What happened
On 25 October 2021, a Boeing 737 aircraft, registered VH-VZT and operated by Qantas, was being prepared for a passenger air transport flight from Perth, Western Australia to Adelaide, South Australia. During a routine external aircraft check (walk-around), the captain identified frost on the wings, resulting from cold fuel from the previous flight in the main tanks. To remove the frost, cold fuel was transferred from the main (wing) tanks to the centre tank, and the main tanks were filled with warmer fuel via refuelling.
Just after reaching the assigned cruise altitude, the flight crew completed a routine procedure to switch the centre tank fuel pumps off and the aircraft began to use main tank fuel. At about 1749, a fuel imbalance alert was displayed, indicating unequal fuel quantities in each main tank. While actioning checklists, the flight crew diagnosed a fuel leak and shut down the left engine. The flight diverted to Kalgoorlie, Western Australia where the aircraft landed without incident. Post-flight inspections determined no fuel leak and the fuel system was serviceable.
What the ATSB found
The licensed aircraft maintenance engineer (LAME) directed the flight crew on the fuel transfer from memory. The procedures required the crossfeed valve to be closed when the operation was completed, but the valve was not closed. This was likely associated with the crew following the LAME's instructions rather than referring to the relevant procedure.
During pre-flight checks, and later during climb and level-off, the flight crew did not notice the crossfeed selector in the open position or the associated dimmed blue indicator light on the fuel panel. In flight, after the centre tank fuel pumps were switched off, fuel was continually pumped from the left main tank to the right engine via the open crossfeed valve due to uneven fuel pump pressures. Although the aircraft's manuals stated this could occur when the crossfeed valve was open in flight and lead to a fuel imbalance, the flight crew did not recall this at the time. The Boeing 737 IMBAL (imbalance) checklist did not provide sufficient guidance for a flight crew to identify an open crossfeed valve as a potential reason for a fuel imbalance. Consequently, the flight crew decided there could be a fuel leak.
Partly as a result of confirmation bias, stress and perceived time pressure with the aircraft approaching an overwater segment, the flight crew abbreviated the checklists. Actions when completing the Fuel leak engine checklist resulted in them mistakenly confirming a fuel leak, but the method used was invalid due to a step performed out of sequence. As a result, the crew unnecessarily shut down an engine during flight.
What has been done
Following the occurrence, Qantas communicated the factors involved to 737 flight crews.
Safety message
Checklists are designed to minimise performance variability under workload and stress, and thereby increase the likelihood all required actions are successfully carried out. The importance of precision when following checklists, especially under stress and time pressure, is highlighted by this occurrence.