The Australian Transport Safety Bureau (ATSB) has released its final report into an incident involving a Qantas cargo flight from Brisbane to Melbourne which was forced to make an emergency landing in Canberra.
Newshub published air traffic control recordings in the days following the incident which took place in August 2018. In it, the pilot can be heard declaring MAYDAY while breathing through his oxygen mask. It was also at this time, his co-pilot had become incapacitated.
The pilot can be heard asking for weather conditions in Canberra as "they had to get on the ground immediately."
The pilot then tells controllers to arrange for ambulance staff to meet the flight when it lands.
The ATSB report says the aircraft developed multiple technical issues the flight crew "could not resolve using the approved non-normal checklist procedures."
As a result the crew conducted an emergency descent and diversion during which they experienced separate incapacitation events.
The 737-300 aircraft had been converted into a cargo aircraft in 2003 and was operated by Express Freighters Australia. Prior to that, it had been operated as a commercial passenger plane by Qantas in New Zealand.
During cruise the crew observed the master caution warning light flickering, as well as an overheating warning signal.
The problems continued despite the crew undertaking a non-normal checklist, so they began troubleshooting the issue with maintenance staff in Sydney.
As the flight flew over Narrandera, the crew identified the cabin pressure was reducing, within seconds both crew said they felt slightly unwell.
As the cabin altitude continued to climb and would soon exceed 10,000 feet, the crew quickly fitted emergency oxygen masks and advised air traffic control that they were beginning an immediate descent.
During the initial phase of descent, the captain became temporarily incapacitated due to ingesting an increased supply of oxygen. The first officer then declared a 'MAYDAY', advising of issues with the aircraft and that they had commenced an emergency descent for Canberra Airport.
After the captain had recovered, the first officer then experienced incapacitating symptoms consistent with hyperventilation.
The captain then declared a 'PAN PAN' radio call to air traffic control, informing of the first officer's incapacitation.
The ATSB's investigation identified that the intermittent flickering of the master caution light and overheat annunciator was likely due to an electrical fault in the right wing-body overheat detection system.
A fault with a valve in the aircraft air conditioning system and a higher than normal cabin leak rate reduced airflow and lowered cabin pressure.
"The flight crew responded to the cabin pressure reduction by donning their oxygen masks and descending the aircraft," ATSB Director Transport Safety Stuart Macleod said.
After landing, both the captain and first officer were transported to hospital by ambulance for medical assessment. Post‑occurrence medical testing and assessments did not identify lasting effects from the flight.
Subsequently, the operator has implemented a range of changes to its maintenance program, including incorporating a functional check of the cabin drain valves - specifically verifying the integrity of the auxiliary power unit ducts, and introducing an enhanced aircraft cabin pressurisation system check.
The investigation also reminded flight crews to be aware that "non‑normal situations can lead to a misapplication of emergency equipment in the moment that it is actually needed."
In this case the level of airflow on the fixed oxygen system resulted in a temporary incapacitation of the captain.
Listen to a recording of the incident unfolding in the video above.