Blocked drain holes which stopped water being discharged caused false airspeed warnings in a Virgin Australia plane, forcing it to return to Perth, an investigation has found.
The Virgin Airbus A320 was climbing away from the Perth airport in September 2015 when its autothrust and autopilot systems disconnected and multiple alerts were generated.
But, in a report released on Thursday, the Australian Transport Safety Bureau said it was the blocked drainage system which caused the false airspeed warnings.
The system uses a device known as a pitot probe to measure the flow of water which allows it to determine the airspeed of a plane.
The ATSB said the false airspeed warnings were not detected by the flight crew, but had been detected by aircraft's software systems.
They had also sparked a number of engine-related alerts which took precedence over the initial issue, because they were programmed to have a higher priority.
Although the crew had to deal with a further stall warning as they returned to Perth, the plane landed without incident.
The ATSB said the Airbus manufacturer was in the process of updating the aircraft's software to ensure that alerts were displayed on the crews' screens in the appropriate order of seriousness.
"Modern aircraft with multiple interacting systems can have many layers between the source information and the flight crew," the safety bureau said in its report.
"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."
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