Aviation safety investigations & reports

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

Investigation number:
Status: Completed
Investigation completed
Phase: Final report: Dissemination Read more information on this investigation phase

Final report

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What happened

On the evening of 15 August 2018, a Boeing 737-376 Special Freighter, registered VH-XMO, operated by Express Freighters Australia, was transporting freight from Brisbane Airport, Queensland to Melbourne Airport, Victoria when the master caution and a right wing-body overheat annunciator illuminated.

The non-normal checklist was actioned followed by further troubleshooting in consultation with maintenance personnel. This resulted in a reduction of cabin pressure. The crew donned oxygen masks and the aircraft was entered into an emergency descent. During the initial part of the descent the captain was temporarily incapacitated by a reaction to the increased supply of breathing oxygen from the mask. A MAYDAY was declared by the first officer and the aircraft was diverted to Canberra Airport, Australian Capital Territory. During the diversion, the first officer also experienced incapacitating symptoms. The aircraft landed at Canberra Airport, under the control of the captain, with no further issues.

What the ATSB found

The ATSB found that faults in the right wing-body overheat detection system likely led to intermittent flickering of the master caution light and illumination of the right wing-body overheat annunciator. The operating flight crew conducted the appropriate non-normal checklist, however the overheat indication could not be rectified due to the fault in the wing-body overheat detection system.

An additional fault with an isolation valve in the aircraft pressurisation system prevented isolation of the right wing-body pressure duct. This 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. During the descent, the captain selected emergency flow on the oxygen mask resulting in an ingestion of gaseous oxygen, causing a temporary incapacitation. After the flight was diverted to Canberra, the first officer experienced symptoms consistent with hyperventilation, leading the captain to declare the first officer incapacitated.

Post‑occurrence medical testing and assessments were carried out on the flight crew with no effects from the flight identified. 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.

What has been done as a result

The operator advised the ATSB that, following the occurrence, amendments were incorporated into the approved scheduled maintenance program to:

  • incorporate a functional check of the cabin drain valves
  • specifically verify the integrity of the auxiliary power unit duct bellows seal
  • introduce an enhanced aircraft cabin pressurisation system check.

The operator implemented an inspection regime to ensure timely detection and rectification of faults compromising the operation of the wing-body overheat detection system.

The operator also advised that in‑flight troubleshooting outside of the non-normal checklist procedures and Flight Crew Operations Manual is now prohibited.

Safety message

This occurrence is a reminder to flight crew of the hazards of dealing with system malfunctions that are not resolved using the approved non-normal checklist procedures. In such circumstances, associated system effects need to be taken into account when electing to conduct further troubleshooting outside of the non‑normal procedures, even with the assistance of external maintenance specialists. Configuration changes to an aircraft system may induce other effects due to underlying unserviceable components that may not be immediately apparent.

The ATSB also reminds flight crew to be cognisant that a non‑normal situation 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 temporary incapacitation of the captain.

Finally, a sequential series of non‑normal events, in conjunction with the use of emergency equipment, can add pressure and workload to the flight crew. Though it would seem unlikely to occur, these pressures may result in hyperventilation, increasing the potential for incapacitation during a critical phase of the flight.


Download Final Report
[Download  PDF: 836KB]

The occurrence


Safety analysis


Safety action

Sources and submissions

General details
Date: 15 August 2018   Investigation status: Completed  
Time: 1326 UTC   Investigation level: Defined - click for an explanation of investigation levels  
Location   (show map): Narrandera, north 19km   Investigation phase: Final report: Dissemination  
State: New South Wales   Occurrence type: Air/pressurisation  
Release date: 24 June 2021   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737-376 Special Freighter  
Aircraft registration VH-XMO  
Serial number 23488  
Operator Express Freighters Australia  
Sector Jet  
Damage to aircraft Nil  
Departure point Brisbane, Queensland  
Destination Melbourne, Victoria  
Last update 24 June 2021