B737 NG
Hydraulics
Boeing B737 NG Systems Summary [Hydraulics]
Introduction
The airplane has three hydraulic systems: A, B and standby. The standby system
is used if system A and/or B pressure is lost. The hydraulic systems power the
following airplane systems:
• flight controls
• wheel brakes
• leading edge flaps and slats
• nose wheel steering
• trailing edge flaps
• thrust reversers
• landing gear
• autopilots.
Either A or B hydraulic system can power all flight controls with no decrease in
airplane controllability.
Each hydraulic system has a fluid reservoir located in the main wheel well area.
System A and B reservoirs are pressurized by bleed air. The standby system
reservoir is connected to the system B reservoir for pressurization and servicing.
Pressurization of all reservoirs ensures positive fluid flow to all hydraulic pumps.
Hydraulic Power Distribution Schematic
AIR
PRESSURE
SYSTEM
A
SYSTEM
B
STBY
SYSTEM
M
AIR
PRESSURE
M
M
POWER
TRANSFER
UNIT
AUTO SLATS
AND
LEADING EDGE
FLAPS & SLATS
PTU CONTROL
VALVE
NUMBER 1
THRUST
REVERSER
RUDDER
NUMBER 2
THRUST
REVERSER
AILERONS
FLIGHT
SPOILERS
ALTERNATE
BRAKES
AUTOPILOT A
NOSE WHEEL
STEERING
GROUND
SPOILERS
ELEVATOR AND
ELEV FEEL
LANDING
GEAR
LANDING GEAR
TRANSFER UNIT
Page 1
FLIGHT
SPOILERS
ALTERNATE NOSE
WHEEL STEERING
AUTOPILOT B
YAW
DAMPER
NORMAL
BRAKES
TRAILING
EDGE FLAPS
Boeing B737 NG Systems Summary [Hydraulics]
A and B Hydraulic Systems
Components powered by hydraulic systems A and B are:
System A
•
•
•
•
•
•
•
•
•
•
•
ailerons
rudder
elevator and elevator feel
flight spoilers
(two on each wing)
ground spoilers
alternate brakes
No. 1 thrust reverser
autopilot A
normal nose wheel steering
landing gear
power transfer unit (PTU)
System B
•
•
•
•
•
•
•
•
•
•
•
•
•
ailerons
rudder
elevator and elevator feel
flight spoilers
(two on each wing)
leading edge flaps and slats
normal brakes
No. 2 thrust reverser
autopilot B
alternate nose wheel steering
landing gear transfer unit.
autoslats
yaw damper
trailing edge flaps.
A and B Hydraulic System Pumps
Both A and B hydraulic systems have an engine–driven pump and an AC electric
motor–driven pump. The system A engine–driven pump is powered by the No. 1
engine and the system B engine–driven pump is powered by the No. 2 engine. An
engine–driven hydraulic pump supplies approximately 4 times the fluid volume of
the related electric motor–driven hydraulic pump.
The ENG 1 (system A) or ENG 2 (system B) pump ON/OFF switch controls the
engine–driven pump output pressure. Positioning the switch to OFF isolates fluid
flow from the system components. However, the engine–driven pump continues
to rotate as long as the engine is operating. Pulling the engine fire warning switch
shuts off the fluid flow to the engine–driven pump and deactivates the related
LOW PRESSURE light.
[Option - Abex electric motor driven hydraulic pumps]
The ELEC 2 (system A) or ELEC 1 (system B) pump ON/OFF switch controls the
related electric motor–driven pump. If an overheat is detected in either system, the
related OVERHEAT light illuminates.
Page 2
Boeing B737 NG Systems Summary [Hydraulics]
[Option - Vickers electric motor driven hydraulic pumps]
The ELEC 2 (system A) or ELEC 1 (system B) pump ON/OFF switch controls the
related electric motor–driven pump. If an overheat is detected in either system, the
related OVERHEAT light illuminates, power is removed from the pump and the
LOW PRESSURE light illuminates.
Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand
on system A, may result in an intermittent LOW PRESSURE light for the
remaining electric motor-driven hydraulic pump. The system A flight
controls LOW PRESSURE light, Master Caution light, and the FLT CONT
and HYD system annunciator lights also illuminate.
Hydraulic fluid used for cooling and lubrication of the pumps passes through a
heat exchanger before returning to the reservoir. The heat exchanger for system A
is located in main fuel tank No. 1 and for system B is in main fuel tank No. 2.
CAUTION: Minimum fuel for ground operation of electric motor-driven
pumps is 760 kgs/1675 lbs in the related main tank.
