Incorrectly calculated take-off speeds caused a Jetstar Airbus A320 to suffer flap and landing gear retraction overspeeds, Australian aviation safety investigators have revealed.

Releasing the final investigation report into the 2018 incident, the Australian Transport Safety Bureau (ATSB) says the flight crew also did not independently verify and cross check their calculations.

The flight crew were preparing for their flight from Sydney to Melbourne on the evening of 29 September 2018, their third leg in the day. 

“This incident highlights the importance of independent validation and cross-check of flight performance data, in particular performance speeds and aircraft weight” - ATSB transport safety director Stuart Godley

During pre-flight preparation, the crew’s electronic flight bags, which helped automate functions such as take-off speed calculations, were not updated at the start of the day, as stipulated by the carrier. As a result, when both crew members calculated the take-off performance data, they received inconsistent data.

The first officer, the pilot monitoring, had updated his software before the flight, but the captain, the pilot flying, had an older software version.

The captain was unable to update his software, so they decided to employ the back-up procedure: calculating speeds manually.

The first officer “recalled that he performed the calculations” and that the captain “agreed with the results”.  The first officer “did not recall [the captain] independently performing the task”, the ATSB notes.

Using manually-calculated data, the crew performed take off from Sydney later that night. Shortly after take off, the maximum flap extended speed was exceeded. Flight data also reveals that the crew did not make the “positive climb” and “gear up” calls, which were usual procedure.

The first officer, who was monitoring, told his colleague that the aircraft was exceeding the maximum flap extended speed. A short while later, the overspeed warning was also triggered.

The captain retarded thrust levers in response – instead of increasing the aircraft pitch – at an altitude of 144ft (43.9m) above ground level. The ATSB notes that this was below the safe thrust reduction altitude of 800ft.

After the flaps were retracted, the crew reported hearing a “buffeting noise.” They initially attributed this to the aircraft’s auxiliary power unit (APU) still being switched on and delivering bleed air.

However, the noise continued after the APU was shut off and the crew realised that the landing gear was still extended. Flight data reveals that when the gear was retracted a retraction overspeed event occured. 

The aircraft continued on to Melbourne without further incident. There were no injuries recorded.

In its safety analysis, the ATSB notes that the use of manual calculations was a “rarely practised” procedure by the crew. “It was unlikely the flight crew completed the full procedure for the manual calculation,” states the bureau.

It found that the calculated rotation speed was based on an aircraft weight that was “significantly heavier” than the actual take-off weight.

It adds that the reduction of speed below the safe altitude “had the potential to affect safety of the flight”.

Following the incident, the ATSB notes that Jetstar has issued its own internal safety summary, with safety reminders to its flight crew.

On the incident, ATSB transport safety director Stuart Godley says: “This incident highlights the importance of independent validation and cross-check of flight performance data, in particular performance speeds and aircraft weight.”

“This investigation emphasises the importance of considering reasonability and accuracy checks, consulting company procedures manuals in the event of electronic flight bag issues, and conducting a normal rotation followed by reference to the Speed Reference System,” Godley adds.

Cirium fleets data indicates that VH-VFX was delivered to the low-cost carrier in November 2013.