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.
The ATSB is satisfied that these proactive safety actions, when available in the A320 fleet, will address the safety issue by providing the flight crew with the best opportunity to identify a transient airspeed disagreement and not disregard stall warnings when activated. The further action of the Unreliable Airspeed Mitigation Means will also provide enhanced awareness of an unreliable airspeed indication situation.
On 23 January 2019, Airbus advised the ATSB that Airbus has launched the following safety actions:
1. The priority of the NAV ADR DISAGREE alert has been increased and will now have a higher priority than the EPR MODE FAULT alerts. In scenarios similar to the event, this alert will become immediately visible to the flight crew, therefore the detectability of a transient airspeed discrepancy will be significantly improved. It is recalled that the FCTM [Flight Crew Training Manual] presents this alert as one of the typical symptoms the flight crews must have in mind in order to detect this situation early and apply the "UNRELIABLE SPEED INDICATION" QRH [Quick Reference Handbook] procedure.
2. Following the full analysis of the AoA [angle of attack] failure cases leading to the triggering of the NAV ADR DISAGREE procedure, it was decided to remove the information line “RISK OF UNDUE STALL WARN” from the ECAM status. Indeed, it corresponded only to theoretical cases not considered as realistic in service. With this modification, if a flight crew determines the source of the NAV ADR DISAGREE alert to be an AoA disagree, the risk of undue stall warning will no more be present on the ECAM and therefore the flight crew will rely on the stall warning. This will address the risk of stall and loss of aircraft control mentioned by the ATSB in the Stall warning section of this report. This modification will be introduced in the FWS [flight warning system] standard F12, conjointly with the change of the NAV ADR DISAGREE alert priority.
3. Finally, Airbus will further enhance the detection of the unreliable airspeed situations with the introduction of the Unreliable Airspeed Mitigation Means (UAMM) function. In the event scenario, the ECAM should display the NAV ALL SPD UNCERTAIN red warning, which will request the ADR CHECK PROC / UNRELIABLE SPEED INDICATION to be applied. This function is intended to be introduced in forward fit in 2019 on the A320 Family, and will also be available for retrofit for eligible aircraft (a minimum computers configuration will be required).
