Incident Overview
On 5 June 2025, a Piper Chieftain PA-31, registered VH-PGO and operated by Shine Aviation, was conducting a passenger transport flight from Carnarvon to Meekatharra, Western Australia. During the approach to Meekatharra Airport, the R LOW FUEL FLOW warning light illuminated, followed shortly by the R FUEL BOOST INOP warning light and surging of the right engine. The pilot performed memory item initial engine failure checks and feathered the right propeller. The approach continued and the aircraft landed without further incident.
Fuel Loss Investigation
Photographs taken by the pilot after landing showed fuel staining immediately aft of the right inboard fuel cap, consistent with fuel leaking from the cap in flight. The pilot observed that the right inboard tank was emptied earlier than expected based on calculated consumption, and that the tank cell had collapsed upwards towards the cap. These indicators suggested that fuel had likely been siphoned through the cap in flight. The pilot also reported that the fuel cap latch was difficult to lift, indicating possible incorrect seating.
During the post-occurrence inspection, the chief engineer found no evidence of other fuel leaks around the right inboard tank. The pilot had removed and reinstalled the cap prior to this inspection, preventing the engineer from determining exactly how the cap was incorrectly fitted. The engineer was unable to reproduce an incorrect installation. However, it was considered likely that the cap had been incorrectly secured, with the attaching lanyard potentially interfering with the cap and seal. No damage was observed to the cap, seal, or lanyard. The engineer replaced the O-ring as a precaution, though the removed O-ring was deemed serviceable. Subsequent flights, including the immediate post-occurrence flight with the engineer observing the cap, did not reproduce the leak. Since the leak did not recur, it was almost certain the cap was incorrectly secured in Carnarvon.
As part of troubleshooting, the engineer refuelled the right inboard tank with 204 L, while the left tank required only 67 L to fill. Given similar usage and accounting for fuel used by the left engine after the right stopped, the discrepancy indicated that approximately 131 L of fuel had been lost through the fuel cap from the right inboard tank.
Fuel Cap Inspection
Shine Aviation’s Policy and Procedures manual required the pilot in command to ensure that fuel caps were securely fitted. The pilot’s operating handbook specified that the pilot in command must visually check the fuel supply and confirm that the fuel caps are secure. Checking the fuel quantity required removing the caps and physically handling them to verify correct fitment. Although the pilot stated that they visually inspected the fuel cap, a closer inspection or physical check would likely have aided in identifying the incorrect installation. Consequently, the pilot did not detect that the right inboard fuel tank cap was incorrectly secured.
Gauge Reading Anomaly
During flight, the rate of fuel siphoning through the cap was likely sufficient to overcome the vent’s ability to equalise pressure in the fuel tank, resulting in negative pressure that collapsed the fuel cell. The cell was secured by a single cord around the upper perimeter, and the collapse caused the base of the cell to lift towards the filler port. One of the right inboard fuel tank float-style sender units was located near the filler port, and it is likely that the lifting floor raised the outboard sender unit’s float, leading to an overreading of the cockpit fuel quantity gauge. ATSB occurrence brief AB-2021-009 identified a similar failure mechanism where fuel cell collapse caused gauge overreading. The Civil Aviation Safety Authority’s advisory circular AC 91-25, section 6, also contained information about collapsing fuel cells interfering with gauge accuracy.
Contributing Factors
The fuel cap on the right inboard fuel tank was almost certainly incorrectly secured, leading to fuel being siphoned overboard in flight. Additionally, the pilot did not check that the fuel caps were secured as required by the pilot’s operating handbook and company procedures, resulting in the aircraft departing with the right inboard fuel cap almost certainly incorrectly secured.