737 Flight Crew Operations Manual
Air Systems -
Bleed Air System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
2.20.2 D6-27370-866-EGP
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.
External Air Connection
An external air cart/source provides an alternate air source for engine start or air
conditioning.
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 back-pressuring the 9th stage
modulating and shutoff valve. This would cause the 9th stage valve to close.
DUAL BLEED Light
The DUAL BLEED light illuminates whenever the APU bleed air valve is open
and the position of the engine bleed air switches and isolation valve would permit
possible backpressure of the APU. Therefore, thrust must be limited to idle with
the DUAL BLEED light illuminated.
May 15, 2008
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