What happened
A Boeing 737-838 operating a scheduled passenger service from Brisbane to Perth departed with the minimum fuel required by the operator. While the initial fuel planning was verified as sufficient, the aircraft flew at a Mach number higher than the planned value during the cruise phase. This increased speed led to an extra fuel consumption of approximately 700–800 kg during the early part of the flight.
Upon detecting this higher burn rate, the flight crew consulted with another aircraft and elected to climb to a different altitude earlier than planned. Although this maneuver subjected the aircraft to stronger headwinds, it successfully stabilized the fuel burn at the intended rate. As the aircraft reached the decision point, the crew determined they still possessed enough fuel to meet arrival requirements and proceeded toward Perth.
However, after passing the decision point, Airservices Australia informed the crew that traffic delays would be approximately 16 minutes, exceeding the previously published 10-minute advisory. Because the aircraft was already close to its destination, the crew had no opportunity to implement further fuel-saving measures. Consequently, the aircraft would have landed with less than the required fixed fuel reserve, leaving the crew with no choice but to declare a fuel MAYDAY to secure priority landing.
The investigation
The ATSB examined the flight data and fuel consumption patterns. Investigators found that the aircraft's speed during the cruise segments exceeded the planned Mach number. It is suspected that the Flight Management Computer (FMC) was set to an ECON speed based on a specific cost index; however, the exact cost index value could not be confirmed because the parameter was not recorded in the flight data.
Regarding the arrival delays, a review of Airservices Australia data confirmed that Air Traffic Control (ATC) had applied the necessary priority to the aircraft. However, the discrepancy between the expected 10-minute delay and the actual 16-minute delay was the critical factor in the fuel shortage.
Findings
- The aircraft flew at a higher speed than planned during the cruise, causing an unplanned consumption of 700–800 kg of fuel.
- The crew's decision to climb early to mitigate fuel burn resulted in encountering stronger headwinds.
- The arrival delay exceeded the published 10-minute advisory, leaving the aircraft with insufficient fuel for the extended holding period.
- The proximity to Perth at the time of the delay notification prevented the crew from executing further fuel-saving maneuvers.