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Digital alerts under scrutiny after A320 incident

The Australian Transport Safety Bureau (ATSB) has flagged a potential safety issue around how aircraft system alerts are prioritised and presented as it finalises its investigation into a 2015 incident involving a Virgin Australia Regional Airlines Airbus A320.

The investigator detailed multiple issues during the 12 September incident that saw the aircraft, registered VH-FNP, return to Perth an hour after departing on a charter flight to Boolgeeda with nine crew and 139 passengers on-board.

As the aircraft was passing through 8,000ft the autothrust system disengaged, followed by the autopilot disengaging. A message on the electronic centralised aircraft monitoring (ECAM) system informed the crew that there was an engine fault, and the thrust control system was operating in a degraded mode.

The captain took manual control of the jet and continued the climb. After several failed attempts to re-engage the autopilot, the captain levelled off at 20,000ft and started troubleshooting procedures.

The crew were able to complete procedures for the engine faults and cleared those alerts from the ECAM. Then, a ‘NAV ADR DISAGREE’ message displayed, signifying that there was a disagreement in the air data reference systems.

“This alert was generated at the same time as the engine alerts, but due to the limited space on the ECAM, was likely off the bottom of the display,” the ATSB says.

Comparing speeds across the three airspeed indicators, which all showed 250kts, led the crew to believe that the disagreement was between the aircraft’s angle-of-attack sensors. That carried the risk of the crew receiving erroneous stall warnings.

After clearing that ECAM message, further alerts showed that the aircraft had reverted to alternate flight control law, and others relating to the rudder travel limiter system.

The crew then spent around 20 minutes working through procedures to clear the alerts, after which the autopilot system was successfully re-engaged. They then elected to return to Perth.

As the A320 descended, the crew noticed that the captain’s airspeed indication was not consistent with the first officer’s and the back-up indicator, and so the captain switched to the latter input.

Shortly after, the autopilot disconnected again, and the rudder travel limited system error message displayed on the ECAM. The crew then continued the flight under manual control, radioing a ‘PAN’ call to air traffic control, informing them that they had “flight control issues”.

During a turn to capture the instrument landing system, a stall warning sounded for six seconds. Nonetheless, the flight landed safely.

An inspection of the aircraft found water in all three pitot systems used to feed airspeed data to the avionics, and blockages in most of the drain ports. A foreign object was also ejected from the standby pitot probe, but no faults were found with the angle-of-attack sensors.

The ATSB is continuing its investigation, with a focus on the “potential broader safety implications of how the flight crew’s understanding of the situation they encountered was influenced by how the aircraft’s alerting system prioritised and, presented those alerts and their associated procedures.”

It adds that the final report is being drafted and will be released once it has been reviewed internally and by those involved.

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