Qantas B738 near Kalgoorlie on Oct 25th 2021, fuel imbalance and engine shut down in flight

Last Update: February 28, 2024 / 11:31:41 GMT/Zulu time

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Incident Facts

Date of incident
Oct 25, 2021



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-800

ICAO Type Designator

A Qantas Boeing 737-800, registration VH-VZT performing flight QF-888 from Perth,WA to Adelaide,SA (Australia) with 88 passengers and 6 crew, was enroute at FL390 about 190nm southeast of Kalgoorlie,WA (Australia) when the crew decided to divert to Kalgoorlie due to a fuel imbalance and decided to shut the left hand engine (CFM56) down as a precaution. The aircraft drifted down to FL260 and landed safely on Kalgoorlie's runway 29 about 45 minutes after leaving FL390.

Passengers reported the cabin became very hot, everybody was asking for water. The captain subsequently announced that the left hand engine had been shut down.

The airline reported the crew observed an issue with the fuel system and diverted to the nearest airport Kalgoorlie. Engineers are inspecting the aircraft.

The ATSB reported: "During cruise, the flight crew received a fuel imbalance warning. During completion of the associated checklists, the flight crew conducted a precautionary shutdown of the affected engine. The aircraft was diverted to Kalgoorlie-Boulder Airport, Western Australia and landed without further incident." The occurrence was rated a serious incident and is being investigated (short investigation anticipated to conclude in 2nd quarter of 2022).

The passengers were flown back to Perth on flight QF-1611 (A320 VH-UVQ) and were accommodated in Perth over night.

The occurrence aircraft was still on the ground in Kalgoorlie about 55 hours after landing.

On Feb 28th 2024 the ATSB released their final report concluding the probable causes of the incident were:

Contributing factors

- Under instruction from the engineer, the flight crew conducted steps to perform a ground transfer of fuel (to remove cold soaked fuel frost from the wings) without referring to the relevant procedures. Consequently, an error was made in not closing the fuel crossfeed valve before flight.

- 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.

- The Boeing 737 IMBAL (imbalance) checklist did not provide sufficient guidance for a flight crew to identify an open crossfeed valve as being a potential reason for a fuel imbalance.

- Partly as a result of confirmation bias, stress and perceived time pressure, the flight crew abbreviated the relevant checklists. As a result, the flight crew’s calculation of the rate of fuel imbalance change was invalid, and they misdiagnosed the fuel imbalance as being the result of a fuel leak, resulting in an unnecessary inflight engine shutdown.

The ATSB analysed:


When carrying out an exterior inspection of the aircraft prior to departure, the captain observed extensive cold soaked fuel frost on the lower surface of both wings. The presence of frost is a risk to flight safety, and the captain and the attending licenced aircraft maintenance engineer agreed on removing the frost by a ground transfer of fuel. This was carried out by the engineer and the first officer (FO). Transferring fuel from the main tanks to the centre tank required the crossfeed selector (in the flight deck) to be opened to commence the transfer and closed once completed.

The procedural step to close the crossfeed valve was not carried out, and the aircraft was dispatched. This resulted in a progressive fuel imbalance, which was misdiagnosed by the flight crew as a fuel leak. As a result, the left engine was shut down in flight unnecessarily and the aircraft was diverted to Kalgoorlie-Boulder.

The analysis will examine the coordination of pre-departure fuel transfer procedures, and the flight crew’s management of the fuel imbalance.

Fuel transfer coordination

The licensed aircraft maintenance engineer (LAME), who was licenced on the aircraft type, drew on their technical knowledge of the aircraft’s systems to facilitate the fuel transfer, and the FO recalled being directed to carry out the required actions in the flight deck. It could not be established when the captain arrived on the flight deck but the captain was present later to approve the additional fuel.

The recollections of the LAME, captain and FO differed in regard to the completion of the fuel transfer. As the captain needed to authorise the change in fuel uplifted, it is likely that both flight crew were on the flight deck at the conclusion of the fuel transfer when the crossfeed valve was to be closed.

However, the procedural step to close the crossfeed valve was not carried out. This was likely associated with the FO and/or captain following the LAME’s 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. In any case, there was insufficient evidence to determine the reasons for the crossfeed valve not being closed.

