A Boeing 737’s crossfeed valve was not closed after a pre-flight fuel transfer, later resulting in a fuel imbalance warning which led to the flight crew unnecessarily shutting down one of the aircraft’s engines, an ATSB investigation report details.
Just after reaching cruise altitude on a Perth to Adelaide flight on 25 October 2021, the captain and first officer of a Qantas 737 were alerted to a fuel imbalance, indicating there were unequal quantities of fuel in the left and right main fuel tanks, located in each wing of the aircraft.
Prior to the flight, the captain had identified extensive cold soaked fuel frost on the wings, due to cold fuel remaining in the main tanks from the aircraft’s previous flight.
“To remedy this, the cold fuel was transferred to the centre tank, and the main tanks were refuelled with additional, warmer fuel,” ATSB Director Transport Safety Dr Stuart Godley said.
“The procedures required the crossfeed valve to be closed when the operation was completed, however, the valve was not closed. This was likely associated with the crew following the maintenance engineer’s verbal instructions rather than referring to the relevant procedure. While this is permissible, referring to procedures is a more reliable method to ensure all steps are carried out.”
“During pre-flight checks, and later during the climb and level-off, the pilots did not notice the crossfeed selector in the open position, or the associated dimmed blue indicator light on the fuel panel,” Dr Godley said.
Once the centre fuel tank was exhausted and its pumps were switched off, the open crossfeed valve allowed fuel to be continually pumped from the left main tank to the right engine, as a result of uneven fuel pump pressures.
While the aircraft’s manuals stated this could occur, the flight crew did not recall this, and the Boeing 737 imbalance checklist, worked through by the flight crew, did not provide sufficient guidance for an open crossfeed valve to be identified as the potential reason for a fuel imbalance.
“This led the flight crew to decide there could be a fuel leak and then, partly as a result of confirmation bias, stress and perceived time pressure as the aircraft approached the Great Australian Bight where it would fly over water, they abbreviated the relevant checklists and mistakenly confirmed a fuel leak as the cause for the imbalance.”
The ATSB’s report notes the flight crew, when working through the fuel engine leak checklist, inadvertently performed a step out of sequence, invalidating the process, and contributing to their conclusion that there was a fuel leak.
As a result of this incorrect confirmation, the flight crew unnecessarily shut down the aircraft’s left engine during flight.
After the flight crew diverted to Kalgoorlie and conducted a single-engine landing, a post-flight inspection revealed there was no fuel leak, and the fuel system was serviceable.
Dr Godley noted that, although the presence of information in the checklist about the effect of an open crossfeed valve probably would have led to a different outcome in this case, a fuel imbalance condition is itself usually a minor condition, and the ATSB considered the checklists adequate to address a more serious condition such as a fuel leak.
“This incident highlights the importance to all pilots of being precise when following checklists, especially when under stress,” Dr Godley said.
“Checklists are designed to minimise performance variability under workload and stress, increasing the likelihood that all required actions are successfully carried out.”
You can find here the report: Fuel imbalance and engine shutdown involving Boeing 737, VH-VZT 135 NM south of Kalgoorlie-Boulder Aerodrome, Western Australia, on 25 October 2021
