Depressurisation and crew incapacitation highlights the hazards of system malfunctions that are not resolved by following checklist procedures

Boeing 737 FreighterBoeing 737 Freighter

A Boeing 737 freighter aircraft developed multiple technical issues that the flight crew could not resolve using the approved non-normal checklist procedures, resulting in the crew conducting an emergency descent and diversion during which they experienced separate incapacitation events, an ATSB investigation details.

The 737-376SF aircraft, operated by Express Freighters Australia, was conducting a scheduled freight flight from Brisbane to Melbourne on the evening of 15 August 2018 with two flight crew on board. During cruise the crew observed the master caution warning light flickering, and then identified that the right wing-body overheat annunciator was illuminating.

In response the flight crew actioned the non-normal checklist. When this did not resolve the overheat indication, the crew then conducted further troubleshooting in consultation with line maintenance operations personnel in Sydney.

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. 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 and advised air traffic control that they had commenced a descent.

During the initial phases of that descent, the captain became temporarily incapacitated due to ingesting an increased supply of oxygen. This was due to the captain selecting the emergency flow setting while manipulating the oxygen mask settings. The first officer then declared a MAYDAY, advising of issues with the aircraft and that they had commenced an emergency decent. The flight was subsequently diverted to Canberra Airport.

After the captain had recovered, the first officer 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 and requesting the attendance of emergency services on arrival at Canberra. The aircraft was landed without further incident.

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 prevented isolation of the right wing-body duct, which led the crew to conduct further troubleshooting during which the cabin air supply was reduced. In conjunction 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,” said ATSB Director Transport Safety Stuart Macleod.

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

“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.”

After landing, both the captain and first officer were transported to hospital via ambulance for medical assessment. Post‑occurrence medical testing and assessments did not identify lasting effects from the flight.

Separately, the aircraft was inspected by maintenance personnel.

“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,” Mr Macleod said.

Subsequent to the occurrence the operator 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 duct bellows seal; and introducing an enhanced aircraft cabin pressurisation system check.

“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,” said Mr Macleod.

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

The investigation also reminds 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 selection of the emergency flow setting on the fixed oxygen system resulted in a temporary incapacitation of the captain.

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

Read the report: Depressurisation and crew incapacitation, Boeing 737-376SF, VH-XMO, 19 km north of Neranderra Airport, New South Wales, on 15 August 2018

Last update 24 June 2021

Final report

Depressurisation and crew incapacitation Boeing 737-376SF, VH-XMO, 19 km north of Narrandera Airport, New South Wales, on 15 August 2018