B757 Air Systems
DO NOT USE FOR FLIGHT Boeing B757 - Systems Summary [Air Systems]
2.20 Air Systems-Air Conditioning System Description
Air Conditioning Introduction
The air conditioning system supplies conditioned bleed air and recirculated cabin
air at a controlled temperature throughout the airplane.
The system supplies conditioned air to the flight deck shoulder heaters.
The system supplies ventilation for the cabin:
• lavatories
• galleys
• individual passenger seat gaspers.
Pack control, zone temperature control, cabin air recirculation, fault detection, and
overheat protection are all automatic.
The airplane is divided into three temperature zones: the flight deck and two cabin
zones.
Air Conditioning Packs
Two identical air conditioning packs cool bleed air from the engines, APU, or high
pressure air from a ground source. Bleed air is precooled before entering the pack.
The packs are controlled by two identical pack controllers. Pack output is
automatically increased during high pack demand periods (to compensate for a
failed pack or recirculation fan), or limited during high bleed air demand periods
(such as with an engine inoperative).
Air Conditioning Automatic Mode
With the pack selector in the AUTO position, pack output temperature is
determined by the cabin temperature controller. Zone temperature is then satisfied
by the zone temperature controllers using trim air.
Air Conditioning Standby Mode
With the pack selector in the standby mode, pack output temperature is determined
by the position of the pack selector:
• N (normal) – constant, moderate temperature
• C (cool) – full cold
• W (warm) – full warm.
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Boeing B757 - Systems Summary [Air Systems]
Ground Conditioned Air Operation
When a ground source of conditioned air is available, it may be used to supply
conditioned air directly to the cabin distribution system, eliminating the need for
pack operation.
Pack Non–Normal Operation
Pack control, fault detection, and overheat protection are all automatic.
The pack INOP light illuminates and the EICAS advisory message L or R PACK
TEMP displays for all pack control system faults and overheats.
When an automatic control system fault or a pack outlet overheat is detected, the
pack continues to operate in an uncontrolled, degraded condition, requiring crew
interaction.
The pack valve closes and the pack is shut down automatically when an internal
pack overheat is detected. The PACK OFF light illuminates and the EICAS
advisory message L or R PACK OFF displays in addition to the PACK TEMP
indications.
If the INOP light remains illuminated after selecting STBY, the fault is a pack
overheat. After the pack has cooled, an attempt to restore pack operation may be
made by pushing the PACK RESET switch.
Air Distribution
Conditioned air from the packs flows into a mix manifold where it mixes with air
from two recirculation fans. Recirculation fans maintain overall cabin air
circulation while allowing a reduction of cabin air ventilation, permitting the
packs to be operated at a reduced flow. The mixed air is then ducted into the cabin.
The flight deck receives 100% fresh conditioned air from the left pack only and is
maintained at a slightly higher pressure than the passenger cabin. This prevents
smoke from entering the flight deck. When the left pack is inoperative, the flight
deck receives air from the mix manifold.
The left recirculation fan draws air through the forward E/E system and returns it
to the mix manifold. Air that moves from the cabin to the lower deck either
exhausts overboard through the outflow valve or is drawn into the right
recirculation fan and returns to the mix manifold.
The right recirculation fan can be turned off for several minutes to provide a more
rapid exchange of air. The left recirculation fan should not be turned off as this
causes the overboard exhaust valve to latch open. Maintenance action is required
to reset the OVBD EX VAL OPEN status message.
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Boeing B757 - Systems Summary [Air Systems]
The recirculation fan INOP light illuminates and the EICAS advisory message L
or R RECIR FAN displays whenever a recirculation fan fails or the switch is off.
A slight increase in fuel consumption occurs for each fan that is off.
Temperature Control
The airplane is divided into three temperature control compartments:
• flight deck
• forward cabin
• aft cabin.
The pack controllers regulate the pack output air temperature to satisfy the
temperature requirement of the compartment requiring the coolest air. Hot trim air
from the bleed air system is added through trim air valves to control the
temperature in each of the other compartments. Each temperature control
compartment has an associated temperature control.