Pressure switches, located in the engine–driven and electric motor–driven pump
output lines, send signals to illuminate the related LOW PRESSURE light if pump
output pressure is low. A check valve, located in each output line, isolates the
related pump from the system. The related system pressure transmitter sends the
combined pressure of the engine–driven and electric motor–driven pump to the
related hydraulic system pressure indication.
System A Hydraulic Leak
If a leak develops in the engine–driven pump or its related lines, a standpipe in the
reservoir prevents a total system fluid loss. With fluid level at the top of the
standpipe, the reservoir quantity displayed indicates approximately 20% full.
System A hydraulic pressure is maintained by the electric motor–driven pump.
If a leak develops in the electric motor–driven pump or its related lines, or
components common to both the engine and electric motor–driven pumps, the
quantity in the reservoir steadily decreases to zero and all system pressure is lost.
System B Hydraulic Leak
If a leak develops in either pump, line or component of system B, the quantity
decreases until it indicates approximately zero and system B pressure is lost. The
system B reservoir has one standpipe which supplies fluid to both the
engine–driven pump and the electric motor–driven pump. However, with fluid
level at the top of the standpipe, fluid remaining in the system B reservoir is
sufficient for power transfer unit operation.
A leak in system B does not affect the operation of the standby hydraulic system.
Page 3
Boeing B737 NG Systems Summary [Hydraulics]
Power Transfer Unit
The purpose of the PTU is to supply the additional volume of hydraulic fluid
needed to operate the autoslats and leading edge flaps and slats at the normal rate
when system B engine–driven hydraulic pump volume is lost. The PTU uses
system A pressure to power a hydraulic motor–driven pump, which pressurizes
system B hydraulic fluid. The PTU operates automatically when all of the
following conditions exist:
• system B engine–driven pump hydraulic pressure drops below limits
• airborne
• flaps are less than 15 but not up.
• flaps not up.
Landing Gear Transfer Unit
The purpose of the landing gear transfer unit is to supply the volume of hydraulic
fluid needed to raise the landing gear at the normal rate when system A
engine–driven pump volume is lost. The system B engine–driven pump supplies
the volume of hydraulic fluid needed to operate the landing gear transfer unit
when all of the following conditions exist:
• airborne
• No. 1 engine RPM drops below a limit value
• landing gear lever is positioned UP
• either main landing gear is not up and locked.
Standby Hydraulic System
The standby hydraulic system is provided as a backup if system A and/or B
pressure is lost. The standby system can be activated manually or automatically
and uses a single electric motor–driven pump to power:
• thrust reversers
• rudder
• leading edge flaps and slats (extend only)
• standby yaw damper.
Manual Operation
Positioning either FLT CONTROL switch to STBY RUD:
• activates the standby electric motor–driven pump
• shuts off the related hydraulic system pressure to ailerons, elevators and
rudder by closing the flight control shutoff valve
• opens the standby rudder shutoff valve
• deactivates the related flight control LOW PRESSURE light when the
standby rudder shutoff valve opens
• allows the standby system to power the rudder and thrust reversers.
Page 4
Boeing B737 NG Systems Summary [Hydraulics]
[737 modified rudder- installed]
• illuminates the STBY RUD ON, Master Caution, and Flight Controls
(FLT CONT) lights.
Positioning the ALTERNATE FLAPS master switch to ARM, (refer to Chapter 9,
Flight Controls for a more complete explanation):
• activates the standby electric motor–driven pump
• closes the trailing edge flap bypass valve
• arms the ALTERNATE FLAPS position switch
• allows the standby system to power the leading edge flaps and slats and
thrust reversers.
Automatic Operation
Automatic operation is initiated when the following conditions exist:
• loss of system A or B, and
• flaps extended, and
• airborne, or wheel speed greater than 60 kts, and
• FLT CONTROL switch A or B Hydraulic System ON
[737 modified rudder- installed]
OR:
• the main PCU Force Fight Monitor (FFM) trips
Automatic operation:
• activates the standby electric motor–driven pump
• opens the standby rudder shutoff valve
• allows the standby system to power the rudder and thrust reversers.
[737 modified rudder- installed]
• illuminates the STBY RUD ON, Master Caution, and Flight Controls
(FLT CONT) lights.
Page 5
Boeing B737 NG Systems Summary [Hydraulics]
Standby Hydraulic System Schematic
[737 modified rudder - not installed]
FLT CONTROL
A
B
STANDBY
HYD
STBY
RUD
STBY
RUD
OFF
OFF
A ON
B ON
LOW
QUANTITY
LOW
PRESSURE
ALTERNATE FLAPS
B
TRAILING
EDGE FLAP
BYPASS
VALVE
TRAILING
EDGE
DRIVE
UNIT
OFF
LOW
PRESSURE
UP
LOW
PRESSURE
OFF
ARM
DOWN
FROM
SYSTEM B
RESERVOIR
STBY
RES
TO THRUST
REVERSERS
STANDBY
YAW DAMPER
M
TO
EXTEND
L.E.
