Tasman Cargo B763 at Auckland and enroute on Jul 27th 2019, fuel imbalance
Last Update: April 22, 2021 / 08:32:58 GMT/Zulu time
The aircraft remained on the ground for about 110 minutes, then departed for the return flight.
The ATSB reported it was later determined "that the alert was due to an anomaly in the fuel transfer from the centre tank to the right wing tank." A short investigation into the occurrence rated an incident has been opened.
An apparently similiar case had been reported by the Canadian TSB 11 days earlier, see Incident: Canada Rouge B763 at Athens on Jul 16th 2019, uncommanded fuel transfer.
On Apr 22nd 2021 the ATSB released their final report concluding the probable causes of the incident were:
- A fault in the fuel system, likely caused by the malfunctioning of one of three fuel system valves, resulted in fuel inadvertently being fed into the right main tank and a gradually increasing fuel imbalance between the left and right main tanks. As the aircraft approached the departure runway, this abnormal fuel system behaviour triggered the FUEL CONFIG caution light, and the associated Engine Indication and Crew Alerting System advisory alert message.
- Having considered the likelihood of a fuel leak and the low priority of the alert, the flight crew decided to address the imbalance once airborne. However, they did not consider the Minimum Equipment List procedural requirements to return to the line for maintenance action.
Other factors that increased risk
- The flight crew became aware of the abnormal fuel system operation shortly after becoming airborne but delayed completion of the associated non-normal checklist. That resulted in continued increase in the fuel imbalance beyond the allowable limit, unnecessarily elevating the safety risk.
- Contrary to the requirements of the operator's policy and procedures manual, the abnormal behaviour of the fuel system was not entered into the aircraft’s technical fault log. This resulted in a delay to maintenance corrective action until after a further two sectors had been flown by the aircraft, and probably impacted identification of the underlying fault.
The ATSB wrote in their abstract (brief summary of the report):
While the Boeing 767, VH-EXZ, was taxiing for departure from Auckland an imbalance in the fuel load between the left and right main tanks developed while the centre tank was providing fuel to both engines. That imbalance triggered the FUEL CONFIG advisory alert message. In response, the flight crew considered whether there was a fuel leak and, having determined this was not the case, decided to depart and correct the out-of-balance condition airborne.
Once airborne, the flight crew delayed the procedure to rebalance the fuel until the centre tank fuel had been depleted. As a result, the fuel imbalance increased to 2.6 t, a weight difference in excess of the fuel imbalance limitation published in the operator’s policy and procedures manual. On arrival at Sydney, the flight crew verbally notified the maintenance personnel of the imbalance but did not enter it into the technical log. The return flight was not loaded with centre tank fuel. The operator’s maintenance organisation did not become aware of the fuel imbalance issue until about 3 days after the occurrence.
What the ATSB found
The ATSB found that the fuel imbalance was the result of abnormal fuel system behaviour, due to a fault within the fuel system, which resulted in fuel being fed into the right main tank from the centre tank. As the imbalance occurred before take-off, a procedure within the Minimum Equipment List (MEL) required the flight crew to action the relevant non-normal checklist and if discontinuation of the flight was not required, then consult the MEL to determine whether maintenance action was required.
Application of the MEL would have required the aircraft to return for maintenance action.
The flight crew had differing knowledge of the MEL requirements however, they shared a common belief that the risk was low enough for the flight to proceed. Consequently, having consulted only the non-normal checklist, the aircraft departed Auckland.
Airborne, the flight crew identified that the abnormal fuel system operation was the result of fuel being pumped into the right main tank. Additionally, the flight crew continued to monitor for a fuel leak and noted that the aircraft’s handling did not appear to be affected by the imbalance. Further, as fuel system guidance and the low priority of the FUEL CONFIG advisory alert message indicated minimal risk from a fuel imbalance condition, the flight crew chose to delay rebalancing. Consequently, the flight crew did not determine whether there was full access to the remaining fuel until they had recommenced the FUEL CONFIG non-normal procedure.
The fuel system unserviceability was verbally notified to engineering, however, contrary to the requirements of the operator's policy and procedures manual, it was not entered into the technical fault log. This delayed maintenance corrective action, and likely hampered determination of the cause of the imbalance.
What has been done as a result
The aircraft’s operator advised the ATSB that an amendment to the MEL has been drafted to include clarification as to crew actions in the event of an Engine Indication and Crew Alerting System (EICAS) message between off-blocks and take-off. This amendment will be situated in the early part of the MEL Introduction section.
The operator has also stated that it will alert flight crew to the procedural requirement through notification of the MEL amendment.
This occurrence highlights the value of flight crews being fully conversant with operating procedures, particularly those related to aircraft unserviceability. Those procedures are critical to the safety of flight operations.
It is also important that any unserviceability is recorded in the aircraft’s technical log to ensure that it is addressed and to provide future reference in case of further, or related, instances.
The ATSB analysed:
The fuel system fault
When the centre tank contains fuel, the fuel system’s normal setup is for the centre tank to supply fuel to both engines, with the main (wing) tanks being available to provide fuel should the centre tank fuel pressure fall. This is achieved through the centre tank having higher pump output pressure than the main tank pumps, and all tanks feeding a common manifold through one-way valves (Figure 1). With normal centre tank operation, fuel is not to be pumped from the centre tank into either the left or right main tank.
The effect of the fuel system fault was that fuel was pumped into the right main tank during centre tank operation. The fault did not affect the left main tank, the content of which remained stable. As the right main tank quantity increased while the left remained unchanged, eventually an imbalance resulted that was sufficient to cause the FUEL CONFIG light to activate. This in turn triggered the EICAS FUEL CONFIG advisory alert message.