A compartment INOP light illuminates and the EICAS advisory message FLT
DECK TEMP, FWD CABIN TEMP, or AFT CABIN TEMP displays to indicate:
• zone temperature controller failure
• zone selector is in the OFF position
• trim air switch is OFF
Temperature Control With Loss of Trim Air System
During operation with the trim air system off, the packs attempt to maintain all
compartments at an average temperature. The trim air OFF light illuminates and
the EICAS advisory message TRIM AIR displays when a TRIM AIR switch is off.
Shoulder and Foot Heaters
Flight crew shoulder heat is provided by electric heaters in the ducts that supply
conditioned air to the side windows. Foot heat is provided by electric heating
elements only, with no airflow. The high setting for both shoulder and foot heat is
available in flight only.
Forward Equipment Cooling System
Forward equipment cooling supplies cooling air to the equipment in the forward
equipment racks and the flight deck avionics. The system has a supply fan that
draws cooling air from the cabin and forces it through the equipment racks and
avionics. The air is then exhausted into the air conditioning mix manifold by the
left recirculation fan. If the differential pressure is low, the overboard exhaust
valve automatically opens and most of the air is exhausted overboard.
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Boeing B757 - Systems Summary [Air Systems]
An auxiliary fan automatically operates to provide cooling for essential avionics
and electrical equipment if the alternate supply fan has been selected but both
supply fans are inoperative. However, non–essential avionics and electrical
equipment are subject to imminent failure. The auxiliary fan can be powered by
the hydraulic driven generator.
On the ground, the OVHT light illuminates, the EQPT OVHT message displays
and the ground crew call horn sounds if insufficient cooling airflow is detected.
Equipment Smoke
The equipment cooling SMOKE light illuminates and the EICAS advisory
message FWD EQPT SMOKE displays when smoke is detected in the equipment
cooling system.
The following items, which may be observed in the flight deck, occur
automatically to help evacuate the smoke:
• one or both recirculation fans turn off
• one or both air conditioning packs switch to high flow
• the overboard exhaust valve is latched open.
When smoke is no longer detected, the recirculation fans and air conditioning
packs automatically return to normal operation.
Cargo Heat System
Cargo compartment heating and ventilation is completely automatic. There are no
controls or indicators in the cockpit.
Page 5
Boeing B757 - Systems Summary [Air Systems]
Air Conditioning System Schematic
COMPT TEMP
C
INOP
INOP
INOP
FLT DK
AUTO
FWD CAB
AUTO
AFT CAB
AUTO
OFF
W
C
OFF
W
C
W
OFF
MIX MANIFOLD
TRIM AIR
RECIRC
FAN
L
ON
ON
OFF
INOP
INOP
L PACK
R PACK
RESET
RESET
INOP
INOP
PACK
OFF
PACK
OFF
AUTO
OFF
AUTO
N
OFF
N
S
CT
B
Y
PACK AIR
EXHAUST AIR
MIX MANIFOLD
AIR
R
ON
W
S
CT
B
Y
W
TRIM AIR
PNEUMATIC
SUPPLY
PNEUMATIC
SUPPLY
Page 6
Boeing B757 - Systems Summary [Air Systems]
2.30 Air Systems-Pressurization System Description
Pressurization Introduction
Cabin pressurization is controlled by adjusting the discharge of conditioned cabin
air through the outflow valve.
Positive pressure relief valves and negative pressure relief doors protect the
fuselage against excessive pressure differential.
The pressurization system has automatic and manual operating modes.
Pressurization System Automatic Operation
The pressurization system is in the automatic mode when the cabin altitude mode
selector is set to AUTO 1 or AUTO 2. If the selected auto mode fails, control is
automatically switched to the other auto mode.
In the automatic mode, the pressurization system uses ambient pressure data from
the air data system in conjunction with the selected cabin auto rate, the takeoff
altitude and the indicated landing altitude to calculate the cabin pressurization
schedule.
Takeoff
For takeoff, the system supplies a small positive pressurization to cause a smooth
cabin altitude transition.
Climb
During climb, cabin altitude increases on a schedule related to the takeoff field
elevation, airplane altitude, and the selected auto climb rate limit.
If the maximum cabin pressure differential is reached, cabin climb rate becomes
a function of airplane climb rate, while maintaining the maximum differential
pressure.
Cruise
Shortly after the airplane levels off, the system enters the cruise mode. The landing
altitude and the scheduled cabin altitude are compared and the higher of these two
is selected for the cruise cabin altitude. If necessary, the cabin climbs to the cruise
cabin altitude at one–half the auto rate or descends to the cruise cabin altitude at
the auto rate.
