737 Flight Crew Operations Manual
Air Systems Chapter 2
Bleed Air System Description Section 20
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 2.20.1
20 Bleed Air System DescriptionBleed Air System DescriptionFile Hig hlight
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
• Aspirated TAT probe
Switches on the air conditioning panel operate the APU and engine bleed air
supply system.
Engine Bleed System Supply
Engine bleed air is obtained from the 5th and 9th stages of the compressor section.
When 5th stage low pressure bleed air is insufficient for the bleed air system
requirements, the high stage valve modulates open to maintain adequate bleed air
pressure. During takeoff, climb, and most cruise conditions, low pressure bleed air
from the 5th stage is adequate and the high stage valve remains closed.
Engine Bleed Air Valves
The engine bleed air valve acts as a pressure regulator and shutoff valve. With the
engine bleed air switch ON, the valve is DC activated and pressure operated. The
valve maintains proper system operating pressure and reduces bleed air outflow in
response to high bleed air temperature.
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.
September 28, 2006
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