Description 

On 27 July 2019, a Boeing 767-300 (VH-EXZ) being operated by Tasman Cargo Airlines on an international cargo flight from Auckland to Sydney was taxiing for departure when a fuel configuration advisory was annunciated. The crew decided there was no evidence of a fuel leak and continued the departure. Their intended in-flight re-balancing was then delayed until the applicable imbalance limits had been exceeded. The fault was not recorded after flight and the aircraft continued to operate with the operator’s maintenance organisation remaining unaware of the fault for several days.      

Investigation 

An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). Relevant DFDR data from the aircraft was successfully downloaded and facilitated a meaningful review of the event. 

The Captain had a total of approximately 19,100 hours flying experience which included 176 hours on type and the First Officer had a total of approximately 9,000 hours flying experience which included 450 hours on type. They reported being well rested and were beginning a two sector day involving a return flight to Sydney. In the 90 days prior to the investigated flight, they had respectively flown about 55 and about 105 hours on type.

What Happened

It was established that the EDTO flight with just the two man flight crew and cargo on board and with the First Officer as PF was approaching the holding point for departure runway at Auckland when the ‘FUEL CONFIG’ caution had begun to intermittently illuminate. Inspection of the fuel contents gauges showed that there was an imbalance between the left and right main tanks, with the left tank low which the First Officer reported initially having attributed to “the refuelling being out-of-balance and extended APU use during pre-flight". 

Then, just after the flight had been cleared to enter the departure runway, the ‘FUEL CONFIG’ light illuminated and a corresponding advisory message was presented. At this point, the centre tank contents had reduced from 8 tonnes to 6.2 tonnes, and the 13 tonnes in the wing tanks had increased to 14.4 tonnes, made up of 6.6 tonnes in the right tank and 7.7 tonnes in the left tank. After checking that the fuel panel was configured correctly and comparing the fuel used totaliser with the FMC fuel remaining figure, they were satisfied that there was no fuel leak. They decided to continue with their departure and deal with the wing tank imbalance en route. This decision was based on the Captain’s wish not to make fuel panel changes when so close to takeoff and the fact that the imbalance was assessed as “not critical to departure” which was imminent. The flight was cleared for takeoff and became airborne four minutes after continuous illumination of the ‘FUEL CONFIG’ light had commenced.

Once airborne, the crew reported that they had reassessed the fuel imbalance and the Captain had stated that it “did not cause any controllability issues and that the aircraft trim remained at zero for the duration of the fuel imbalance”. He reported having continued to check for any indications of a fuel leak and found none whilst observing that the cause of the imbalance was fuel being fed into the right tank. The centre tank fuel quantity was seen to be decreasing but the left wing tank quantity remained unchanged. The Captain stated that since rebalancing would involve changes to the fuel panel which would have triggered the ‘FUEL CONFIG’, he had “decided to delay rebalancing until the centre tank was nearly empty” so the appropriate non-normal procedure required in response to the alert was not commenced until about half an hour after departure. 

Having reached the planned cruise level of FL 360, fuel contents rebalancing was then commenced when the centre tank’s left and right fuel pump low lights were illuminated, indicating that the fuel remaining in the centre tank had reduced to about 400 kg. At this time, the imbalance between the left and right wing tanks had reached 2.6 tonnes. On completion of rebalancing, which took almost 40 minutes the wing tanks remained balanced for the remainder of the flight.

On arrival at Sydney, the abnormal fuel system behaviour was reported to the maintenance engineer in Auckland who had dispatched the aircraft by telephone and was “also discussed with the maintenance engineer who met the aircraft in Sydney” but the defect was not entered in the aircraft Technical Log and no maintenance action to address it occurred.

No additional fuel was loaded in the centre tank for the return flight to Auckland and the crew reported that the fuel system operated normally throughout and no related entry was made in the Technical Log after that flight either. Only three days later did a maintenance investigation into imbalance in the fuel system commence.   

Why it happened and how operational risk was managed

The Investigation sought to establish the cause of the fault, examine the response of the flight crew to it and consider the significant delay which occurred before any maintenance whilst the aircraft continued in service. 

