.  
flaps are less than 15 but not up.

Landing Gear Transfer Unit
The purpose of the landing gear transfer unit is to supply the pressurized hydraulic fluid needed to raise the landing gear at the normal rate when system A is lost due to No. 1 engine RPM loss. The system B engine–driven pump supplies the pressurized 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. It uses a single electric motor–driven pump to power:
.  
thrust reversers

.  
rudder

.  
leading edge flaps and slats (extend only).

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.

.  
(after RSEP modification) illuminates the STBY RUD ON, Master
Caution, and Flight Controls (FLT CONT) lights.

Copyright . The Boeing Company. See title page for details.
13.20.4   December 5, 2003

Hydraulics -System Description 737 Operations Manual
Positioning the ALTERNATE FLAPS master switch to ARM, (see the Flight Controls chapter 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, or rudder pressure reducer system fails in low
pressure mode, and

.  
flaps extended, and

.  
airborne, or wheel speed greater than 60 kts, and

.  
FLT CONTROL switch A or B Hydraulic System ON

OR (after RSEP modification)
.  The main PCU Force Fight Monitor (FFM) trips.
Automatic operation: . open’s the standby rudder shutoff valve
.  
activate’s the standby electric motor–driven pump

.  
allow’s the standby system to power the rudder and thrust reversers.

.  
(after RSEP modification) illuminates the STBY RUD ON, Master
Caution, and Flight Controls (FLT CONT) lights.

Hydraulics -System Description

Standby Hydraulic System Schematic

(before RSEP modification)
Copyright . The Boeing Company. See title page for details.
13.20.6  December 5, 2003

Hydraulics -System Description 737 Operations Manual

(after RSEP modification)
Copyright . The Boeing Company. See title page for details.
December 5, 2003  13.20.7
Hydraulics -System Description 737 Operations Manual

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 64% full (between 1/2 & RFL on non–EIS equipped airplanes).
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.
Copyright . The Boeing Company. See title page for details.
　
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