There are numerous tasks that may be carried out without referring to a procedure. However, for tasks that are infrequently performed or that may be unfamiliar, the use of a procedure increases the likelihood they will be accomplished successfully.

Flight with open crossfeed valve

During subsequent preparations for the flight, the captain and FO did not identify that the crossfeed selector had remained in the open position, and at the time its accompanying ‘valve open’ light would be dimmed. As the fuel panel was out of the normal field of view, the selector position and indicator light would not be readily detectable.

Neither flight crew member noticed the crossfeed selector was positioned to open or the dimmed blue ‘valve open’ light immediately above it on the overhead fuel panel during the first part of the flight. There was no requirement for the selector or indicator to be checked in normal flight.

The IMBAL (imbalance) alert does not usually require immediate attention as an imbalance condition is itself a relatively minor concern, and not a strong indicator of a serious problem. It is presented as an amber warning on the fuel tank quantity indication and is not associated with an aural alert. Associated with this manner of indication, the flight crew probably did not notice the alert immediately.

Use of checklists in response to the in-flight fuel imbalance


The correct response to an IMBAL alert is to commence the IMBAL (imbalance) checklist. The objective of this checklist was to decide if a fuel leak is suspected and, if not, balance the fuel.

With a suspected fuel leak, the IMBAL checklist led to the Fuel leak engine checklist. The objective of this checklist was to confirm the existence of an engine fuel leak and either shut down the affected engine or continue with normal operations if there was no fuel leak.

The following sections discuss the flight crew’s application of these checklists and the reasons for the diversion and erroneous shutting down of the aircraft’s left engine.

Not recognising crossfeed issue when using the IMBAL (imbalance) checklist
When the imbalance alert was triggered, the flight crew commenced actioning the QRH IMBAL non-normal checklist. The IMBAL checklist stated that the alert may be caused by a fuel leak, an inoperative crossfeed valve or a fuel imbalance; it did not state that it could be the result of the crossfeed valve being open. This meant that a critical piece of information was absent from the flight crew’s awareness at the time; had they recalled this knowledge or if it had been included in the checklist as a note or step, they may have re-evaluated the possibility of a leak and continued the IMBAL checklist. This would have directed the flight crew to perform steps to verify the functionality of the crossfeed valve and then rectify the imbalance if the valve was serviceable.

Although the presence of crossfeed information probably would have led to a different outcome in this case, a fuel imbalance condition is itself usually minor and a flight crew erroneously progressing to the Fuel leak engine checklist should still usually be able to detect or rule out a more serious condition such as a fuel leak.

Misdiagnosing fuel leak

The IMBAL checklist stated that a fuel leak should be suspected if the total fuel remaining is less than planned, or if an engine has excessive fuel flow. Neither of these conditions existed, but the crew reported they felt time pressure to take action prior to the aircraft crossing the coastline.

Consequently, although there was no evidence that the remaining fuel was less than expected and no leak was visible, the flight crew formed a mental model that there was a fuel leak, originating with and/or reinforced by the rapid reduction of fuel in the left main tank. Consequently, the flight crew progressed to the Fuel leak engine checklist.

The Fuel leak engine checklist had a step to check that the crossfeed valve was closed. It was not possible to conclusively determine whether, or at what time, the flight crew did this due to the limitations of the recorded data and the flight crew’s recollections. However, until around the time the left engine was shut down, more fuel was being used from the left tank than the right. From 1753-1755, the average imbalance rate reduced to about 225 kg/h, which may indicate that the crossfeed valve may have been closed during all or part of this period. However, the imbalance continued to increase through this period, with each engine having a comparable fuel flow, and the imbalance increased again after this period. This indicates that if the crossfeed valve had been closed at about 1753 it was likely also opened again, but the evidence suggests that it was more likely than not open throughout this period. Also, fuel flow rates after this indicate that the crossfeed valve was closed at about the same time the left engine was shut down (1807), and then reopened about 4 minutes later, staying open for the rest of the flight. In either case, the step of the Fuel leak engine checklist to close the crossfeed valve (and keep it closed) was not carried out correctly. If there had been an actual leak from the left engine fuel system, having the crossfeed valve open would have resulted in a greater loss of fuel than would otherwise have occurred.