Maintenance investigation was unable to determine the source of the fault. However, Boeing stated that the likely cause was a fault in one of three fuel system valves.
During the taxi for departure, the first officer observed the FUEL CONFIG light on the overhead panel briefly flicker. On checking the fuel tank gauges, an imbalance condition was identified, which was marginally around the value necessary to intermittently trigger the FUEL CONFIG advisory alert message.
The first officer rationalised the cause of the imbalance to be extended APU usage and an imbalanced fuel load. While a reasonable assumption, it was incorrect and further examination of available information and discussion between the crew members could have resolved the cause of the imbalance at this stage. Departure procedures and clearance to enter the runway for departure followed, by which time the abnormal fuel system behaviour had increased the imbalance sufficiently to activate the FUEL CONFIG advisory alert message.
The flight crew’s response to the FUEL CONFIG alert
The activation of an EICAS alert message required the flight crew to initiate the associated nonnormal checklist. The EICAS prioritised alerts in a manner that indicated the safety impact of the triggering fault or condition. In the hierarchy of alerts, the advisory alert was the lowest priority. It identified a need for crew awareness, and that corrective action may be required.
The checklist actions in response to an EICAS FUEL CONFIG advisory alert message were found in the Quick Reference Handbook (QRH). The FUEL CONFIG checklist identified that a possible cause of an imbalance was a fuel leak, and in its first item it required determination of whether a fuel leak existed through the conduct of a specific procedure. The flight crew performed this check.
The remainder of checklist required adjustment of the fuel panel to bring the fuel tanks back into balance. As the alert was of a low priority, the flight crew conferred and agreed to depart and address the imbalance condition airborne. The low priority of the alert and the impending departure, however, appears to have influenced investigation of the cause of the imbalance and the fault indications that were present.
The Minimum Equipment List procedure
The actions required of the flight crew in response to the FUEL CONFIG advisory alert message before departure were not limited to the conduct of the relevant non-normal checklist. The operator’s Minimum Equipment List (MEL) for VH-EXZ also contained a procedure required to be conducted by the flight crew in the event of equipment failure occurring post-dispatch but before take-off. That procedure first required completion of the relevant QRH checklist. If that checklist did not require a return for maintenance action, then the MEL item for that equipment failure was to be examined. If the requisite MEL relief required maintenance action, then the aircraft was to be returned for that action. There were also other safety related considerations to be completed that also required return for maintenance.
On this occasion the captain did not recall the MEL procedure relating to the management of defects that occurred between dispatch and take-off. While the first officer did report an awareness of this procedure, both flight crew were influenced by the QRH being the primary document for response to the alert. After applying the required QRH checklist to the point where the procedure called for changing the aircraft’s fuel system configuration, they decided to take-off.
That action was probably the result of a common belief that the risk was low enough for the flight to proceed. That assessment could be supported by the advisory status of the alert message as well as Boeing’s guidance regarding fuel imbalance and on delaying balancing during critical phases of flight.
However, non-compliance with the MEL procedure meant that a risk control designed to prevent the aircraft departing with faulty equipment was not applied.
Departure decision with fuel imbalance
The aircraft departed Auckland with a fuel system that was operating in an abnormal manner as a result of a fault. This raised the risk that further fault could affect the safety of the flight. The departure was also made without the required MEL consideration.
Flight crew’s actions airborne
The flight crew did not identify the abnormal behaviour of the fuel system until after the aircraft had departed Auckland and was established in the climb. Having identified that fuel was being transferred into the right main tank and was causing an increasing imbalance, the flight crew decided to continue to monitor the fuel system and delay rebalancing the fuel distribution until after the fuel in the centre tank was depleted. This decision was based on:
- the rebalancing procedure requiring the centre tank pumps to be switched off, which in turn would result in the FUEL CONFIG advisory alert message
- Boeing guidance on fuel imbalance, which indicated that the fuel imbalance alert was a compromise between minimising crew attention to rebalancing and minimising excess fuel usage due to trim considerations
- the aircraft’s trim indicating that the aircraft was not affected by the imbalance condition
- the low priority of the FUEL CONFIG advisory alert message.
While these considerations indicated minimal risk from delaying rebalancing, the Policy and Procedures Manual required the flight crew to observe the Aircraft Flight Manual fuel imbalance limitation. This limitation was significantly exceeded because of the delay in rebalancing. Further, while the flight crew exhibited a level of concern about the system’s operation, as demonstrated by the captain’s continued check for a fuel leak, they did not determine whether the fuel system fault affected access to all fuel in the tanks. As a result, the flight crew’s actions once airborne presented an increased risk to the aircraft's operation.
Maintenance notification requirements
All faults and abnormal system behaviour were required to be reported in the aircraft’s technical log to ensure corrective maintenance action was completed. The responsibility for this rested with the aircraft’s captain. While the captain verbally notified the maintenance engineers in Sydney and Auckland about the abnormal behaviour, the engineering system’s structure and the absence of the technical log entry resulted in a significant delay in maintenance action to identify and correct the fault.
Further, that delay resulted in the aircraft being dispatched for an Auckland to Sydney and return service without corrective maintenance action being undertaken. While centre tank fuel was not taken on that flight, the delay in maintenance action directly affected the likelihood of the maintenance being able to identify and correct the fault that caused the imbalance to occur.
This article is published under license from Avherald.com. © of text by Avherald.com.
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