Freighter Flight Crew Incapacitated After Excess Oxygen Ingestion

A Boeing 737 Captain and First Officer experienced separate incapacitation events while operating a Qantas freighter service over New South Wales in 2018. According to a report released on Thursday by the Australian Transport Safety Bureau (ATSB), the incident highlights the hazards of system malfunctions that are not resolved by following checklist procedures.

Pilots on a freighter flight operated for Qantas became incapacitated during a pressurization incident in 2018. Photo: George Canciani via ATSB

Checklists fail to resolve inflight alerts

On August 15, 2018, a Boeing 737-376 Special Freighter (registered as VH-XMO and operated by Express Freighters Australia) experienced an incident. The flight was between Brisbane (BNE) and Melbourne (BNE) with only the Captain and First Officer onboard. Express Freighters Australia operates freighter flights on behalf of Qantas.

Over New South Wales, the pilots noticed the master caution warning light flickering. They then saw the right wing-body overheat annunciator was illuminating. The pilots commenced the non-normal checklist. This did not resolve the problem. The pilots conducted further troubleshooting while liaising with Qantas maintenance personnel in Sydney.

Captain becomes incapacitated after ingesting too much oxygen

The pilots became aware the cabin pressure was reducing. Both were feeling unwell. With cabin altitude set to exceed 10,000 feet, the pilots put on oxygen masks and began a descent.

On this first descent, the Captain became incapacitated. He selected the emergency flow setting while manipulating the oxygen mask setting and ingested too much oxygen. As a result, the First Officer declared a MAYDAY, commenced an emergency descent, and diverted to Canberra Airport (CBR).

The Captain recovered, but then the First Officer began experiencing incapacitating symptoms consistent with hyperventilation. The Captain declared a PAN PAN and requested emergency services meet the Boeing in CBR.

Recorded data from the August 2018 flight. Source: ATSB

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ATSB says incident a timely reminder for pilots

The 737-376 went on to land safely. Ambulances took the Captain and First Officer to hospital. Fortunately, they experienced no lasting effects from the flight.

“This occurrence is a reminder to flight crews of the hazards of dealing with system malfunctions that are not resolved using the approved non-normal checklist procedures,” says ATSB Director Transport Safety Stuart Macleod.

“Configuration changes to an aircraft system may induce other effects due to underlying unserviceable components that may not be immediately apparent.”

The subsequent ATSB investigation found 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.

The report also found a fault with a valve in the aircraft air conditioning system that prevented isolation of the right wing-body duct. As a result, the pilots conducted further troubleshooting, during which the cabin air supply was reduced. With a higher than normal cabin leak rate, the reduced airflow also lessened the cabin pressure.

“The flight crew responded to the cabin pressure reduction by donning their oxygen masks and descending the aircraft,” Macleod said.

“During the descent, the captain selected emergency flow on the oxygen mask resulting in ingestion of gaseous oxygen, causing their temporary incapacitation.

“Maintenance 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.”

Several freight airlines operate Qantas branded freighter flights. Photo: Aero Icarus via Wikimedia Commons

Express Freighters Australia makes changes to its maintenance procedures

Following the incident, Express Freighters Australia made changes to its maintenance procedures. This includes a functional check of the cabin drain valves, specifically verifying the integrity of the auxiliary power unit duct bellows seal. Express Freighters Australia also introduced an enhanced aircraft cabin pressurization system check.

With the release of the incident report on Thursday, the ATSB says flight crews need to be aware that non‑normal situations can lead to a misapplication of emergency equipment when it is actually needed. The selection of the emergency flow setting on the fixed oxygen system resulted in the pilots becoming incapacitated.

“A series of non‑normal events, in conjunction with the use of emergency equipment, can add pressure and workload to the flight crew,” the ATSB Director said.