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is used during descent and other low power setting operations.
To prevent damage to ducting and equipment downstream, the pressure regulating
valve (PRV) limits bleed air pressure. The fan air precooler regulates bleed air
temperature.
With an Engine Bleed Air switch ON, system logic allows bleed air to open the
HP bleed valve and the PRV and allows bleed air to open the respective engine
bleed air valve and flow into the bleed air duct.. The bleed air valves are pressure
actuated and remain closed until engine bleed air pressure causes forward flow.
The engine bleed air valve regulates the engine bleed air to provide normal bleed
air system pressure. It also prevents reverse flow of bleed air from the duct, except
during the engine start sequence. When air pressure in the bleed air duct from
another source (APU, ground air cart, another engine) is higher than the bleed air
from an engine, the engine bleed air valve closes to prevent reverse flow.
During engine start, the engine bleed air valve opens and allows reverse flow of
air from the bleed air duct to open the start valve. The PRV is positively closed to
prevent reverse air flow into the engine compressor sections. Any time the engine
start valve is not fully closed, the PRV remains positively closed. After N2
increases past 50%, the start valve closes which enables the PRV to open and the
engine bleed air valve reverse flow prevention is enabled.
If the engine start valve fails to close, bleed air is isolated from the engine starter
because both the PRV and the engine bleed air valve remain closed. Nacelle
anti-ice is not available for the respective engine.
• air conditioning
• pressurization
• wing and engine anti–ice
• engine start
• leading edge flaps
• aft cargo heat
• cargo smoke detection
• hydraulic reservoir
pressurization
• potable water tank
pressurization
• air driven hydraulic demand
pumps
• thrust reversers
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
2.40.2 D6-30151-425
Air Systems -
Bleed Air System Description
If an Engine Bleed Air switch is off, the respective engine bleed air valve, PRV,
and HP bleed valve are closed.
If a bleed air overheat is detected, the PRV and HP bleed valves close. Pushing an
Engine Bleed Air switch from off to ON resets the engine bleed fault detection
system.
Bleed air is available for nacelle anti-ice operation with the Engine Bleed Air
switch ON except when:
• the PRV has failed closed, or
• the PRV has been closed due a bleed air overheat, or
• the start valve is not closed
Bleed air is available for nacelle anti-ice operation with the Engine Bleed Air
switch off except when:
• the PRV has failed closed, or
• the PRV has been closed due a bleed air overheat, or
• the start valve is not closed, or
• the HP bleed valve is failed open
Bleed air is available for thrust reverser operation with the Engine Bleed Air
switch either ON or off except when:
• the PRV has failed closed, or
• the PRV has been closed due a bleed air overheat, or
• the start valve is not closed
APU Bleed Air Supply
APU bleed air is used primarily during ground operations for pack operation and
engine starting. APU bleed air is available in flight.
With the APU bleed air switch ON, the APU bleed valve opens when the APU can
supply bleed air. The EICAS memo message APU RUNNING is displayed when
APU N1 is 95% and higher. APU bleed air is supplied through the center section
of the bleed air duct. The check valve in the APU supply line prevents reverse flow
of bleed air from the duct into the APU.
Bleed duct overheat protection is provided in the APU bleed air system to detect
leaks. If the APU shuts down for an overheat, it cannot be restarted.
Ground Bleed Air Supply
External connectors are provided to connect a ground source of high pressure air
directly to the bleed air duct.
Check valves prevent reverse flow of bleed air from the bleed air duct to the
connectors.
October 1, 2000
747 Operations Manual
Air Systems -
Bleed Air System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 2.40.3
Bleed Air Duct System
The left and right isolation valves separate the bleed air duct into three sections:
left, center, and right. The system normally operates with the isolation valves
open. The valves are controlled by the left and right isolation valve switches.
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