the site and supervised, on behalf of the ATSB, the relocation of the aircraft from the tidal mudflats to dry land. That necessitated removal of the engines from the airframe, and each was separately lifted out by helicopter. The aircraft and its engines were cordoned off at a vacant block of land and a security guard was in attendance until investigators examined the aircraft.
The propellers had continued to windmill after the engines failed, operating the engine-driven hydraulic pumps, which provided hydraulic pressure to enable retraction of the landing gear.
Both propellers exhibited damage consistent with little or no power being produced by the engines during the wheels-up landing. Neither propeller had been feathered. The wing flaps were not extended.
The quantity of fuel contained in each inboard tank was not sufficient to enable the tank contents to be visually assessed through the filler neck. Fuel from the inboard tanks would not flow under gravity from the engine firewall fuel line fitting when the fuel line was disconnected. A check of another Chieftain fuelled to a similar quantity confirmed that this was a normal characteristic of the fuel system design.
A total of approximately 236 L of fuel was recovered from the aircraft's fuel tanks, which comprised 101 L from the left inboard tank (main), 105 L from the right inboard tank (main) and about 15 L from each outboard tank. Each inboard tank had a capacity of 212 L, including about 11 L unusable. The capacity of each outboard tank was 152 L, including about 8 L unusable.
Inspection of the fuel system confirmed the correct operation of the fuel selector controls, the fuel filters were clear of contaminants and the tank venting system functioned normally. No water or other contaminant was recovered from the fuel system drains and collector points. There was no evidence to indicate that fuel had been lost from the aircraft during flight.
Inspection of the engines and their systems did not identify any defect that may have influenced the circumstances of the occurrence. The engines and their accessories operated normally in an engine test stand.
Fuelling records
Trip and fuelling records indicated that the inboard fuel tanks were fully fuelled (capacity 402 L usable fuel) during the morning of 9 August 2004. Fuel consumption of 180 L was recorded during a subsequent two-sector return flight of 1.1 hours duration and 222 L of fuel remained in the inboard tanks. That pilot reported that he did not use the outboard tanks during those flights.
The occurrence pilot requested that the aircraft be refuelled with 80 L on the morning of the occurrence. Fuelling records indicated that this fuel was delivered to the aircraft. The pilot estimated that after refuelling, the inboard tanks contained about 300 L, sufficient fuel to complete the return flight to Bathurst Island.
The aircraft had flown 0.6 hours since departing Darwin and was completing the second sector at the time of the occurrence. Based on the trip records and the calculated fuel consumption, the aircraft main tanks should have contained about 160 L.
The pilot reported that on the day of the occurrence he did not operate either tank on 'crossfeed'4, nor did he select the outboard tanks.
Fuel testing
Laboratory analysis of fuel samples from the aircraft's fuel tanks confirmed that the fuel complied with the relevant specifications for AVGAS 100. Although minor traces of water and fine particulates were detected, that sample was obtained from a fuel line disconnected during recovery of the aircraft. The quantity of those contaminants was not significant.
Descent profile
Analysis of the radar data indicated that the aircraft's descent profile was normal until reaching left base for runway 11. Soon after the turn onto the base leg, the aircraft's groundspeed reduced significantly, and the rate of descent increased. The final descent to the emergency landing area took about 1 minute.
Data from the aircraft manufacturer indicated that the aircraft's glide ratio was approximately 12.5:1 and accordingly, from a height of 1,000 ft, the aircraft could glide about 2 NM.5 The investigation concluded that the aircraft was not within gliding range of the airport when the engines failed.
Operational information
The pilot had logged approximately 1,000 hours on PA-31 type aircraft. The pilot also regularly operated the company's Embraer E110-P1 Bandeirante aircraft in single pilot operations. The pilot's practice was to select the Bandeirante's engine fuel condition levers to LO IDLE during his pre-landing checks. The configuration of those engine controls was similar to that of the Chieftain's fuel mixture controls. Moving the Chieftain's mixture controls to a position consistent with LO IDLE for a Bandeirante would stop the flow of fuel to both engines and result in a sudden and complete loss of engine power.
- Bathurst Island is located approximately 40 NM north-west of Darwin.
- Mayday is an internationally recognised call for urgent assistance.
- A protection order is issued under the provisions of section 45 of the Transport Safety Investigation Act 2003 for the purpose of protecting evidence that might be relevant to an investigation.
- Crossfeed describes an abnormal operating configuration, where fuel from tanks on one side of the aircraft is provided to the opposite engine, therefore operating both engines from the same tank of fuel.
- In nil wind, at the airspeed for best glide, wheels and flap retracted and propellers of both inoperative engines feathered.