737 Flight Crew Operations Manual
Hydraulics -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
13.20.4 D6-27370-866-EGP
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.
• 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.
September 28, 2006
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