Bleed Air System
Bleed air is provided by a centrifugal load
compressor. It can be used on the ground
to start the engines or to supply air to the air conditioning for comfort. During takeoff, the supply of APU bleed air to the air conditioning can help avoid the use of engine bleed air when takeoff performance is crucial. Inflight,bleed air can be used as a backup to the air conditioning or if there is a need to restart an engine.
Under no circumstances can APU bleed air be used for wing anti-ice! Bleed air generated by the centrifugal load compressor is directed to a butterfly- type Bleed Control Valve (BCV). This BCV discharges this compressed air to the exhaust when the valve is in the closed position or delivers it to the pneumatic system when it is in the open position. When the APU BLEED pushbutton on the overhead panel is pressed, the ECB commands the BCV in the open position to deliver compressed air to the pneumatic system. In the ON position, bleed air is always at 100 percent and HIGH FLOW for the PACKS is automatically selected, regardless of the selection of the PACK FLOW rotary switch. APU bleed air pressure is displayed on the APU page. If ADIRU 2 is not powered or selected OFF, the pressure is replaced by two amber X's because the reference pressure is measured by ADIRU 2.
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