The cruise cabin altitude does not change for minor altitude variations. A
significant altitude change causes the system to re–enter the climb mode or to
enter the descent mode.
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Boeing B757 - Systems Summary [Air Systems]
Descent and Landing
During descent, cabin altitude decreases to slightly below the selected landing
altitude. This ensures that the airplane lands pressurized. Landing altitude
barometric pressure correction comes from the captain’s altimeter.
At touchdown, the outflow valve opens to depressurize the airplane.
Non–Normal Indications
If the cabin altitude climbs above 10,000 feet, the CABIN ALT and CABIN
ALTITUDE lights illuminate and the EICAS warning message CABIN
ALTITUDE displays. The lights extinguish and the EICAS message blanks when
the cabin altitude descends below 8,500 feet.
The AUTO INOP light illuminates and the EICAS caution message CABIN
AUTO INOP displays when automatic pressurization control fails or when cabin
altitude MODE SELECT is MAN. Manual operation is required if the automatic
system fails.
Only the automatic mode is equipped with an aneroid switch that automatically
closes the outflow valve when the cabin altitude exceeds 11,000 feet. The manual
mode has no auto closure feature.
To operate the pressurization system manually:
• set the cabin altitude MODE SELECT to MAN
• hold the cabin altitude MANUAL control to CLIMB to move the outflow
valve toward open and cause the cabin altitude to climb
• hold the cabin altitude MANUAL control to DESCEND to move the
outflow valve toward closed and cause the cabin altitude to descend.
Page 8
Boeing B757 - Systems Summary [Air Systems]
2.40 Air Systems-Bleed Air System Description
Bleed Air Introduction
Bleed air can be supplied by the engines, APU, or a ground air source.
Bleed air is used for:
• air conditioning
• pressurization
• engine start
• wing and engine anti–ice
• hydraulic reservoir
pressurization.
Engine Bleed Air Supply
Engine bleed air comes from either the high pressure (HP) or the low pressure
(LP) engine compressor sections. LP air is used during high power setting
operations. HP air is used during descent and other low power setting operations.
The engine bleed air valves are armed when the engine bleed air switches are
selected ON. The valves are pressure actuated and remain closed until engine
bleed air pressure is sufficient to cause forward flow. The valves may close when
the APU is starting either engine. The valves may close when ground pneumatic
air is connected or during periods of low engine bleed air demand, such as when
the air conditioning packs are OFF. The engine bleed air OFF light illuminates and
the EICAS advisory message L or R ENG BLEED OFF displays when the engine
bleed air valve is closed.
The HI STAGE light illuminates and the EICAS advisory message L or R ENG
HI STAGE displays when engine bleed air pressure is excessive and the high
pressure bleed air valve automatically locks closed.
The engine BLEED light illuminates and the EICAS Caution message L or R
ENG BLEED VAL displays if the engine bleed air temperature is excessive.
APU Bleed Air Supply
APU bleed air is used primarily during ground operations for air conditioning
pack operation and engine starting. In flight, APU bleed air is available up to
approximately 17,000 feet.
The check valve in the APU supply line prevents reverse flow of bleed air from
the duct into the APU.
The APU bleed air VALVE light illuminates and the EICAS advisory message
APU BLEED VAL displays when the APU Bleed Valve position disagrees with
the commanded position.
Page 9
Boeing B757 - Systems Summary [Air Systems]
Ground Pneumatic 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.
Bleed Air Duct System
The duct pressure indicator displays the pressure in the left and right bleed air
ducts.
The isolation valve separates the bleed air ducts into isolated segments. The
isolation valve is normally closed except for engine start or single bleed source
operation.
The isolation VALVE light illuminates and the EICAS advisory message BLEED
ISLN VAL displays when the isolation valve position disagrees with the
commanded position.
The DUCT LEAK light illuminates and the EICAS caution message L or R BLD
DUCT LEAK displays when a high temperature bleed air leak is detected.
