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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.
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.

Copyright . The Boeing Company. See title page for details.
April 28, 2000  13.20.3
Hydraulics -System Description 737 Flight Crew Operations Manual

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.
YD651 - YK104

.  
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.

 

Copyright . The Boeing Company. See title page for details.
13.20.4   March 28, 2005

Hydraulics -System Description 737 Flight Crew Operations Manual
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

YD651 - YK104 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.
　
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