The document summarizes the hydraulic systems on a Boeing 737 NG, including: - There are three hydraulic systems - A, B, and a standby system that acts as backup if the other systems lose pressure. - Systems A and B each have an engine-driven pump and electric pump, while the standby only has an electric pump. - The systems power various flight controls and other aircraft components. The standby system can power the rudder, thrust reversers, and leading edge flaps if needed. - The document describes components, indications, and manual or automatic activation methods for the standby system in the event of issues with systems A or B.

1. B 737 NG Ground School.
See the aircraft study guide at www.theorycentre.com
The information contained here is for training purposes only. It is of a general nature it is
unamended and does not relate to any individual aircraft. The FCOM must be consulted for
up to date information on any particular aircraft.

2. HYDRAULIC POWER

3. 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
• leading edge flaps and slats
• trailing edge flaps
• landing gear
• wheel brakes
• nose wheel steering
• thrust reversers
• 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.

5. FWD
MAIN WHEEL WELL
A SYSTEM
HYD
RESERVOIR
B SYSTEM
HYD
RESERVOIR STANDBY SYS
RESERVOIR
RIGHT HAND
MAIN GEAR.

6. A Hydraulic System
Components powered by hydraulic system A are:
• 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)

7. B Hydraulic System
Components powered by hydraulic system B are:
• ailerons
• rudder
• elevator and elevator feel
• flight spoilers
(two on each wing)
• leading edge flaps and slats
• autoslats
• trailing edge flaps.
•normal brakes
• No. 2 thrust reverser
• autopilot B
• alternate gear retraction only.
• alternate nose wheel steering
• yaw damper

8. 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 (Full extend only)
• standby yaw damper.

9. 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 No. 1
engine and system B engine–driven pump is powered by No. 2 engine. An
engine–driven hydraulic pump supplies a much larger fluid volume than 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 switch shuts off
the fluid flow to the engine–driven pump and deactivates the related LOW
PRESSURE light.

11. Controls solenoid on the
pump. NORMALLY ON
Solenoid cold. Extends life.

12. Controls solenoid on the
pump. NORMALLY ON
Solenoid cold. Extends life.
Off energises solenoid
and blocks flow from
the pump.
Re-routes to the case
drain and Fuel oil heat
exchanger.EDP 36 GPM (145 L) at 3,000 PSI

13. EDP output less than 1,300 PSI.

14. EDP output less than 1,300 PSI.
Inhibited if FIRE SWITCH PULLED!

15. A and B Hydraulic System 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, on some aircraft the power
may be removed from the pump and the LOW PRESSURE light will illuminate.
NOTE; System A electric pump is labelled ELECT 2 because it is powered by AC
transfer bus 2.

16. TRANSFER BUS No 2
5.7 GPM (23.0 L) at 3,000 PSI

17. 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 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.

18. Elect 2 output pressure
less than 1,300 PSI

19. Return line cooling fluid more
than 107° C.
MOTOR case more than 113° C.

20. Pulling the engine fire switch shuts off the
fluid flow to the engine–driven pump and
deactivates the related LOW
PRESSURE light.
HYDRAULIC SHUTOFF VALVES

21. 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.
EDP Supply
ELECT 2 supply
100% 21.6 L (5.7 Gal)
76% 17.7 L (4.7 Gal) RF
20% 8.5 L (2.3 Gal) Standpipe

22. 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.
EDP Supply
ELECT 1 supply
PTU supply
100% 31.1 L (8.2 Gal)
76% 26.0 L (6.9 Gal) RF
72% 25.1 L (6.6 Gal) Balance line
0% 4.9 L (1.3 Gal) Standpipe

23. 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.

24. Standby Hydraulic System
The standby hydraulic system is provided as a backup if system A and/or B
pressure is lost. The standby system uses a single electric motor–driven pump to
power:
• thrust reversers
• rudder
• leading edge flaps and slats (extend only)
• standby yaw damper.
The standby system can be activated two ways manually or two ways automatically

26. LOW QUANTITY light
indicates 50% or less in the
STBY Reservoir.
When full the reservoir holds 13.3 litres
of fluid (3.6 Gal)

27. Standby by pump is low pressure.
Only armed when Pump is
commanded to run.

28. Standby Rudder SOV is open and
STBY RUD PCU is pressurised.

29. 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.
• illuminates the STBY RUD ON, Master Caution, and Flight Controls
(FLT CONT) lights.

30. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
STBY RUD switch Normal guarded ON.
OFF position used for maintenance only
Closes six flight control valves in the main wheel well.

31. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
1. Either STBY RUD switch to STBY RUD.
Stand by rudder valve opens.
Standby pump runs.

33. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
Standby PUMP delivers 3.7 GPM @ 2,700 PSI
AC TRANSFER BUS No 2

34. Positioning the ALTERNATE FLAPS master switch to ARM
• 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.

35. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
2. Alternate Flaps Master switch to ARM.

