A and B Hydraulic System Pumps
Both A and B hydraulic systems have an engine driven pump and an AC electric
motor driven pump. The system A engine–driven pump is powered by the No. 1 engine and
the system B engine driven pump is powered by the No. 2 engine. An engine driven
hydraulic pump supplies approximately 4 times the fluid volume of the related electric motor
driven hydraulic pump.
The ENG 1 (system A) or ENG 2 (system B) pump ON/OFF switch controls the engine
driven pump output pressure. Positioning the switch to OFF isolates fluid flow from the
system components. However, the engine driven pump continues to rotate as long as the
engine is operating. Pulling the engine fire warning switch shuts off the fluid flow to the
engine–driven pump and deactivates the related LOW PRESSURE light.
Hydraulic fluid used for cooling and lubrication of the pumps passes through a heat
exchanger before returning to the reservoir. The heat exchanger for system A is located in
main fuel tank No. 1 and for system B is in main fuel tank No. 2.
CAUTION: Minimum fuel for ground operation of electric motor-driven pumps is 1675 lbs or
760 kg in the related main tank.
Pressure switches, located in the engine–driven and electric motor–driven pump output
lines, send signals to illuminate the related LOW PRESSURE light if pump output pressure
is low. A check valve, located in each output line, isolates the related pump from the
system. The related system pressure transmitter sends the combined pressure of the
engine–driven and electric motor–driven pump to the related hydraulic system pressure
indication.
