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 B.  The auxiliary hydraulic pumps are installed on the aft side of the No. 1 and No. 4 strut torque box. The pumps are used for ground operations only. They operate continuously when the switch is in the AUX position and the engine-driven pump (EDP) pressure is less than 1,400 psi.
 C.  Electrical power for the auxiliary hydraulic pumps is supplied by the
 ground handling bus and the 28-volt dc bus No. 1 and No. 4.Auxiliary Hydraulic Pump (Fig. 1)
________________________
 A.  The auxiliary hydraulic pump has a variable displacement piston pump and an alternating current (ac) electric motor. The electrical motor uses a self-contained centrifugal pump to control the temperature of the motor. The centrifugal pump lets hydraulic fluid flow on the motor windings.
 B.  The auxiliary hydraulic pump will operate at 5,800 rpm. With a supply
 pressure of 40 psi the pump will have an output of 5.7 gpm at 2,700 psi.Case Drain Filter (Fig. 1)
_________________
 A.  A case drain filter is installed aft of the hydraulic reservoir in the No. 1 and No. 4 strut. Hydraulic fluid from the case drain hose passes through the filter before it is sent back to the hydraulic system.
Operation (Fig. 1 and 2)
_________
 A.  Functional Description
 (1)  
Hydraulic fluid for the auxiliary hydraulic pump is supplied from the No. 1 or No. 4 system reservoir. The pressure hose supplies the brake system of the airplane with hydraulic pressure. Case drain fluid is used to lubricate and control the temperature of the pump. Fluid then goes to the case drain filter, the return module, and back to the reservoir.

 (2)  
Electrical power for the ac electric motor is supplied from the ac ground handling bus and the dc power bus. The APU can also supply power for the motor. The motor can only operate when the airplane is on the ground.

 NOTE: 28 V dc power operates the switch which supplies 115 V ac
____
 power to the AUX pump.

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747-400
MAINTENANCE MANUAL

ENGINE NACELLE STRUT 
FWD 
MODULE CASE DRAIN  SEE  A  SEE  B 
AIR-DRIVEN PUMP (REF) 
FILTER CASE DRAIN AC PUMP  A  FWD  HOSE CASE DRAIN HYDRAULIC PUMP AUXILIARY 
PRESSURE HOSE 
SUPPLY HOSE 
FWD B (RIGHT SIDE SHOWN) 
Auxiliary AC Hydraulic System Components Figure 1 

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747-400
MAINTENANCE MANUAL

HYDRAULIC CONTROL MODULE

Auxiliary Hydraulic Pump Control Switch
Figure 2

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MAINTENANCE MANUAL

 (3)  The auxiliary hydraulic pump will operate when:
 -The airplane is on the ground, and
 -Electrical power is supplied, and
 -The control switch is in the AUX position, and
 -System 1 or 4 EDP pressure is less than 1,400 psi.
 (4)  If the temperature of the auxiliary case drain fluid increases above 105° C, it will cause an advisory message. The advisory message HYD OVHT SYS (1 or 4) will be shown on the main EICAS screen. This message can also be an indication that the EDP or air-driven pump case drain temperatures are above 105° C.
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