Incident Overview
On 8 March 2024, a GAM Air Aero Commander 500-S, registered VH-MEH, was operating a cargo return flight from Bankstown Airport to Parkes Regional Airport, New South Wales, with intermediate stops. The pilot, on the first sector, discovered the fuel cap was off but secured by a retention chain. After re-securing it, the cap was again found off upon landing at Parkes. A maintenance engineer identified a fault preventing the cap from locking and rectified it.
Later that day, the pilot began the return sectors. Shortly after departing Bathurst for the final leg to Bankstown, both engines lost power. The pilot executed a forced landing in a field. The aircraft sustained no damage and the pilot was uninjured.
ATSB Findings
The Australian Transport Safety Bureau (ATSB) investigation found that during the first two sectors, the fuel cap was incorrectly installed, with the retention chain lodged in the anti-siphon valve, causing the cap to dislodge in flight and siphon fuel overboard. At Parkes, the pilot noticed a discrepancy between expected and indicated fuel quantity but did not refuel or update the flight log. Consequently, the aircraft departed Parkes with insufficient fuel for the remaining flights.
After departing Parkes, the pilot likely did not monitor the fuel gauge, continued calculations based on incorrect fuel quantity, and did not refuel at Bathurst. This led to fuel exhaustion shortly after departure from Bathurst. The ATSB also noted that GAM Air's Quick Turn Around – Pre-Start checklist omitted a fuel quantity check before start, contrary to the aircraft flight manual.
Operator Actions
GAM Air published a pilot notice reinforcing fuel management for the Aero Commander 500 series, emphasizing fuel cap security and potential fuel loss. It provided guidance on fuel planning and log entries. The operator also began periodic audits of pilot fuel calculations and planned to discontinue the Quick Turn Around checklist. The fuel supplier at Bankstown was reminded to allow fuel to settle when refueling.
Safety Message
The ATSB highlighted that an unsecured fuel cap can cause rapid fuel loss even with an anti-siphon valve. Such events should be treated as emergencies, with immediate diversion. In aircraft with rubber fuel cells, fuel gauge readings may overread due to cell deformation. Filling to a visually confirmed quantity is necessary for accurate fuel measurement. The Civil Aviation Safety Authority's Advisory Circular 91-15 emphasizes verifying fuel quantity using at least two methods.
Sequence of Events
Before the flights, the pilot estimated fuel based on visual assessment below the anti-siphon valve level. The quantity was less than full due to insufficient settling during refueling or ground running. Despite likely overestimating by up to 70 L, the aircraft departed Bankstown with adequate fuel for planned sectors.
After finding the cap off at Orange and Parkes, it was likely unsecured in flight during those sectors. The engineer found the cap chain held the anti-siphon valve slightly open, preventing locking, and noted fuel dye staining indicating leakage. After rectification, the cap remained secure. The ATSB analysis indicated about 200 L of fuel siphoned overboard during sectors to Parkes, leaving approximately 143 L on landing—insufficient for the return.
At Parkes, the pilot, aware of the cap issue but unable to visually confirm fuel quantity, identified a discrepancy between calculated and gauge readings. Operator procedures required refueling to full in such cases, but the pilot instead recalculated based on the gauge reading without updating the flight log, incorrectly determining sufficient fuel. The pilot's decision may have been influenced by an expectation that the anti-siphon valve would prevent siphoning and a typical fuel excess on that route.
The ATSB assessed whether the gauge was erroneous. Though CASA guidance suggests possible overreading in rubber cell aircraft, the operator and manufacturer stated this is not possible in Aero Commander due to cell construction. No prior gauge issues were reported, and post-incident checks confirmed correct operation, with minor calibration adjustments for fuller quantities. Thus, the gauge likely indicated correctly at Parkes, showing a significant discrepancy. The pilot's misunderstanding led to departure with insufficient fuel. The pilot then failed to monitor fuel state or refuel at Bathurst, resulting in fuel exhaustion.
The operator's Quick Turn Around checklist lacked a fuel quantity check before start, though this was not considered contributory.