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747-400 MAINTENANCE MANUAL
 ENGINE BLEED AIR PRESSURE INDICATING SYSTEM - DESCRIPTION AND OPERATION
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General
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 A.  The pressure in the pneumatic manifold is sensed by two duct pressure transmitters. Pressure signals from the transmitters are sent to the the CMCS and EICAS via the ASCTU. The display of system pressure is shown on the main EICAS display, ECS synoptic page and on the ECS maintenance page.
 B.  The overpressure switch provides system fault indication on EICAS during an overpressure condition. A signal is sent to the ASCTU logic circuits to illuminate the SYS FAULT light and generate a BLEED 1, 2, 3 or 4 EICAS advisory message along with its associated BLEED OVPRESS 1, 2, 3 or 4 EICAS status message.
 C.  The bleed air pressure sensor continuously monitors the engine duct pressure downstream of the precooler and provides information to the CMCS.
Duct Pressure Transmitter
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 A.  There are two pressure transmitters (L, R) installed in the pneumatic crossover manifold, both transmitters are located upstream of their respective wing isolation valves. The transmitters can be accessed through the air conditioning bay, near packs 1 and 3. The pneumatic manifold duct pressure is shown on the main EICAS display, ECS synoptic page and on the ECS maintenance page.
 B.  The transmitters are strain gage based pressure transducers. Pressure signals from the transducers are sent to the EICAS and CMCS via the ASCTU.
Bleed Air Pressure Sensor
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 A.  The bleed air pressure sensor is connected by a pneumatic sense line to a fitting at the precooler outlet. It senses the engine duct pressure and sends the pressure information to the ASCTU. The engine duct pressure is shown on the ECS maintenance page.
 (1) To monitor the engine duct pressure, look at the ENG DUCT PRESSURE
 displayed on the ECS maintenance pages (AMM 45-10-00/201).Bleed Air Overpressure Switch
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 A.  There are four bleed air overpressure switches installed in the wing leading edge. Access to the overpressure switch is through a panel on the wing leading edge inboard of the engine strut. The overpressure switch is located next to the pressure sensor.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

SEE A
SEE B
A
DUCT PRESSURE TRANSMITTER
____ THE RIGHT DUCT PRESSURE
NOTE: TRANSMITTER IS SHOWN. THE LEFT DUCT PRESSURE TRANSMITTER IS OPPOSITE.
FWD
B
Duct Pressure Transmitter Installation Figure 1
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

BLEED AIR PRESSURE SENSOR
BLEED AIR
OVERPRESSURE
ENGINE 3

SWITCH
BLEED AIR

WING RIB
PRESSURE SENSOR DRAIN HOLES
ENGINE 4 SEE A
BLEED AIR PRESSURE SENSOR
SEE B
FWD
ENGINE 3 BLEED AIR PRESSURE SENSOR (ENGINE 2 BLEED AIR PRESSURE SENSOR IS OPPOSITE)
A
BLEED AIR
PRESSURE

BLEED AIR
SENSOR

OVERPRESSURE SWITCH
DRAIN HOLES
WING RIB
FWD
ENGINE 4 BLEED AIR PRESSURE SENSOR
(ENGINE 1 BLEED AIR PRESSURE SENSOR IS OPPOSITE)

B
Bleed Air Pressure Sensor and Bleed Air Overpressure Switch Figure 2
L98689
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