An open crossfeed valve on a Qantas Airways Boeing 737-800 led to a fuel imbalance in flight, with the crew misidentifying the problem as a fuel leak, and then shutting down an engine and diverting.

The incident, which the Australian Transport Safety Bureau labels as ‘serious’, occurred on 25 October 2021 as the aircraft, VH-VZT, operated a scheduled service from Perth to Adelaide.

Qantas 737-800

Source: Wikimedia Commons

A Qantas 737-800 on final approach to Perth in October 2021

Prior to departing Perth in the late afternoon, the captain conducted a walkaround and saw frost on the wings caused by cold fuel in the main tanks. As this presented a safety risk, the crossfeed valve was opened, moving cold fuel from the wings into the centre tank. The wing tanks where then filled with warm fuel.

“During pre-flight checks, and later during the 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,” says the ATSB’s final report into the incident.

“In flight, after the centre tank fuel pumps were switched off, fuel had been continually pumped from the left main tank to the right engine via the open crossfeed valve as the result of uneven fuel pump pressures.”

The aircraft’s manual indicated that this could cause a fuel imbalance, but the crew did not recall this fact. Also, the aircraft’s imbalance checklist provided “insufficient guidance.”

This resulted in the crew misidentifying the issue as a fuel leak. They shut down the left-hand engine and diverted to Kalgoorlie.

“Partly as a result of confirmation bias, stress and perceived time pressure with the aircraft approaching an overwater segment of the flight, the flight crew abbreviated the relevant checklists,” adds the ATSB.

“Flight crew actions when completing the fuel leak engine checklist resulted in them mistakenly confirming the presence of a fuel leak, but unknown to the flight crew at the time, the method used to determine the presence of a fuel leak was invalid due to a step being inadvertently performed out of sequence.”

The ATSB says that the incident highlights the importance of checklists.

“Checklists are designed to minimise performance variability under workload and stress, and thereby increase the likelihood that all required actions are successfully carried out,” it says.

“The importance of precision when following checklists, especially when under stress and time pressure, is highlighted by this occurrence.”