Aviation safety authorities have revealed shocking details of a mayday incident three years ago that left both the captain and co-pilot incapacitated.

The Australian Transport Safety Bureau (ATSB) released a report detailing an incident onboard a Boeing 737-376 Special Freighter operated by Express Freighters Australia which resulted in an emergency landing.

On 15 August 2018, the plane was flying from Brisbane Airport to Melbourne Airport when its master caution light began to flicker.

After troubleshooting with a Sydney Line Maintenance Operations engineer, the crew noted a rising cabin altitude and both the captain and first officer, or co-pilot, said they began to feel unwell.

The two donned emergency oxygen masks and contacted air traffic control to request descent, fearing they would become incapacitated due to fumes in the cockpit.

“During the initial stages of the descent the captain reported that after manipulating the mask settings an ingestion of pressurised oxygen occurred,” the ATSB report said.

“This resulted in a choking and gagging response from the captain. The first officer, cognisant that they had previously discussed feeling unwell, observed the captain slumped forward, gagging, and gasping for air.

“The first officer recalled checking on the captain during this episode but not receiving a response.”

The first officer broadcasted a mayday to air traffic control and commenced an emergency descent.

After the flight was diverted to Canberra, the first officer experienced symptoms consistent with hyperventilation, and the captain declared the first officer incapacitated and requested emergency services to meed the flight upon landing.

Once the plane had safely landed in Canberra, aviation rescue fire fighters conducted an air sample test that detected no fumes.

Both the captain and first officer were transported to hospital via ambulance for medical assessment.

During post-flight inspections, Qantas engineers identified a range of serviceability issues with the aircraft fuselage cabin drain valves, fuselage door seal, and the auxiliary power unit duct bellow seal that affected the capacity for the aircraft to hold cabin pressure.

The ATSB said Express Freighters, which is owned by Qantas, introduced enhancements and amendments to its maintenance and inspection regimes.

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