Aviation safety investigations & reports

Unreliable airspeed indication and stall warning involving an Airbus A320, VH‑FNP, near Perth, Western Australia, on 12 September 2015

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

Final Report

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What happened

On 12 September 2015, when a Virgin Australia Regional Airlines Airbus A320 aircraft, registered VH-FNP, was passing through about 8,500 ft on departure from Perth Airport, Western Australia, 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 levelled out to troubleshoot the issues before returning to Perth. During the approach, when the flight crew were aligning the aircraft with the instrument landing system, they received a stall warning. The warning stopped after six seconds and the approach continued for a successful landing.

What the ATSB found

The ATSB found that blocked drain holes in the pitot probes prevented water from being effectively discharged, resulting in erroneous airspeed measurements in all three systems at various times during the take-off and climb. The erroneous airspeeds were not detected by the flight crew, but had been detected by the system, resulting in the autothrust and autopilot disconnecting, and the generation of multiple alerts, including a NAV ADR DISAGREE alert. That alert required the flight crew to crosscheck the three airspeed indications and the result would indicate if they had an airspeed or angle of attack disagreement. Due to the limited space in the alert message area, the NAV ADR DISAGREE alert was initially pushed off the screen by engine related alerts that were programmed to have a higher priority.

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. 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.

The ATSB also found that the NAV ADR DISAGREE alert and the associated procedure in the Airbus A320 may lead the flight crew to incorrectly identify the source of the alert (for example, angle of attack instead of airspeed) when there is a short-term disagreement in the airspeeds.

What's been done as a result

The aircraft manufacturer is in the process of updating the aircraft’s software so that the NAV ADR DISAGREE alert has a higher priority than the associated engine alerts. In the case of multiple alerts, it will take precedence over the other associated alerts and be immediately visible to the flight crew. In addition, the ‘risk of undue stall warning message’ will be removed from the aircraft status related to the NAV ADR DISAGREE alert.

Safety message

Modern aircraft with multiple interacting systems can have many layers between the source information and the flight crew. In such systems, where there is erroneous information from an information source, it is important that alerts and procedures be designed to ensure that the flight crew can correctly diagnose the source of the erroneous information. This is particularly important when the information may be erroneous for a short period.

Airbus A320, VH-FNP
Airbus A320, VH-FNP Source: ATSB


Source: ATSB

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The occurrence


Safety analysis


Safety issues and actions

Aircraft details

Sources and submissions



Safety Issues

Go to AO-2015-107-SI-01 - Go to AO-2015-107-SI-02 -

Priority of NAV ADR DISAGREE alert

Although the NAV ADR DISAGREE had more immediate safety implications relating to unreliable airspeed, the ECAM alert priority logic placed this alert below the engine-related faults. As a result, the NAV ADR DISAGREE alert was not immediately visible to the flight crew due to the limited space available on the ECAM display.

Safety issue details
Issue number: AO-2015-107-SI-01
Who it affects: Operators of Airbus A320 aircraft
Status: Adequately addressed


A NAV ADR DISAGREE alert can be triggered by either an airspeed discrepancy, or angle of attack discrepancy. The alert does not indicate which, and 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.

Safety issue details
Issue number: AO-2015-107-SI-02
Who it affects: Operators of Airbus A320 aircraft
Status: Adequately addressed
General details
Date: 12 September 2015   Investigation status: Completed  
Time: 0654 AWST   Investigation level: Systemic - click for an explanation of investigation levels  
Location   (show map): near Perth   Investigation phase: Final report: Dissemination  
State: Western Australia   Occurrence type: Avionics/flight instruments  
Release date: 04 April 2019   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer Airbus  
Aircraft model A320-231  
Aircraft registration VH-FNP  
Serial number 0429  
Operator Virgin Australia Regional Airlines  
Type of operation Charter  
Sector Jet  
Damage to aircraft Nil  
Departure point Perth, WA  
Destination Boolgeeda Airport, WA  
Last update 03 April 2020