DEVICES
STANDBY RUDDER
SHUTOFF VALVE
STANDBY
RUDDER PCU
FLIGHT CONTROL
SHUTOFF VALVE
A
RUDDER
ELEVATOR AND
ELEVATOR FEEL
COMPUTER
AILERON
CONDITION:
SYSTEM A LOST, STANDBY SYSTEM AND SYSTEM B PRESSURIZED
Page 6
FLIGHT CONTROL
SHUTOFF VALVE
B
Boeing B737 NG Systems Summary [Hydraulics]
[737 modified rudder - installed]
FLT CONTROL
A
B
STANDBY
HYD
STBY
RUD
STBY
RUD
OFF
OFF
A ON
B ON
LOW
QUANTITY
LOW
PRESSURE
STBY RUD
ON
ALTERNATE FLAPS
B
OFF
LOW
PRESSURE
OFF
ARM
TRAILING
EDGE FLAP
BYPASS
VALVE
TRAILING
EDGE
DRIVE
UNIT
UP
LOW
PRESSURE
DOWN
FROM
SYSTEM B
RESERVOIR
STBY
RES
M
TO THRUST
REVERSERS
STANDBY
YAW DAMPER
STANDBY
RUDDER PCU
FFM
RUDDER
FLIGHT CONTROL
SHUTOFF VALVE
A
ELEVATOR AND
ELEVATOR FEEL
COMPUTER
AILERON
CONDITION:
SYSTEM A LOST, STANDBY SYSTEM AND SYSTEM B PRESSURIZED
STBY RUD ON LIGHT ILLUMINATED
Page 7
TO
EXTEND
L.E.
DEVICES
STANDBY RUDDER
SHUTOFF VALVE
STBY HYD
AUTO ON
LOGIC
FLIGHT CONTROL
SHUTOFF VALVE
B
Boeing B737 NG Systems Summary [Hydraulics]
Standby Hydraulic System Leak
If a leak occurs in the standby system, the standby reservoir quantity decreases to
zero. The LOW QUANTITY light illuminates when the standby reservoir is
approximately half empty. System B continues to operate normally, however, the
system B reservoir fluid level indication decreases and stabilizes at approximately
72% full.
Variations in Hydraulic Quantity Indications
During normal operations, variations in hydraulic quantity indications occur
when:
• the system becomes pressurized after engine start
• raising or lowering the landing gear or leading edge devices
• cold soaking occurs during long periods of cruise.
These variations have little effect on systems operation.
If the hydraulic system is not properly pressurized, foaming can occur at higher
altitudes. Foaming can be recognized by pressure fluctuations and the blinking of
the related LOW PRESSURE lights. The MASTER CAUTION and HYD
annunciator lights may also illuminate momentarily.
Page 8
Boeing B737 NG Systems Summary [Hydraulics]
Hydraulic Panel
LOW
PRESSURE
OVERHEAT
OVERHEAT
LOW
PRESSURE
LOW
PRESSURE
LOW
PRESSURE
ENG 1 ELEC 2
ELEC 1 ENG 2
ON
ON
OF
A
HYD PUMPS
OFF
B
FORWARD OVERHEAD PANEL
Electric Hydraulic Pump OVERHEAT Lights
Illuminated (amber) – Hydraulic fluid used to cool and lubricate the
corresponding electric motor driven pump has overheated or the pump itself has
overheated.
Hydraulic Pump LOW PRESSURE Lights
Illuminated (amber) – output pressure of associated pump is low.
Note: When an engine fire warning switch is pulled, the low pressure light is
deactivated.
ELECTRIC HYDRAULIC PUMPS Switches
ON – provides power to associated electric motor–driven pump.
OFF – electrical power removed from pump.
ENGINE HYDRAULIC PUMPS Switches
ON – de–energizes blocking valve in pump to allow pump pressure to enter
system.
Note: Should remain ON at shutdown to prolong solenoid life.
OFF – energizes blocking valve to block pump output.
Page 9
Boeing B737 NG Systems Summary [Hydraulics]
Hydraulic Indications
[Option - Side by Side display]
A
3
2
B
1
95
4
3
4
0
2
1
HYD P
HYD Q %
0
70 RF
UPPER DISPLAY UNIT
HYDRAULIC System PRESSURE Indications
Indicates system pressure:
• displayed (white) - normal operating range
• displayed (amber) - caution range
• displayed (red) - operating limit reached.
Note: When both pumps for a system are OFF, respective pointer reads zero.
