Bleed Trip Sensors
Bleed trip sensors illuminate the respective BLEED TRIP OFF light when engine bleed air
temperature or pressure exceeds a predetermined limit. The respective engine bleed air
valve closes automatically.
Duct Pressure Transmitters
Duct pressure transmitters provide bleed air pressure indications to the respective (L and
R) pointers on the bleed air duct pressure indicator. The indicator is AC operated.
Differences between L and R duct pressure on the bleed air duct pressure indicator are
considered normal as long as there is sufficient air for cabin pressurization.
Isolation Valve
The isolation valve isolates the left and right sides of the bleed air duct during normal
operations. The isolation valve is AC operated. With the isolation valve switch in AUTO,
both engine bleed air switches ON, and both air conditioning pack switches AUTO or HIGH,
the isolation valve is closed. The isolation valve opens if either engine bleed air switch or
air conditioning pack switch is positioned OFF. Isolation valve position is not affected by the
APU bleed air switch.
APU Bleed Air Valve
The APU bleed air valve permits APU bleed air to flow to the bleed air duct. The valve
closes automatically when the APU is shut down. The APU bleed air valve is DC controlled
and pressure operated. With both the APU and engine bleed air valves open, and the
engines operating at idle thrust, there is a possibility of APU bleed air backpressuring the
9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.