After this step, the checklist required crews to record the fuel quantities and ‘the current time’ and monitor for a change in imbalance of more than 230 kg ‘within 30 minutes or less', and to check for visual indications of fuel spray. If either or both of these conditions are met, a fuel leak was to be considered ‘confirmed’.

The relevant step in the checklist, to ‘record the main tank fuel quantities and the current time’, required that the fuel record was to be contemporaneous. However, the flight crew calculated the rate of imbalance change using the fuel quantity that was recorded prior to commencing this checklist, so it included a period of time during which the crossfeed valve was open. Therefore, the imbalance was continuing to increase because the fuel for both engines was mostly being drawn from the left fuel tank.

While it is prudent to record fuel quantities as soon as practicable after a fuel leak is suspected, the effect of changing the order of the two checklist steps was probably not apparent to the crew.

The decision to use the quantity that they previously recorded was probably influenced by perceived time pressure of approaching the coastline, at night, with a suspected fuel leak. Had the flight crew recorded the fuel quantities again they would have had to wait several more minutes to assess the rate of imbalance change. Furthermore, the monitoring condition could be misread or misunderstood by a flight crew experiencing stress and time pressure.

During this period, the imbalance changed by more than 230 kg within 10 minutes. As a result, one of the conditions to confirm a fuel leak appeared to be met, but unknown to the crew at the time, was based on incorrect information as the relevant checklist was not followed precisely.

This is likely an example of confirmation bias, which is the tendency for people to seek information that confirms their hypotheses, interpret ambiguous evidence as supporting their hypotheses, and either discount or not seek information that contradicts their hypotheses (Wickens and others 2013).

Although the flight crew may have realised that the crossfeed valve had been open when they later closed the crossfeed valve, by this time they had erroneously ‘confirmed’ the presence of a fuel leak based on what information they considered at the time, and likely did not recall the systems knowledge about the effect of an open crossfeed valve and did not have enough contradictory information to question their now well-established mental model of a fuel leak, nor to question the validity of the imbalance calculation. Accordingly, the flight crew decided to follow the checklist steps that ultimately led to an engine shutdown.

With ample fuel on board for the 46-minute descent and landing, even accounting for the hypothetical potential to lose all fuel from the left main tank, in principle the flight crew now had time to reconsider the situation. However, they did not have enough information at hand to give them a compelling reason to perform a new calculation of the imbalance rate, or of the fuel remaining compared with the expected amount. Further, while it is likely that these actions would have provided correct information, it would also have contradicted the information previously used and potentially confused the flight crew further.

Stress and time pressure

Both flight crew members reported stress and time pressure with diagnosing the reason for the fuel imbalance. Research has shown that stress may result in ‘attentional narrowing’, resulting in people drawing upon fewer information sources (Staal 2004), and narrowing their perception of the most relevant or threatening cues (Burian and others 2005, Wickens and Hollands 2000).

The ability to carry out complex tasks and access working memory is impaired (Burian and others 2005). People can also act more impulsively (Dismukes and others 2007), and the capacity to retrieve information from long term memory is affected (Dismukes and others 2015). When under time pressure, trained and experienced individuals often make decisions based on their recognition of a situation rather than methodically reviewing all available options (Klein 1998).

This occurrence is an example of how stress and time pressure can affect decision-making by reducing the effective search for information and the ability to evaluate a solution. This impacted the flight crew’s ability to recall and gather information, such as checking the crossfeed selector or recalling the effect of it being open, carefully evaluating the fuel used against the expected amount.

This led to a cautious and conservative but somewhat hasty decision to suspect a fuel leak. In turn, and further associated with stress and time pressure, a step in the Fuel leak engine checklist—to record the fuel quantities at the current time—was skipped or overlooked in favour of using the fuel quantities recorded earlier. This inadvertently led to an incorrect fuel imbalance calculation and, ultimately, an unnecessary engine shutdown./i>
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Incident Facts

Date of incident
Oct 25, 2021



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-800

ICAO Type Designator

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