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A
737-300/400/500MAINTENANCE MANUAL
 (7)  If system B EDP hydraulic power is lost, the PTU pressure switch senses the loss of hydraulic pressure and causes system A to drive the power transfer unit (PTU) to provide system B pressure for operation of leading edge flaps/slats. For further information on the PTU, refer to 29-22-00.
 B. Functional Description - Standby System (Fig. 19)
 (1)  
The pressure source for the standby system is one EMDP.

 (2)  
The standby system reservoir supplies fluid under pneumatic pressure to the EMDP. The pneumatic pressure is provided through the balance line from system B reservoir. The output of the pump flows through a filter in the standby system pressure module before going to the operating systems. Fluid from the operating systems returns directly to the reservoir.

 (3)  
If system pressure becomes too high, a pressure relief valve in the system pressure module opens to allow fluid to return to the standby reservoir.

 (4)  
Fluid flows through the case of the EMDP for pump cooling and lubrication. This case drain fluid is filtered by the case drain filter and returned directly to the standby system reservoir.

 (5)  
The standby hydraulic pump relay is of dual coil construction, reacting to the power sensing relay to provide 115-volt, 400 Hz, 3 phase power to the EMDP from generator 1, or from generator 2 if generator 1 fails.

 (6)  
The standby EMDP runs when:

 (a)
The FLT CONTROL A switch is positioned to STDBY RUD, or

 (b)
The FLT CONTROL B switch is positioned to STDBY RUD, or

 (c)
The ALTERNATE FLAPS switch is positioned to ARM, or

 (d)  
Automatically, with the following conditions: 1) Either FLT CONTROL A or FLT CONTROL B switch positioned to ON, and 2) Either flight control A or flight control B low pressure switch activated (low pressure in system), and 3) A wheel speed of greater than 60 knots, or an AIR mode at

 AIR SENSING relay (R278, E-11 panel), and 4) Flaps not up.
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A 737-300/400/500MAINTENANCE MANUAL

Standby Hydraulic System
Figure 19

F42929
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A

737-300/400/500
MAINTENANCE MANUAL

SELF-SEALING
(OR QUICK TYPE)
DISCONNECT

HYDRAULIC FUSE

FLOW LIMITER
(ONE DIRECTION ONLY)

FLOW RESTRICTOR
(BOTH DIRECTIONS)

FILTER WITH BYPASS & DIFFERENTIAL PRESSURE INDICATOR
HEAT EXCHANGER-FINNED TYPE, FLUID COOLED
LRU WHICH IS PART OF SYSTEM BEING SHOWN
LRU WHICH IS NOT PART OF SYSTEM BEING SHOWN
PRESSURE SENSOR

TEMPERATURE SENSOR
Hydraulic System Schematic Symbols
Figure 20

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T95055
A
737-300/400/500MAINTENANCE MANUAL
 HYDRAULIC SYSTEMS A, B, AND STANDBY - TROUBLE SHOOTING
______________________________________________________
 1.  General_______
 A.  When checking for leakage, pressurize applicable systems and examine all components, hydraulic fittings and connecting lines. Pin hole leaks may be detected by noting damp spots on the airplane and tracing run back fluid to locate source of leak. When disconnecting hydraulic lines during removal of components take care to prevent spillage of hydraulic fluid. Should fluid leak on the airplane or spill while performing maintenance, the affected area must be decontaminated (Ref 12-40-00).
　
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