737 MAX Flight Crew Operations Manual
Hydraulics -
System Description
MN-FLT-OH-201
13.20.5
If a leak develops in the electric motor–driven pump or its related lines, or
components common to both the engine and electric motor–driven pumps, the
quantity in the reservoir steadily decreases to zero and all system pressure is lost.
System B Hydraulic Leak
If a leak develops in either pump, line or component of system B, the quantity
decreases until it indicates approximately zero and system B pressure is lost. The
system B reservoir has one standpipe which supplies fluid to both the
engine–driven pump and the electric motor–driven pump. However, with fluid
level at the top of the standpipe, fluid remaining in the system B reservoir is
sufficient for power transfer unit operation.
A leak in system B does not affect the operation of the standby hydraulic system.
Power Transfer Unit
The purpose of the PTU is to supply the additional volume of hydraulic fluid
needed to operate the autoslats and leading edge flaps and slats at the normal rate
when system B engine–driven hydraulic pump is inoperative. The PTU uses
system A pressure to power a hydraulic motor–driven pump, which pressurizes
system B hydraulic fluid. The PTU operates automatically when all of the
following conditions exist:
• system B engine–driven pump hydraulic pressure drops below limits
• airborne
• flaps not up.
Landing Gear Transfer Unit
The purpose of the landing gear transfer unit is to supply the volume of hydraulic
fluid needed to raise the landing gear at the normal rate when system A
engine–driven pump volume is lost. The system B engine–driven pump supplies
the volume of 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
and uses a single electric motor–driven pump to power:
• thrust reversers
• rudder
October 26, 2021