36. 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 (Normal
guarded position.)

37. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
3. Auto Activated
 Low pressure in SYS A or B.
 Flaps Not UP. With;
 Wheel speed more than 60 Knots.
 Or Air mode.
This automatically activates the standby
rudder in a critical flight phase.

38. Automatic Operation
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.
• illuminates the STBY RUD ON, Master Caution, and Flight
Controls (FLT CONT) lights.

40. STANDBY HYDRAULIC SYSTEM CAN BE ACTIVATED IN 4 WAYS
4. Auto Activated If Main Rudder PCU Force Fight
Monitor trips.
Automatically activates the STBY rudder
if system A rudder PCU and system B
PCU are trying to move the rudder in
opposite directions (Force fight)

41. Indicates low hydraulic pressure to flight controls.
Rudder
Ailerons
Elevator

42. Indicates low hydraulic pressure to flight
controls.
Rudder
Ailerons
Elevator
INHIBITED; If Standby Rudder
PCU is Pressurised.
<1,300 PSI

43. 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.

44. 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
Less than 2,350 PSI
• airborne
• flaps are less than 15 but not up. (Some aircraft flaps not up)

45. POWER TRANSFER UNIT
(PTU)
Hydraulic MotorHydraulic Pump
HYD SYS AHYD SYS B
PRESS
ReturnSuction
PRESS

46. PTU; Used to assist the movement of the leading edge Flaps
and Slats. Will assist Auto slat operation.

47. PTU; Used to assist the movement of the leading edge Flaps
and Slats. Will assist Auto slat operation.
Control valve OPEN
Air Mode.
T/E Flaps between UP
and 15
EDP PRESSURE LESS
THAN 2,350 PSI

48. 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 due to an engine failure. The system B engine–driven
pump supplies the volume of hydraulic fluid needed to operate the landing gear
transfer unit only when all of the following conditions exist:
• airborne
• No. 1 engine RPM drops below a limit value (N2 less than 50%)
• landing gear lever is positioned UP
• either main landing gear is not up and locked.
The electrical pump is able to raise the landing gear but only slowly. With an engine
failure between V1 and gear up a clean aircraft is very desirable as quickly as
possible.

49. How is Hydraulic system oil cooled?

50. How is Hydraulic system oil cooled?
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 in the related main tank.

52. The hydraulic indication RF will be displayed.
a) Only in flight with a reservoir quantity less than 76%.
b) On the ground or in flight with a reservoir quantity less than 76%.
c) On the ground with both engines shutdown.
d) On the ground if any of the three reservoirs quantities are less than 76%.

53. The hydraulic indication RF will be displayed.
a) Only in flight with a reservoir quantity less than 76%.
b) On the ground or in flight with a reservoir quantity less than 76%.
c) On the ground with both engines shutdown.
d) On the ground if any of the three reservoirs quantities are less than 76%.

54. How can a loss of fluid from the standby system be identified?

55. How can a loss of fluid from the standby system be identified?
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.
50%
72%

56. The PTU provides a back up source of hydraulic power to operate:
a) Rudder, thrust reversers and L.E flaps.
b) Autoslats.
c) Autoslats and thrust reversers.
d) Landing gear retraction.

57. The PTU provides a back up source of hydraulic power to operate:
a) Rudder, thrust reversers and L.E flaps.
b) Autoslats.
c) Autoslats and thrust reversers.
d) Landing gear retraction.

58. What flap settings will enable automatic operation of the PTU.
a) Flap 15 only.
b) Flaps less than 15 but not up.
c) Any flap setting less than 15.
d) Flaps more than 15.

59. What flap settings will enable automatic operation of the PTU.
a) Flap 15 only.
b) Flaps less than 15 but not up.
c) Any flap setting less than 15.
d) Flaps more than 15.

60. PTU; Used to assist the movement of the leading edge Flaps
and Slats. Will assist Auto slat operation.
Control valve OPEN
Air Mode.
T/E Flaps between UP
and 15
EDP PRESSURE LESS
THAN 2,350 PSI

61. How is the reservoir for the standby hydraulic system pressurised.
a) By hydraulic system B.
b) Directly from B system reservoir.
c) Directly from A system reservoir.
d) Directly from the pneumatic system manifold.

62. How is the reservoir for the standby hydraulic system pressurised.
a) By hydraulic system B.
b) Directly from B system reservoir.
c) Directly from A system reservoir.
d) Directly from the pneumatic system manifold.
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.

63. What can the Standby hydraulic system power?

64. What can the Standby hydraulic system power?
L.E Flaps and slats Full extend
No1 and No 2 Thrust reversers
Standby rudder
Standby Yaw Damper.

65. The END of Hydraulic Power
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Two 16-cylinder engines combined generate 4400 hp, powering 14 hydraulic pumps.
The reservoirs hold up to 3400 gallons of hydraulic fluid
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