737 MAX Flight Crew Operations Manual
Hydraulics -
System Description
13.20.4
MN-FLT-OH-201
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 6 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 switch shuts off
the fluid flow to the engine–driven pump and deactivates the related LOW
PRESSURE light.
1B401, 1B402, 1B801 - 1B824
The ELEC 2 (system A) or ELEC 1 (system B) pump ON/OFF switch controls the
related electric motor–driven pump. If an overheat is detected in either system, the
related OVERHEAT light illuminates, power is removed from the pump and the
LOW PRESSURE light illuminates.
Note:
Loss of an engine-driven hydraulic pump and a high demand on the
system may result in an intermittent illumination of the LOW PRESSURE
light for the remaining electric motor-driven hydraulic pump. The flight
control LOW PRESSURE light, Master Caution light, and the FLT CONT
and HYD system annunciator lights also illuminate.
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 760 Kgs 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 pumps to the
hydraulic pressure indication on the MAX Display System (MDS). The related
system fluid quantity is displayed on indicators at the reservoir and on MDS.
System A Hydraulic Leak
If a leak develops in the engine–driven pump or its related lines, a standpipe in the
reservoir prevents a total system fluid loss. With fluid level at the top of the
standpipe, the reservoir quantity displayed indicates approximately 1/4 full or
20% full on the MDS. System A hydraulic pressure is maintained by the electric
motor–driven pump.
October 26, 2021