Page 10
Boeing B757 - Systems Summary [Air Systems]
Bleed Air System Schematic
R PACK
L PACK
GROUND
PNEUMATIC
GROUND
PNEUMATIC
BLEED AIR
60
80
20
0
DUCT
PRESS
PSI
40
DUCT
LEAK
DUCT
LEAK
ISOLATION
BLEED
HI STAGE
BLEED
HI STAGE
VALVE
L WING ANTI-ICE
HYDRAULIC
RESERVOIRS
R WING ANTI-ICE
HYDRAULIC
RESERVOIRS
L ENG
APU
V
A
L
V
E
O
F
F
ENGINE
ANTI-ICE
STARTER
R ENG
O
F
F
ENGINE
ANTI-ICE
APU
LP
LP
HP
HP
PANEL FLOW LINES
FLOW LINES NOT ON PANEL
Page 11
STARTER
Boeing B757 - Systems Summary [Air Systems]
L PACK
R PACK
GROUND
PNEUMATIC
BLEED AIR
GROUND
PNEUMATIC
60
40
20
80
DUCT
PRESS
PSI
0
DUCT
LEAK
DUCT
LEAK
ISOLATION
BLEED
HI STAGE
BLEED
HI STAGE
VALVE
L WING ANTI-ICE
HYDRAULIC
RESERVOIRS
R WING ANTI-ICE
HYDRAULIC
RESERVOIRS
L ENG
O
F
F
ENGINE
ANTI-ICE
STARTER
APU
R ENG
V
A
L
V
E
O
F
F
APU
ENGINE
ANTI-ICE
LP
LP
HP
HP
STARTER
PANEL FLOW LINES
FLOW LINES NOT ON PANEL
Page 12
Boeing B757 - Systems Summary [Air Systems]
2.50 Air Systems-EICAS Messages
Air Systems EICAS Messages
The following EICAS messages can be displayed.
Message
Level
Light
Aural
Condition
APU BLEED VAL
Advisory
VALVE
L BLD DUCT LEAK
Caution
DUCT
LEAK
BLEED ISLN VAL
Advisory
VALVE
Bleed isolation valve
position disagrees with
the commanded position.
CABIN ALTITUDE
Warning
CABIN
Siren
ALTITUDE,
CABIN
ALT
Cabin altitude excessive.
CABIN AUTO INOP
Caution
AUTO
INOP
Automatic pressurization
control has failed or the
cabin altitude mode
selector is in manual.
AFT CABIN TEMP
Advisory
INOP
A fault in the zone
temperature controller or
trim air switch OFF.
Advisory
OFF
Engine bleed air valve is
closed for a system fault.
Caution
BLEED
Advisory
HI STAGE
Engine bleed system
pressure is excessive and
the high pressure bleed
air valve has
automatically locked
closed.
Advisory
OVHT
Forward equipment
cooling system
inoperative.
R BLD DUCT LEAK
APU bleed valve position
disagrees with the
commanded position.
Beeper
Beeper
FWD CABIN TEMP
L ENG BLEED OFF
R ENG BLEED OFF
L ENG BLEED VAL
R ENG BLEED VAL
L ENG HI STAGE
R ENG HI STAGE
EQPT OVHT
Page 13
Beeper
A high temperature bleed
air leak is detected.
Engine bleed air
temperature excessive.
Boeing B757 - Systems Summary [Air Systems]
Message
Level
Light
Aural
Condition
FWD EQPT SMOKE
Advisory
SMOKE
Smoke detected in the
forward equipment
cooling ducts.
FLT DECK TEMP
Advisory
INOP
Duct overheat, fault in
the zone temperature
controller, or trim air
switch off.
L PACK OFF
Advisory
PACK OFF
Pack valve is closed.
Advisory
INOP
Automatic control system
fault or overheat.
Advisory
INOP
Fan is failed or not
operating.
Advisory
OFF
Trim air switch off.
R PACK OFF
L PACK TEMP
R PACK TEMP
L RECIR FAN
R RECIR FAN
TRIM AIR
Page 14
2.10Boeing
Air Systems-Controls
andSummary
Indicators [Air Systems]
B757 - Systems
Air Conditioning System
Air Conditioning Control Panel
COMPT TEMP
1
2
INOP
INOP
INOP
FLT DK
AUTO
FWD CAB
AUTO
AFT CAB
AUTO
3
C
OFF
C
W
OFF
4
6
OFF
ON
ON
ON
OFF
INOP
INOP
L PACK
RESET
INOP
INOP
8
PACK
OFF
PACK
OFF
AUTO
10
R PACK
RESET
7
OFF
W
RECIRC FAN
R
L
TRIM AIR
5
C
W
N
9
W
S
CT
B
Y
OFF
AUTO
N
W
S
CT
B
Y
OVERHEAD PANEL
1
Compartment Temperature (COMPT TEMP) Indicators
Displays actual temperature sensed in the compartment.
