Aviation safety investigations & reports

Fuel imbalance involving Boeing 767, VH-EXZ during flight from Auckland, NZ to Sydney, New South Wales, on 27 July 2019

Investigation number:
Status: Completed
Investigation completed
Phase: Final report: Dissemination Read more information on this investigation phase

Final report

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Safety summary

What happened

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.

Safety message

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.

Download Final report
[Download  PDF: 971KB]
Alternate: [Download  DOCX: 704KB]

The occurrence


Safety analysis


Safety action

Sources and submissions

General details
Date: 27 July 2019   Investigation status: Completed  
Time: 1200 NZST   Investigation level: Defined - click for an explanation of investigation levels  
Location   (show map): Auckland International Airport   Investigation phase: Final report: Dissemination  
State: International   Occurrence type: Fuel systems  
Release date: 22 April 2021   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 767-3JHF  
Aircraft registration VH-EXZ  
Serial number 37808  
Type of operation Air Transport High Capacity  
Sector Jet  
Damage to aircraft Nil  
Departure point Auckland, NZ  
Destination Sydney, NSW  
Last update 22 April 2021