pressure requirements to operation of the primary
where limited actuation is required. Most other
e s s e n t i a l hy d r a u l i c a l l y o p e r a t e d s y s t e m s h ave
circuit. Operation of the isolation valve is both
emergency power systems that are powered by a hand
automatic and manual.
p u m p , e l e c t r i c m o t o r- d r ive n p u m p , r a m - a i r
The reservoir pressurization system provides the
turbine-driven pump, or a pneumatic compressor.
reservoir with a differential pressure of 40 psi to
On some aircraft, the hand pump is a part of the
prevent engine-driven pump cavitation. The pressure
auxiliary hydraulic system and is not considered as
is maintained at 40 psi by the air regulator. In the event
part of the emergency power systems. The hand pump
of regulator failure, the relief valve installed between
is used for ground operation of the canopy, extensible
t h e r eg u l a t o r a n d t h e r e s e r vo i r p r eve n t s
electronics platform, nose radome opening, and to
overpressurization. The relief valve opens at 50 psi.
recharge the brake accumulator. These systems may
The chemical air drier removes excessive moisture
be operated by aircraft system pressure or, if the
from the bleed air. Dry, clean air is sent to the reservoir
aircraft is shutdown, they may be powered by the
through the check valve, air regulator, and relief valve.
auxiliary electric motor-driven hydraulic pump or the
Two bleeder valves are installed in the flight and
hand pump.
combined system reservoirs. One is found on the air
Q12-1. A system that combines the use of hydraulics
side of the reservoir and the other on the fluid side. The
and pneumatics is known by what term?
air side valve permits the bleeding of air pressure
during system maintenance. It allows the bleeding of
Q12-2. Hydraulic flight control system design
any hydraulic fluid seepage past seals to the air side.
specifications require what total number of
The fluid side bleeder reduces excessive fluid level and
separate systems for operation of the primary
bleeds air from the fluid side.
flight controls?
Quick-disconnect fittings in the hydraulic power
Q12-3. What type of hydraulic system has fluid flow
systems permit easy pump or engine removal without
but no pressure in the system when the
loss of fluid to the system. The fittings connect ground
actuating mechanisms are idle?
hydraulic test stands for maintenance purposes. The
Q12-4. What is the major difference between an
pump disconnects should not be forced together
open-center hydraulic system and a
against the backpressure of a pressurized reservoir or
closed-center hydraulic system?
system. Forcing disconnects together may result in
damaged seals in the male ends of the disconnects.
Q12-5. What is the advantage of the open-center
When the disconnects do not slide in smoothly, they
hydraulic system over the closed-center
should be removed and checked for proper seating of
hydraulic system?
the O-rings. Replace seals if they are damaged. The
Q12-6. The hydraulic control valves and actuators
seal goes on top of the O-ring. When the disconnects
that operate the primary flight controls are of
are uncoupled, the ends not being used should be
what type construction?
properly protected from dirt and other contamination.
Use only approved metal closures.
HYDRAULIC COMPONENTS
EMERGENCY POWER SYSTEMS
LEARNING OBJECTIVE: Identify the
According to the military specifications, which
va r i o u s h y d r a u l i c s y s t e m c o m p o n e n t s .
establish the requirements for aircraft hydraulic
Recognize the procedures required for their
systems, all hydraulically operated systems
maintenance.
considered essential to flight safety or landing must
have provisions for emergency actuation. The
Various types of hydraulic components make up a
specifications further state that these emergency
power system. The components discussed here are
systems may use hydraulic fluid, compressed gas,
representative of those with which you will most likely
directed mechanical linkage, or gravity for their
be working. Values such as pressure, temperature, and
actuation.
instructional tolerances have been given to provide
detail in the coverage.
Some aircraft use mechanical linkage or gravity in
conjunction with pneumatic pressure for emergency
When actually performing the maintenance
actuation of landing gear and other actuating systems
procedures, you will find the exact location and make
12-5