▪    The fuel system fault. Transfer of fuel to the wing tanks when the centre tank is in use is automatic and occurs because the centre tank has a higher pump output pressure than the main tank pumps and all three tanks feed a common manifold through one-way valves. Under normal conditions, no  fuel is pumped from the centre tank into either the left or right wing tanks but during the investigated flight, DFDR data showed that the imbalance was the result of fuel being fed into the right main tank while the engines were being fed fuel from the centre tank. This began at about the time the engines were started and continued until the centre tank pumps were switched off. The maximum difference between left and right wing tank contents of 2.6 tonnes occurred at about the same time as the centre tank pumps were turned off and rebalancing of the wing tanks was commenced. The effect of the fault was that fuel was pumped into the right main tank during centre tank operation. The fault did not affect the left wing tank, the contents of which remained stable. Maintenance investigation was unable to determine the source of the fault but Boeing stated that the likely cause was a fault in one of three fuel system valves.

▪    The flight crew response. The activation of the EICAS alert required that the associated non-normal checklist should be actioned. This first required that whether a fuel leak existed should be determined and the specified procedure for this was completed. The remainder of the checklist required that the tanks should be brought back into balance but was deferred. This action was applicable to any indicated system malfunction occurring after dispatch but before takeoff. If completing these actions did not require a return for maintenance input, then the MEL item for that equipment failure was to be consulted and if the specified MEL relief required maintenance action, then the aircraft was to be returned for such action. There were also a number of other safety-related circumstances which, if identified, would also require a return for maintenance.

     The Captain “did not recall” the MEL procedure in relation to the management of defects that occurred between dispatch and take-off but whilst the First Officer did do so, both pilots “were influenced by the QRH being the primary document for response to the alert”. However, failure to follow this MEL procedure was considered to have removed a risk control which was expressly designed to prevent an aircraft departing with faulty systems. It was considered of note that “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 transfer from the centre tank to the right wing tank was causing the imbalance and was continuing to increase it, the crew decision to allow this to continue until centre tank fuel was depleted was contrary to the limitations contained in the AFM with which unconditional compliance was understandably required in the operator’s Policy and Procedures Manual. In addition, their crew’s inadequate understanding of why (as opposed to how) the incorrect transfer was occurring meant that they could not determine whether the fuel system fault affected access to all fuel in the tanks which alone meant that their response to the problem once airborne had presented an increased risk to the safe operation of the aircraft.

• The delay in investigating the fault. On arrival in Sydney, the Captain’s failed to record the evident malfunction of the fuel system as a defect in the aircraft Technical Log but nevertheless felt it of sufficient significance to speak with the attending line engineer at Sydney and the line engineer at Auckland where the aircraft was due to return later the same day. He then signed the acceptance of the aircraft for this return flight knowing that despite not loading centre tank fuel, in the absence of any engineering assessment of the cause of the transfer fault, it was still perfectly possible that a similar fuel system malfunction could occur either on that flight or when the centre tank was next in use. It was considered that the delay in facilitating an engineering assessment of cause “directly affected the likelihood of maintenance being able to identify and correct the fault which had caused the imbalance to occur”.

Two Contributory Factors which were identified in respect of the fuel imbalance were as follows: 

• 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.

Two Other Factors that increased risk were also identified: 

• The flight crew became aware of the abnormal operation of the fuel system shortly after the flight became airborne but delayed completion of the associated non-normal checklist. That resulted in a continued increase in the fuel imbalance to 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 Technical Log as a defect. This resulted in a delay to maintenance corrective action until after a further two sectors had been flown by the aircraft and probably impacted on the difficulty in identifying the underlying fault.

Safety Action notified to the ATSB one month prior to the release of the Investigation Report as having been taken by Tasman Cargo Airlines was the drafting of an amendment to the 767 MEL to include clarification as to crew actions in the event of EICAS message between off blocks and take-off. When issued, it was stated that the amendment would be situated in the early part of the MEL Introduction section with a corresponding alert to flight crew.

The Final Report was released on 22 April 2021. No Safety Recommendations were made.

Related Articles

• Aircraft Fuel Systems
• Minimum Equipment List (MEL)
• Emergency and Abnormal Checklist
• Aircraft Technical Log