The Australian Transport Safety Bureau did not conduct an on-scene investigation of this occurrence.
Sequence of events
On 12 November 2004, the air traffic control (ATC) sector controller advised the pilot of VH-PTA, a Cessna Aircraft Company 414A Chancellor, of an active restricted area up to flight level (FL) 260, ahead of the aircraft. The pilot requested permission to climb to FL260.
A short time later, while climbing through FL160, the pilot advised the controller that he was unable to climb to FL260. The controller advised the pilot of an alternative route and cleared him to track via that route. However, the pilot had difficulty reading back the amended route clearance and the controller noted that his speech was slurred. Another pilot who was monitoring the frequency suggested that the pilot may be hypoxic (suffering from oxygen deprivation). The controller declared an alert phase.
The pilot later commented that as the aircraft was climbing through FL160, the passengers expressed concerns regarding their comfort and that the air traffic controller queried him regarding the possible onset of hypoxia. The pilot reported that he checked the cabin pressurisation settings and noted that the pressurisation system was not switched on. He switched it on, but the needle went off the scale, so he switched it off and descended the aircraft to 9,000 ft.
An engineering check of the aircraft's pressurisation system found that the system was capable of normal operation and that it had probably been switched off following maintenance on the aircraft avionics. The engineers subsequently informed the pilot that even though the pressurisation gauge needle exceeded the upper limit, if he had left the system switched on the cabin pressure would have stabilised.
Following an accident investigation involving a pressurised aircraft in 2000, the ATSB issued two recommendations regarding aircraft pressurisation systems. The report ( BO/200003771) and the recommendations (R20000288 and R20000289) are available on the ATSB website or from the Bureau on request.