The cabin of the Beech 200 was pressurised with environmental air taken from the compressor bleed air outlets of both engines. The bleed air supply was controlled using two three-position switches mounted side by side on the co-pilot's 'environmental' sub-panel. The bleed air switches were of a different shape to most other toggle switches on the instrument panel, which could be discerned by touch. The switches were detented so that they required pulling before changing position (Refer Figure 1).

The switches were placarded bleed-air valve (left and right), and the individual switch positions were (as read from the top selection to the bottom) OPEN, ENVIR OFF, and INSTR & ENVIR OFF. When switched to either the ENVIR OFF or INST AND ENVIR OFF positions, the bleed air valves that controlled the supply of environmental bleed air to the cabin were closed. When switched to the OPEN position, pressurised environmental bleed air flowed to the cabin for air conditioning and pressurisation. The RAAF reported that the switch detents were worn and the switches could be operated without being pulled. The cabin pressurisation controller automatically adjusted an outflow valve in the rear of the cabin to maintain a preset cabin altitude. The pressurisation controller was located on the centre pedestal in the cockpit.

The cabin pressurisation instruments were positioned low on the centre instrument panel, and were partially obscured by the engine and propeller control levers in flight.

The aircraft also had two vent blowers that forced air through underfloor ducts to assist with cabin ventilation. The vent fans were switched on when the aircraft was on the ground to prevent the ducts from overheating. As the aircraft climbed through 10,000ft the aft blower was normally switched off, and the vent blower was normally switched from HI to LOW. The vent fan switches were positioned directly above and below the right bleed air switch on the co-pilot's environmental sub-panel. The switches were of a similar shape to most other toggle switches on the instrument panel, and did not require pulling out of a detent before changing position. The switches were smaller and dissimilar in shape to the nearby bleed air switches.

ATSB

DC or AC instruments?

The complete loss of AC electrical power, said NTSB investigators, would have rendered most of the pilot's flight instruments inoperative. The only instruments that would have been available to the pilot were those that were operated by the pitot static or vacuum systems. On the left side of the cockpit, only the airspeed indicator and the turn and slip indicator would have been operational. On the right side of the cockpit, the airspeed indicator, turn and slip indicator, altimeter and attitude indicator would have been operational. Despite the loss of the airplane's primary flight instruments, the pilot made no radio transmissions to the controller after Mode C information was lost. (Between one minute, 33 seconds and one minute, 36 seconds elapsed between the time of the last Mode C return and airplane impact.) The airplane's air data computer also produces electrical output signals for the autopilot. Thus, the complete loss of AC power would have caused the autopilot to cease operating if it were being used.

IASA