What happened
While cruising at Flight Level 2D60, the crew of an Express Freighters Australia Boeing 737-300 noticed the master caution light flickering and a right wing-body overheat warning. To address the potential for hot bleed air leaks, the crew followed non-normal checklists to isolate the right engine bleed air. However, a faulty isolation valve prevented the duct from closing completely, leaving it partially pressurised.
In an attempt to resolve the issue, the crew performed additional troubleshooting in consultation with maintenance engineers. During this process, the right air conditioning pack switch was moved to the AUTO position. This action inadvertently transitioned the left air conditioning pack from high flow to low flow, significantly reducing the volume of air entering the cabin. Combined with existing leaks from the APU duct bellows, cabin drain valves, and door seals, the aircraft began a gradual depressurisation. The crew donned oxygen masks and initiated an emergency descent to Canberra Airport.
During the descent, the captain used the emergency purge setting on the oxygen mask. The resulting inhalation of high-pressure, 100 per cent oxygen triggered a gagging reflex, leading to temporary incapacitation. Simultaneously, the first officer suffered from symptoms of hyperventilation driven by intense anxiety, eventually leading the captain to declare the first officer incapacitated.
The investigation
Technical examinations revealed that the initial overheat warnings were likely caused by an electrical fault in the detection system, specifically an out-of-specification sensor conductor. Investigators also confirmed that the isolation valve failure was a primary mechanical issue that prevented the crew from successfully isolating the right bleed duct.