2
Compartment Temperature Inoperative (INOP) Lights
Illuminated (amber) –
• fault in the zone temperature controller
• the Compartment Temperature Control is OFF
• the trim air switch is OFF.
Page 15
11
Boeing B757 - Systems Summary [Air Systems]
3
Compartment Temperature Controls
AUTO –
• provides automatic compartment temperature control
• rotating the control toward C (cool) or W (warm) sets the desired
temperature between 18 degrees C and 30 degrees C.
OFF –
• closes the compartment trim air valve
• the compartment temperature INOP light illuminates.
4
TRIM AIR Switch
ON – the trim air valve is commanded open.
Off (ON not visible) – the trim air valve is commanded closed.
5
Trim Air OFF Light
Illuminated (amber) – the TRIM AIR switch is off.
6
PACK RESET Switches
Push – resets an overheated pack if the pack has cooled to a temperature below the
overheat level.
7
Pack Inoperative (INOP) Lights
Illuminated (amber) –
• the pack is overheated
• fault in the automatic control system.
8
PACK OFF Lights
Illuminated (amber) – the pack valve is closed.
9
PACK Control Selectors
OFF – closes the pack valve.
AUTO – the pack is automatically controlled.
STBY–
• N (normal) – regulates the pack outlet temperature to a constant, moderate
temperature
• C (cool) – sets the pack to full cold operation
• W (warm) – sets the pack to full warm operation.
Page 16
Boeing B757 - Systems Summary [Air Systems]
Recirculation Fan (RECIRC FAN) Switches
• ON – the recirculation fan operates
• Off (ON not visible) – the recirculation fan does not operate.
10
11
Recirculation Fan Inoperative (INOP) Lights
Illuminated (amber) –
• the recirculation fan is failed or not operating
• the RECIRC FAN switch is off.
Shoulder and Foot Heaters
HEATERS
FOOT
HI
LOW
OFF
1
SHOULDER
HI
LOW
OFF
2
LEFT AND RIGHT SIDEWALL
1
FOOT HEATERS Switch
• HIGH – the under–floor electric heater operates on high setting (in flight
only).
• LOW – the under–floor electric heater operates on low setting.
• OFF – the under–floor electric heater is not operating.
SHOULDER HEATERS Switch
• HIGH – the electric heater adds heat at high setting to the conditioned air
flowing at shoulder level (in flight only).
• LOW – the electric heater adds heat at low setting to the conditioned air
flowing at shoulder level.
• OFF – the electric heater is not operating (no heat added to the
conditioned air flowing at shoulder level.).
2
Page 17
Boeing B757 - Systems Summary [Air Systems]
Equipment Cooling Panel
EQUIP COOLING
1
ALTN
SMOKE
OVHT
2
3
OVERHEAD PANEL
1
Equipment Cooling SMOKE Light
Illuminated (amber) – smoke is detected in the forward equipment cooling ducts.
2
Alternate (ALTN) Equipment Cooling Switch
ALTN –
• activates the alternate supply fan
• activates the auxiliary fan, if the alternate supply fan fails.
3
Equipment Cooling Overheat (OVHT) Light
Illuminated (amber) –
• insufficient airflow (on the ground)
• the supply fan is inoperative
• the alternate supply fan is inoperative.
Page 18
Boeing B757 - Systems Summary [Air Systems]
Pressurization System
Cabin Altitude Controls
1
CABIN ALTITUDE CONTROL
VALVE
OP
CLIMB
MANUAL
AUTO RATE
5
2
MIN
CL
MAX
DESCEND
LDG ALT
3
6
AUTO INOP
00340
MODE SELECT
AUTO 2
AUTO 1
4
MAN
7
OVERHEAD PANEL
1
Cabin Altitude MANUAL Control
Spring – loaded to center.
Controls cabin outflow valve position with the Cabin Altitude Mode Selector in
manual (MAN) mode.
CLIMB – moves outflow valve toward open.
DESCEND – moves outflow valve toward closed.
2
Outflow VALVE Position Indicator
OP – Open.
