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BOEING 737 SYSTEMS REVIEW
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PNEUMATICS
1. GENERAL
¡ Pneumatic supply can be provided by the engines, APU, or a ground service source.
¡ The APU or ground source pressurizes the pneumatics manifold until engine start. The engines
normally supply bleed air for pneumatics after engine start.
2. BLEED AIR SYSTEM
The following systems use the pneumatic system for operation:
Air conditioning / pressurization
Wing & engine thermal anti-icing
Engine starting
Hydraulic reservoirs pressurization
Water tank pressurization
Aspirated TAT probe.
Engine Bleed System Supply
¡ Bleed air is obtained from the 5th or 9th stage of the compressor section of the engine.
¡ When 5th low stage pressure is insufficient (at low thrust settings) , the 9th stage modulating and
shutoff valve opens to maintain proper pressure. During takeoff, climb, and most cruise conditions,
the pressure available from the low stage port is adequate and the 9th stage modulating and shutoff
valve remains closed.
Engine Bleed Air Valves
¡ The Engine Bleed Valve combines the following functions :
- Pressure Regulator Valve
- Shutoff Valve
- Reverse Flow Check Valve.
- Reduces pneumatic outflow in case of high bleed air temperature downstream of the precooler.
With the Engine Bleed Air Switch ON, it is DC activated and pneumatically operated.
Bleed Trip Sensors
¡ Bleed trip sensors illuminate the respective BLEED TRIP OFF Light, when engine bleed air
temperature or pressure exceeds a predetermined limit, and close the respective engine bleed air
valve automatically.
Precooler and Precooler Control Valve
¡ A precooler, or heat exchanger, cools engine bleed air. Engine fan air is ducted through the
precooler at a rate determined by a thermostatic precooler control valve.
¡ If the temperature of the engine bleed air leaving the precooler increases, the precooler control valve
modulates open.
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BOEING 737 SYSTEMS REVIEW
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Starter Valve
¡ The starter valve opens when the Engine Start Switch is placed to GRD. The APU, ground air source
or engine bleed air is then used to start the engine. The engine starter valve is DC 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.
¡ Differences between L & R duct pressure on the bleed air duct pressure indicator are considered
normal as long as there is sufficient air for cabin pressurization.
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 pneumatic manifold during normal operation.
¡ The isolation valve is AC operated.
¡ In the AUTO position the valve is :
- closed (normal operation) if both engine bleed air switches ON and both air conditioning pack
switches AUTO or HIGH.
- open if any engine bleed air switch or air conditioning pack switch is in OFF.
Ground Service Pneumatic Connection
¡ A pneumatic ground source provides an alternate pneumatic source for engine start or air
conditioning.
APU Bleed Air Valve
¡ The APU bleed air valve regulates APU bleed air to the pneumatic manifold.
¡ The APU bleed air may be used on ground and in flight.
¡ The valve closes automatically when the APU is shut down. The APU bleed valve is DC controlled
and pneumatically operated.
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BOEING 737 SYSTEMS REVIEW
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3. CONTROLS & INDICATORS
¡ WING-BODY OVERHEAT TEST SWITCH
PRESS :
- Tests the wing-body overheat detector circuits.
- Both WING-BODY OVERHEAT Lights illuminate.
¡ ISOLATION VALVE SWITCH
AC powered; fails in last position.
CLOSE :
- Closes the isolation valve.
AUTO :
- Closes the isolation valve if all Engine Bleed Air and Air conditioning Pack Switches are ON.
- Opens the isolation valve automatically if either Engine Bleed Air or Air conditioning Pack Switch is
positioned OFF.
OPEN :
- Opens the isolation valve.
¡ TRIP RESET SWITCH
PRESS :
If the fault condition has been corrected, (does not reset the wing body overheat) resets :
B737 Classics :
- BLEED TRIP OFF Light(s)
- PACK TRIP OFF Light(s)
- DUCT OVERHEAT /ZONE TEMP Light(s).
Lights remain illuminated until reset.
B737 NG :
- BLEED TRIP OFF Light(s)
- PACK OFF Light(s)
- ZONE TEMP Light(s).
Lights remain illuminated until reset.
¡ ENGINE BLEED AIR SWITCH
DC powered but requires air to open
OFF :
- Closes the engine bleed air valve.
ON :
- Opens the engine bleed air valves when the engine is operating.
¡ APU BLEED AIR SWITCH
DC powered but requires air to open
OFF :
- Closes the APU bleed air valve.
ON :
- Opens the APU bleed air valve if the APU is operating.
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BOEING 737 SYSTEMS REVIEW
4. FAULTS & INDICATIONS
VALID FOR
LIGHT
BLEED
TRIP OFF
DUAL
BLEED
INDICATION
300
400
500
600
700
800
900
BLEED TRIP LIGHT (amber)
ILLUMINATED :
- Indicates excessive engine bleed air temperature or
pressure.
- Associated engine bleed air valve closed
automatically and requires reset.
X
X
X
X
X
X
DUAL BLEED LIGHT (amber)
ILLUMINATED :
- APU bleed air valve OPEN and No. 1 Engine Bleed
Switch ON.
- No. 2 Engine Bleed Switch ON, APU bleed air valve
& isolation valve OPEN.
X
X
X
X
X
X
X
X
X
X
X
X
Note : With both APU & Engine Bleed air valves open
And the engines operating at idle thrust, there is a
th
Possibility of APU Bleed air backpressuring the 9
Stage modulating & shutoff valve. This would cause
th
The 9 stage valve to close.
WING-BODY
OVERHEAT
WING-BODY OVERHEAT LIGHT (amber)
LEFT LIGHT ILLUMINATED :
- Indicates overheat from a bleed air duct leak in the
left engine strut, left wing leading edge, left air
conditioning bay, keel beam or APU bleed air duct.
RIGHT LIGHT ILLUMINATED :
- Indicates overheat from a bleed air duct leak in the
right engine strut, right wing leading edge or right
air conditioning bay.
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Ludovic ANDRE / version 00
Smartcockpit.com
BOEING 737 SYSTEMS REVIEW
Copyright  Smartcockpit.com
Ludovic ANDRE / version 00
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