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Copyright . The Boeing Company. See title page for details.
13.10.6  September 26, 2003

737 Flight Crew Operations Manual

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

Copyright . The Boeing Company. See title page for details.
Hydraulics -System Description 737 Flight Crew Operations Manual

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.
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.
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.
Copyright . The Boeing Company. See title page for details.
13.20.2   September 30, 2002

Hydraulics -System Description
737 Flight Crew Operations Manual
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.
System A Hydraulic Leak
　
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