CL – Closed
3
Landing Altitude (LDG ALT) Indicator
Feet.
Displays selected landing altitude.
4
Landing Altitude (LDG ALT) Selector
Rotate – sets landing altitude indicator.
Page 19
Boeing B757 - Systems Summary [Air Systems]
Cabin Altitude AUTO RATE Control
5
Rotate –
• sets limit for cabin altitude rate of climb or descent during auto control
• index mark establishes approximately 500 fpm climb and 300 fpm
descent.
AUTO Inoperative (INOP) Light
6
Illuminated (amber) –
• AUTO 1 and AUTO 2 cabin altitude control functions are inoperative
• MAN mode is selected.
Cabin Altitude MODE SELECTOR
7
Auto 1 –
• activates Auto 1 cabin altitude control for automatic operation
• outflow valve positioned automatically.
Auto 2 –
• activates Auto 2 cabin altitude control for automatic operation
• outflow valve positioned automatically.
MAN (Manual) –
• outflow valve position is controlled by the cabin altitude MANUAL
control
• AUTO INOP light illuminates.
Cabin Altitude Indicators
CABIN
1
10
8
0
6
2
4
DIFF PSI
0
2
25
4
20
15
6
10 8
ALT
1 2
1 2
FT X
1000
2
OVERHEAD PANEL
Page 20
3
0
FPM X
RATE 1000
CABIN
ALTITUDE
4
PRESS DIFF
LIMIT:
TO/LDG
.125 PSI
5
Boeing B757 - Systems Summary [Air Systems]
1
Cabin Differential Pressure Indicator
Pounds per square inch (psi).
2
Cabin Altitude
Feet x 1,000.
3
Cabin Altitude Rate
Feet per minute (fpm x 1,000).
4
Cabin Altitude Light
Illuminated (red) –
• cabin altitude exceeds 10,000 feet
• extinguishes when cabin altitude descends below 8,500 feet.
5
Pressure Differential (PRESS DIFF) Limit Placard
Specifies differential pressure limitation for takeoff and landing.
Cabin Altitude (CABIN ALT) Light
CABIN
ALT
1
CENTER FORWARD PANEL
1
Cabin Altitude Light
Illuminated (red) –
• cabin altitude exceeds 10,000 feet
• extinguishes when cabin altitude descends below 8,500 feet.
Page 21
Boeing B757 - Systems Summary [Air Systems]
Bleed Air System
Bleed Air Control Panel
BLEED AIR
60
1
80
40
0
20
2
DUCT
PRESS
PSI
DUCT
LEAK
DUCT
LEAK
ISOLATION
BLEED
HI STAGE
BLEED
HI STAGE
3
VALVE
L ENG
O
F
F
4
APU
V
A
L
V
E
R ENG
O
F
F
5
OVERHEAD PANEL
1
Duct Pressure (DUCT PRESS) Indicators
Pounds per square inch (psi).
2
ISOLATION Switch
On (bar in view) – commands isolation valve to open.
Off (bar not visible) – commands isolation valve to close.
3
Isolation VALVE Light
Illuminated (amber) – isolation valve position disagrees with commanded
position.
4
Engine (ENG) Bleed Air Switches
On (bar in view) – the engine bleed air valve opens when engine bleed air is
available.
Off (bar not visible) – valve is manually commanded closed.
2.10.8
Copyright © The Boeing Company.
Page See
22title page for details.
D632N001-200
Boeing B757 - Systems Summary [Air Systems]
5
Engine Bleed Air OFF Lights
Illuminated (amber) – engine bleed air valve is closed:
• automatically due to a system fault
• the switch is OFF
• the engine is not running.
6
DUCT LEAK Lights
Illuminated (amber) – a high temperature bleed air leak is detected.
7
Engine BLEED Lights
Illuminated (amber) – the engine bleed air temperature is excessive.
8
Engine High Stage (HI STAGE) Lights
Illuminated (amber) –
• the engine bleed system pressure is excessive
• the engine high pressure bleed air valve is automatically locked closed.
9
APU Bleed Air Switch
On (bar in view) – the APU bleed air valve is automatically controlled
Off (bar not visible) – commands APU bleed air valve to close.
10
APU Bleed Air VALVE Light
Illuminated (amber) – APU bleed air valve position disagrees with commanded
position.
Page 23