Airbus is working on the A320 software update after Virgin Australia Regional Airlines’ close-call near Perth (Australia) in 2015. The update concerns alert priority in times when pilots receive multiple alerts and high workload, according to the Australian Transport Safety Bureau (ATSB).

In particular, A NAV ADR DISAGREE alert on Airbus A320 aircraft can be triggered by either airspeed or angle of attack error. The problem here is that the alert does not indicate the source, leaving it to flight crews to identify the problem.

In this type of event, the associated procedure may lead flight crews to incorrectly diagnosing the source of the alert when the airspeed is erroneous for a short period and no airspeed discrepancy is present when the procedure is carried out.

"When they received the stall warning during the approach, the flight crew considered it spurious and disregarded that warning. However, there was nothing wrong with the angle of attack and the warning was real”

-ATSB report

This is precisely what happened to Virgin Australia Regional on September 12, 2015.

On that day, Virgin Regional’ A320 (VH-FNP) was carrying a charter flight from Perth Airport to Boolgeeda Airport, Western Australia. As the aircraft departed Perth, passing through about 8,500 ft, the autothrust and autopilot disconnected, and multiple alerts were generated. The flight crew continued the climb to an altitude of 20,000 ft, where they leveled out to troubleshoot the issues.

The pilots requested a return to Perth. On a way back, during the approach, pilots received a  stall message as they working to align the aircraft with the instrument landing system. Dismissing it as a possible false alarm, pilots ignored it. After six seconds the warning stopped and the aircraft landed successfully at Perth.

During the investigation, ATSB has found out that erroneous airspeed measurements in all three systems at various times during the take-off and climb resulted from blocked drain holes in the pitot probes that prevented water from being effectively discharged.

While the flight crew was unaware of erroneous airspeeds, they were detected by the aircraft system, resulting in the autothrust and autopilot disconnecting. Multiple alerts were generated, including a NAV ADR DISAGREE alert. But because there were also engine related alerts that have higher priority, the NAV ADR DISAGREE alert was initially pushed off the screen.

“The engine related alerts did not require immediate actions by the flight crew, and because of their high-workload, the flight crew did not clear them and action the NAV ADR DISAGREE procedure until after the airspeeds had corrected themselves, and all displayed the same value” the report reads. “This led the flight crew to diagnose it as an angle of attack disagreement, which the procedure informed them, had the ‘risk of undue stall warning’. When they received the stall warning during the approach, the flight crew considered it spurious and disregarded that warning. However, there was nothing wrong with the angle of attack and the warning was real”.

Inspecting this situation, ATSB identified two main safety issues. Besides the above-mentioned problem that NAV ADR DISAGREE alert can be triggered by two sources, investigators also point out that Virgin Australia Regional’s case, NAV ADR DISAGREE had more immediate safety implications, but the flight crew could not immediately see it. The alert was placed below engine-related faults because of the ECAM alert priority logic.

“The ATSB’s safety message from this investigation is where there is erroneous information from an information source, it is important that alerts and procedures be designed to ensure that the pilots can correctly diagnose the source of the erroneous information,” ATSB Executive Director for Transport Safety, Nat Nagy, is quoted in a statement on April 4, 2019.  

“Airbus is currently in the process of updating the A320’s software so that the NAV ADR DISAGREE alert has a higher priority than the associated engine alerts,” according to the statement. “In the case of multiple alerts generated by unreliable airspeed, it will take precedence over the other associated alerts and be immediately visible to the pilots. In addition, the ‘risk of undue stall warning’ message will be removed from the aircraft status related to the NAV ADR DISAGREE alert”