What happened
Prior to departure, the captain identified frost on the lower surfaces of the wings caused by cold-soaked fuel. To mitigate this flight safety risk, the crew and a maintenance engineer decided to perform a ground fuel transfer from the main tanks to the centre tank. During this process, the crossfeed valve was opened to facilitate the transfer, but the crew failed to close it once the task was finished.
During the flight, a progressive fuel imbalance developed because the valve remained open. The crew did not notice the position of the selector or the dimmed indicator light on the overhead panel during pre-flight or early flight phases. When an imbalance alert was triggered, the crew misidentified the cause as a fuel leak. Driven by stress and time pressure, the crew abbreviated their checklists and ultimately performed an unnecessary shutdown of the left engine, necessitating a diversion to Kalgoorlie-Boulder.
The investigation
The investigation focused on the coordination of the fuel transfer and the crew's subsequent management of the imbalance. Investigators found that the fuel transfer was conducted under the direction of the maintenance engineer without the crew referring to the formal written procedures. This lack of procedural adherence led to the crossfeed valve remaining open.
Furthermore, the investigation examined the effectiveness of the Boeing 737 imbalance checklist. It was noted that the checklist did not explicitly list an open crossfeed valve as a potential cause for the alert. This omission, combined with the crew's failure to notice the dimmed blue indicator light, contributed to the misdiagnosis of a leak.