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BCD PRESSURIZATION OUTFLOW SCHEMATIC ...............56
AUTO MODE OPERATION
(AIRCRAFT WITH DCPCS >@BCD) ...................................57
BCD MANUAL MODE OPERATION
(AIRCRAFT WITH DCPCS) .........................................................59
BCD AIRCRAFT WITH DCPCS ...............................................59
>@BCD DIGITAL CABIN PRESSURE CONTROL
SYSTEM (DCPCS) CONTROLS AND INDICATORS.................60
AIR SYSTEMS
Sec. 6.2 TOC-4 737
Rev. 11/15/02 #41 Continental Flight Manual
INTENTIONALLY LEFT BLANK
AIR SYSTEMS
737 Sec. 6.2 Page 1
Flight Manual Continental Rev. 11/15/02 #41
BLEED AIR SYSTEM DESCRIPTION
INTRODUCTION
Air for the bleed air system can be supplied by the engines, APU, or an external
air cart / source. The APU or external cart supplies air to the bleed air duct
prior to engine start. After engine start, air for the bleed air system is normally
supplied by the engines.
The following systems rely on the bleed air system for operation:
· Air conditioning / pressurization
· Wing and engine thermal anti-icing
· Engine starting
· Hydraulic reservoirs pressurization
· Water tank pressurization
Switches on the pneumatic panel operate the APU and engine bleed air supply
system.
ENGINE BLEED SYSTEM SUPPLY
Bleed air is obtained from the 5th and 9th stages of the compressor section of
the engine. When 5th low-stage pressure is insufficient, the 9th high-stage valve
modulates open to maintain proper pressure. During takeoff, climb, and most
cruise conditions, the pressure available from the low-stage port is adequate.
The high-stage valve remains closed when 5th stage pressure is adequate.
Engine Bleed Air Valves
The engine bleed valve acts as a pressure regulator, shutoff valve. With the
engine bleed air switch ON, it is normal DC activated and pneumatically
operated. The valve acts as a pressure regulator, maintaining system pressure
below the preset outflow pressure.
The bleed valve also reduces pneumatic outflow in response to a signal of high
bleed air temperature downstream of the precooler.
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.
AIR SYSTEMS
Sec. 6.2 Page 2 737
Rev. 11/15/02 #41 Continental Flight Manual
Precooler And Precooler Control Valve
A precooler, or heat exchanger, precools engine bleed air. Engine fan air is
ducted through the precooler at a rate determined by a thermostatic precooler
valve. If the temperature of the engine bleed air leaving the precooler increases,
the precooler control valve modulates open. The fan air from the precooler
control valve extracts heat from the crossflow heat exchanger, then is
discharged into the engine core cavity and then eventually overboard.
Starter Valve
The starter valve opens when the engine start switch is placed to GRD. The
APU, ground air cart, or engine bleed air is then used to start the engine. The
engine start valve is DC (battery bus) activated and pneumatically operated.
Duct Pressure Transmitters
Duct pressure transmitters provide pneumatic duct pressure indications to the
respective (L and R) pointers on the pneumatic duct pressure indicator. The
indicator is AC operated.
Thermal Anti-Icing (TAI)
Engine bleed air is used for both wing and engine cowl lip anti-icing.
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.
AIR SYSTEMS
737 Sec. 6.2 Page 3
Flight Manual Continental Rev. 11/15/02 #41
APU Bleed Air Valve
The APU bleed valve furnishes APU bleed air to the pneumatic manifold and
may be used on the ground or in the air. The valve closes automatically when
the APU is shut down. The APU bleed valve is DC (battery bus) activated and
pneumatically operated.
With the APU bleed air valve open and the engines operating at idle thrust,
there is a possibility of APU bleed air backpressuring the 9th stage modulating
　
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