Virgin Australia A320 near Perth on Sep 12th 2015, automated flight system issue
Last Update: April 4, 2019 / 14:26:59 GMT/Zulu time
Date of incident
Sep 12, 2015
ICAO Type Designator
Australia's TSB (ATSB) reported on Sep 15th 2015, that air traffic control noticed the aircraft was deviating from its assigned flight level, the crew subsequently reported an automated flight system issue and returned to Perth. The occurrence was rated an incident and is being investigated by the ATSB.
On Sep 22nd 2015 the French BEA reported in their weekly bulletin, that the flight crew noticed uncommanded altitude deviations during cruise and returned to Perth. The occurrence was rated an incident and is being investigated by the ATSB.
On Aug 7th 2018 the ATSB reported the investigation into this occurrence turned out much more complex than anticipated and is still ongoing, the drafting of the final report is currently in progress. The occurrence is now rated an unreliabe airspeed and stall warning.
The ATSB now reported that the aircraft was climbing through FL080 out of Perth's runway 21 when the autothrust system disconnected. 10 seconds later the autopilot disconnected and the master warning activated. The ECAM messages confirmed the autopilot had disconnected and the engines had identified a fault degrading their thrust control from EPR to N1 mode. The captain decided to stop the climb at FL200 due to the likelihood to return to Perth and the crew worked the related check lists. About 8 minutes after the autopilot had disconnected the relevant checklists for the engine and corresponding faults had been completed, however, now the ECAM followed up with a NAV ADR DISAGREE message. This alert had been issued at the same time as the engine fault, however, due to the limited space on the ECAM had not been displayed so far. The crew checked all three airspeed indications and found them all showing 250 KIAS. Working further through the relevant checklists the crew identified the disagree message was due to the AoA sensors. The checklist advised that due to this type of disagreement an undue stall warning might occur. After clearing that fault off the ECAM the ECAM message indicating the fly by wire had reverted to Alternate Law came up and then several others, the crew took about 20 minutes to work through the messages. After the final message concerning the rudder travel limiter had been cleared according to the checklists, the crew was able to re-engage the autopilot. The crew turned for the return. During the descent the crew noticed the captain's air speed indication dropped and disagreed with the other air speed indications. The captain switched his air data source to the alternate (which is the same system the first officer's indications were supplied from), the autopilot disconnected again and rudder travel limiter faults re-occurred. The captain decided they needed some more time to work through the issues. While in the orbit the crew declared PAN advising they had flight control problems and were in alternate law. The crew subsequently left the orbit, while applying the flaps and intercepting the localizer the crew received a stall warning for about 6 seconds, the crew continued the approach and landed safely on runway 21.
During a post flight inspection water was found in all three pitot probes. One of the two drain ports were found blocked in two pitot probes, both drain ports were found blocked in the third pitot probe. A foreign object was blown away while the standby pitot probe was cleaned, the maintenance engineers were not able to catch the object.
The ATSB thus wrote:
The ATSB has identified that the interactions between the sources of the system faults, and the presentation of the alerts and their associated procedures was much more complex than was initially apparent. The ATSB has been investigating how these interactions affected the flight crew’s interpretation of the state of the aircraft and how they then managed the situation.
As a result of the detailed understanding obtained during the investigation, the ATSB is considering 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.
On Apr 4th 2019 the ATSB released their final report into the serious incident concluding the probable causes were:
Unreliable airspeed indications
- Drains in all three pitot probes were blocked, preventing water contamination from being effectively discharged.
- Before and during the flight, water temporarily obstructed all three of the aircraft’s pitot probes, resulting in erroneous airspeed indications. Differences in the airspeeds across the three air data reference systems consequently affected the engine control, flight control and auto flight systems, degrading their functionality and generating multiple system alerts.
Diagnosis of NAV ADR DISAGREE alert source
- The flight crew’s workload following multiple system failures was high, affecting their ability to process information quickly. This, combined with maintaining safe flight, resulted in the flight crew taking about 8 minutes to attend to the engine alerts and action the NAV ADR DISAGREE procedure.
- When the flight crew actioned the NAV ADR DISAGREE procedure, the airspeeds were consistent on all indicators, leading them to incorrectly diagnose that the system failure was the result of an angle of attack discrepancy rather than erroneous airspeeds. The procedure informed them that in this situation, there was a risk of undue stall warning.
- 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]
- 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]
Other factors that increased risk
- Believing it to be an erroneous warning due to an angle of attack discrepancy, the flight crew disregarded a real stall warning during the approach.
- The source of the foreign material blocking the pitot probe drain holes could not be identified.
The ATSB summarized the event as follows:
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.
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.
Date of incident
Sep 12, 2015
ICAO Type Designator
This article is published under license from Avherald.com. © of text by Avherald.com.
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