Riverina air freight emergency above Narrandera revealed in Australian Transport Safety Bureau report

The freight aircraft with two crew members, the captain and a first officer, aboard sent out a mayday call while flying 19 kilometres north of Narrandera Airport on August 15, 2018.

The Australian Transport Safety Bureau (ATSB) released its final report into the incident on Thursday, which occurred on a flight operated by a subsidiary of Qantas Freight between Brisbane and Melbourne.

At about 1pm on the day of the incident the aircraft displayed a master caution light for an overheating right wing and the crew's attempt to deal with the issue, under remote instruction from Qantas, reduced airflow to the cockpit.

"As the flight progressed towards Narrandera, the crew identified that the cabin pressure was reducing, commenting to each other that they both felt slightly unwell," the report stated.

"As the cabin altitude continued to climb and anticipating that it would exceed 10,000 feet, the crew elected to don the emergency oxygen masks."

The crew begun their descent at 1.26pm but the captain's mask oxygen flow rate was set too high.

"During the descent, the captain selected emergency flow on the oxygen mask resulting in an ingestion of gaseous oxygen, causing their temporary incapacitation," ATSB Director Transport Safety Stuart Macleod said.

"After the flight was diverted to Canberra, the first officer then experienced symptoms consistent with hyperventilation, leading the captain to declare the first officer incapacitated."

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After landing at Canberra just after 2pm, both the captain and first officer were transported to hospital via ambulance for medical assessment which did not identify any lasting effects from the flight.

Firefighters boarded the aircraft and it was later inspected by maintenance personnel.

The ATSB found that an electrical fault caused the wing overheat caution light to illuminate.

An additional fault with an isolation valve in the aircraft pressurisation system led the crew to take actions that reduced air supply the cabin while they tried to cut off the right wing's pressure duct in an attempt to respond to the caution.

The flight's operator, Express Freighters Australia, has informed the ATSB that it has amended it maintenance schedule with enhanced checks to aircraft pressurisation systems and prohibited air crews from using actions outside of specific checklists with dealing with in-flight issues.

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