Hydraulic Pressure Amber Bands/Redlines
Displayed (amber) - low/high hydraulic pressure caution range.
Displayed (red) - low/high hydraulic pressure operating limit.
HYDRAULIC System QUANTITY Indications
Indicates digital percentage (0% to 106%) of hydraulic quantity.
Note: Quantity also displayed at each reservoir.
REFILL Indication (RF) (white)
Illuminated (white) – hydraulic quantity below 76%.
Note: Valid only when airplane is on ground with both engines shutdown or after
landing with flaps up during taxi–in.
Page 10
Boeing B737 NG Systems Summary [Hydraulics]
[Option - Over/Under display]
MFD
ENG
SYS
CENTER FORWARD PANEL
HYDRAULIC
A
QTY %
PRESS
98
3000
B
67 RF
3000
LOWER DISPLAY UNIT
MFD System (SYS) Switch
Push – SYS
• displays hydraulic indications on lower DU; or if the lower DU is
unavailable, displays it on upper DU or inboard DU based on the position
of the display select panel selector
• second push blanks lower DU.
HYDRAULIC System QUANTITY Indications (white)
Indicates digital percentage (0% to 106%) of hydraulic quantity.
Note: Quantity also displayed at each reservoir.
HYDRAULIC System PRESSURE Indications (white)
Indicates system pressure:
• Normal pressure – 3000 psi
• Maximum pressure – 3500 psi.
Note: When both pumps for a system are OFF, the indication may read hydraulic
system reservoir pressure, normally less than 100 psi.
REFILL Indication (RF) (white)
Illuminated (white) – hydraulic quantity below 76%.
Note: Valid only when airplane is on ground with both engines shutdown or after
landing with flaps up during taxi–in.
.
Page 11
Boeing B737 NG Systems Summary [Hydraulics]
Flight Control Panel
[B737 modified rudder - not installed]
STANDBY
HYD
FLT CONTROL
A
B
STBY
RUD
STBY
RUD
OFF
OFF
A ON
B ON
LOW
QUANTITY
LOW
PRESSURE
ALTERNATE FLAPS
OFF
LOW
PRESSURE
UP
LOW
PRESSURE
OFF
SPOILER
A
B
ARM
OFF
OFF
ON
ON
DOWN
FEEL
DIFF PRESS
SPEED TRIM
FAIL
YAW DAMPER
MACH TRIM
FAIL
YAW
DAMPER
AUTO SLAT
FAIL
OFF
ON
FORWARD OVERHEAD PANEL
Page 12
Boeing B737 NG Systems Summary [Hydraulics]
[737 modified rudder - installed]
STANDBY
HYD
FLT CONTROL
A
B
STBY
RUD
STBY
RUD
OFF
OFF
A ON
B ON
LOW
QUANTITY
LOW
PRESSURE
STBY
RUD ON
ALTERNATE FLAPS
OFF
LOW
PRESSURE
UP
LOW
PRESSURE
OFF
SPOILER
A
B
ARM
DOWN
OFF
OFF
FEEL
DIFF PRESS
ON
ON
SPEED TRIM
FAIL
YAW DAMPER
MACH TRIM
FAIL
YAW
DAMPER
AUTO SLAT
FAIL
OFF
ON
FORWARD OVERHEAD PANEL
FLIGHT CONTROL Switches
STBY RUD – activates standby pump and opens standby rudder shutoff valve to
pressurize standby rudder power control unit.
OFF – closes flight control shutoff valve isolating ailerons, elevators and rudder
from associated hydraulic system pressure.
ON (guarded position) – normal operating position.
.
Page 13
Boeing B737 NG Systems Summary [Hydraulics]
Flight Control LOW PRESSURE Lights
Illuminated (amber) –
• indicates low hydraulic system (A or B) pressure to ailerons, elevator and
rudder
• deactivated when associated FLIGHT CONTROL switch is positioned to
STBY RUD and standby rudder shutoff valve opens.
STANDBY HYDRAULIC LOW QUANTITY Light
Illuminated (amber) –
• indicates low quantity in standby hydraulic reservoir
• always armed.
STANDBY HYDRAULIC LOW PRESSURE Light
Illuminated (amber) –
• indicates output pressure of standby pump is low
• armed only when standby pump operation has been selected or automatic
standby function is activated.
ALTERNATE FLAPS Master Switch
OFF (guarded position) – normal operating position.
ARM – closes trailing edge flap bypass valve, activates standby pump, and arms
ALTERNATE FLAPS position switch.
STBY RUD ON Light
[737 modified rudder - installed]
Illuminated (amber) - indicates the standby hydraulic system is commanded on to
pressurize the standby rudder power control unit.
Page 14