Air system event - Beechcraft King Air C90, VH-TAM, 74 km north-east of Perth Airport, Western Australia, on 16 July 2009

On 16 July 2009 the pilot of a Beechcraft King Air C90 aircraft, registered VH-TAM, departed Perth Airport on a flight to Wiluna, Western Australia with one passenger on board.

Sometime after becoming established at flight level (FL) 210, the pilot became affected by hypoxia, which resulted in him becoming fixated on the 'distance-to-run' figures on the aircraft's Global Positioning System equipment display and incorrectly interpreting those figures as the aircraft's 'groundspeed'. That confusion resulted in the pilot interpreting the lower-than-expected figures as a significant headwind and in him descending the aircraft to escape the winds. Once established at FL150 for a significant period of time, he realised that that he had been affected by hypoxia. The pilot descended further before landing at his destination.

The investigation identified problems with the aircraft's left landing gear squat switch that prevented the aircraft from pressurising in flight. In addition, the cabin altitude warning system was non‑operational due to the incorrect connection of the switch wiring during previous maintenance.

Following this occurrence, the aircraft manufacturer changed the aircraft type's maintenance manuals and documentation and the Civil Aviation Safety Authority (CASA) issued a letter to owners and operators of Australian-registered pressurised aircraft that proposed mandating the fitment of aural cabin pressure warning systems in those aircraft. As a result of that industry consultation, CASA determined that a uniquely Australian installation requirement could not be justified.

Notwithstanding, as a result of the ongoing risk of serious incidents and fatal accidents in which the occupants of single-pilot, turbine‑powered, pressurised aircraft have been affected by, or have succumbed to unrecognised hypoxia in an unpressurised cabin, the Australian Transport Safety Bureau has issued a safety advisory notice. That notice encourages all operators of such aircraft to consider the installation of an aural cabin altitude pressure warning system that operates separately to their aircraft's visual warning system.