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 20% full. System A hydraulic pressure
is maintained by the electric motor–driven pump. 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 volume is lost. 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 are less than 15 but not up.
