Introduction
The following analysis details technical and operational issues encountered by the crew of a freight flight that led to pilot incapacitation during an emergency descent and diversion to Canberra Airport.
Depressurization Sequence
While cruising at Flight Level 260, the flight crew observed the illumination and rapid flickering of the master caution light. Approximately 2 minutes later, the first officer identified that the annunciator light for the right wing-body overheat was also illuminated. The crew was concerned that hot bleed air gases could leak within the wing cavity, damaging wiring and internal structure. Recommended actions to extinguish the warnings were unsuccessful, and the flickering continued for about 10 minutes. The alerts eventually extinguished, but later maintenance found that a center conductor from a sensor within the overheat detection system was out of specification, causing erroneous behavior.
Actioning the non-normal checklist for the right wing-body overheat required isolating the bleed air systems from the right engine. However, after completing the checklist, the flight crew observed that the right duct remained partially pressurized, indicating incomplete isolation. Technical examination later identified a faulty isolation valve. The residual pressure prompted further troubleshooting with Sydney Line Maintenance, during which the right air conditioning pack switch was selected to AUTO from OFF. This changed the left (operating) pack from high flow to low flow, significantly reducing available pressurization air.
Additionally, the aircraft had significant air leaks from the auxiliary power unit duct bellows, cabin drain valves stuck open, and minor leaks from door seals. With both air conditioning packs operating, cabin pressure could be maintained, but after the troubleshooting actions reduced air supply, the combination of leaks and reduced input led to gradual depressurization. The crew donned oxygen and conducted an emergency descent.
Pilot Incapacitation
During the descent while wearing an oxygen mask, the captain selected the emergency purge setting, which supplied pressurized 100% oxygen. According to the flight crew operations manual, this setting was intended to remove contaminants from the mask. However, the selection produced a gagging reflex, leading to incapacitation. The first officer assumed control and continued the diversion to Canberra Airport.
Conclusion
The depressurization resulted from reduced air supply due to troubleshooting actions combined with pre-existing air leaks. The captain's oxygen mask setting caused incapacitation during the emergency descent.