AUTO FAIL Light
Illuminated (amber) – automatic pressurization system failure detected:
.
indicates a single controller failure when ALTN light is also illuminated

.
indicates a dual controller failure when illuminated alone.

OFF Schedule (SCHED) DESCENT Light
Illuminated (amber) – airplane descended before reaching the planned cruise altitude set in the FLT ALT indicator.

Flight Altitude (FLT ALT) Indicator
.
indicates selected cruise altitude

.
set before takeoff.

Flight Altitude Selector
Rotate – set planned cruise altitude.(-1,000 ft. to 42,000 ft. in 500 ft. increments).
Copyright . The Boeing Company. See title page for details.
Air Systems -Controls and Indicators 737 Flight Crew Operations Manual

Landing Altitude (LAND ALT) Indicator
.
indicates altitude of intended landing field

.
set before takeoff.

Landing Altitude Selector
Rotate – select planned landing field altitude. (-1,000 ft. to 14,000 ft. in 50 ft. increments).

MANUAL Light
Illuminated (green) – pressurization system operating in the manual mode.

Alternate (ALTN) Light
Illuminated (green) – pressurization system operating in the alternate automatic mode:
.  
Illumination of both ALTN and AUTO FAIL lights indicates a single
controller failure and automatic transfer to ALTN mode

.
pressurization mode selector in ALTN position.

Outflow VALVE Position Indicator
. indicates position of outflow valve
. operates in all modes.

Outflow Valve Switch (spring–loaded to center)
CLOSE – closes outflow valve electrically with pressurization mode selector in MAN position.
OPEN – opens outflow valve electrically with pressurization mode selector in MAN position.

Pressurization Mode Selector
AUTO – pressurization system controlled automatically.
ALTN – pressurization system controlled automatically using ALTN controller.
MAN –
.
pressurization system controlled manually by outflow valve switch

.
both auto controllers bypassed.

Cabin /Flight Altitude (CAB ALT)(FLT ALT) Placard
Used to determine setting for cabin altitude when operating in manual mode.
Copyright . The Boeing Company. See title page for details.
2.10.16   September 26, 2003

737 Flight Crew Operations Manual

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
YA721, YA722

.  
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.
Copyright . The Boeing Company. See title page for details.
Air Systems -Bleed Air System Description 737 Flight Crew Operations Manual

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.
　
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