Unreliable airspeed indication and stall warning

Airbus is proactively updating software on its A320 aircraft to ensure pilots receive alerts at an appropriate level of priority during periods of multiple alerts and high workload.

Airbus A320

The update follows an ATSB investigation into an unreliable airspeed indication and stall warning involving a Virgin Australia Regional Airlines Airbus A320 near Perth Airport on 12 September 2015.

While passing through 8,500 ft, the aircraft’s autothrust and autopilot disconnected and multiple system alerts were generated. The captain took manual control of the aircraft and continued the climb to 20,000 ft, and levelled off to troubleshoot the issues and plan a return to Perth.

On approach to Perth Airport while aligning with the instrument landing system, the stall warning activated. The warning stopped after six seconds and the approach was continued for a successful landing.

The ATSB found the autothrust and autopilot disconnect was the result of erroneous airspeed indications during the take-off and climb due to blocked pitot tubes. The erroneous airspeeds were not detected by the pilots but had been detected by the aircraft’s systems, which had triggered the disconnect and generated multiple alerts including a ‘NAV ADR DISAGREE’ alert.

… it is important that alerts and procedures be designed to ensure that the pilots can correctly diagnose the source of the erroneous information…

This alert requires the pilots to crosscheck the three airspeed indications and assists them in determining if the source of the alert is an airspeed or angle of attack disagreement. However, limited space in the alert message area meant it was initially pushed off the screen for engine-related alerts programmed with a higher priority but in this case not requiring immediate action by the crew.  

The crew’s high workload meant the procedures for these alerts initially were not actioned and they were unable to address the ‘NAV ADR DISAGREE’ alert for about eight minutes, by which time the airspeed discrepancies had corrected themselves.  

The ATSB found this sequencing of alert priorities and the alert’s associated procedure may have led the pilots to incorrectly identify the source of the alert as an angle of attack discrepancy, which the NAV ADR DISAGREE procedure advised had a risk of triggering an undue stall warning.

Combined with the multiple system alerts, which to the flight crew appeared to be unrelated, the flight crew thought the stall warning that activated during the approach was spurious and as such did not apply the stall recovery procedure. Stall warnings are triggered by angle of attack, not airspeed, and there were no indications that the angle of attack system was not functioning correctly.

ATSB Executive Director, Transport Safety, Mr Nat Nagy, said modern aircraft with multiple interacting systems can have many layers between the source information and the pilots. 

“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,” Mr Nagy said.

“Further, unless it is absolutely clear that it is erroneous, pilots should appropriately respond to stall warning alerts.”  

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. 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.

Read the report: Unreliable airspeed indication and stall warning involving Airbus A320, VH-FNP, near Perth, Western Australia

Last update 06 June 2019