What happened
During a series of flight sectors, an Aero Commander experienced a significant loss of fuel due to an improperly secured fuel cap. Following a visual assessment of fuel levels prior to departure from Bankstown, the aircraft flew to Orange and then to Parkes. During these initial legs, the fuel cap was not properly locked, as the cap chain was obstructing the anti-sally valve. This prevented the locking mechanism from engaging and allowed the cap to become unsecured during flight.
As a result of the unsecured cap, approximately 200 L of fuel siphoned out of the tank while in flight. Upon arrival at Parkes, the pilot noted a discrepancy between the calculated fuel remaining and the actual gauge reading. Although the operator's procedures required refueling to a known quantity when such discrepancies occur, the pilot chose to recalculate the fuel state based on the gauge reading without refueling. This decision was likely influenced by an expectation that the anti-siphon valve would prevent further loss.
After departing Parkes, the aircraft subsequently flew to Bathurst. The pilot failed to effectively monitor the fuel state or perform a necessary refueling at Bathurst. Consequently, the aircraft suffered fuel exhaustion shortly after departing Bathurst.
The investigation
Investigators examined the fuel system and the maintenance history of the aircraft. A maintenance engineer in Parkes discovered that the fuel cap chain had been interfering with the anti-siphon valve, which prevented the cap from locking. Evidence of fuel dye staining was also found, confirming a leak had occurred.
Technical analysis focused on whether the fuel quantity gauge was providing inaccurate readings. While there was consideration of whether the fuel cell construction could cause the sender to overread, the manufacturer and operator confirmed this was not possible for this Aero Commander model. Post-incident testing confirmed the gauge was functioning correctly, meaning the pilot was presented with an accurate, albeit confusing, indication of low fuel that was not properly addressed.
Findings
- The fuel cap was incorrectly installed, with the chain lodged in the anti-siphon valve, which allowed the cap to dislodge and permitted fuel to siphon overboard.
- The pilot identified a discrepancy in fuel readings at Parkes but did not follow procedures to refuel the aircraft to a known quantity.
- The pilot's decision-making was impacted by an incorrect assumption that the anti-siphon valve would mitigate fuel loss.
- The operator's 'Quick Turn Around' checklist was abbreviated and lacked a mandatory fuel quantity check, which might have otherwise identified the discrepancy during a period of lower workload.