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
- Hydroulic 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 take off, clim, 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 bledd 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.
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.
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 open 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 atar 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 dhut down. The APU bleed air valve is DC controlled and pressure operateds.
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 backpressuring the 9th stage modulaitng 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.
Bleed Air System Schematic
[B737 - 600/700 with Non-Aspirated TAT or with Port-Capped TAT]
[B737 - 800/900 with Aspirated TAT]
Wing-Body Overheat Duct and Lights
A wing-body overheat condition is caused by a bleed air duct leak. It is sensed by the overheat sensors located as shown.
Wing-Body Overheat Ducts and Lights
[B737 - 600/700]
[B737 - 800/900]