system. The aircraft has three independent hydraulic
The manually engaged and pressure disengaged
power systems. The two primary systems are the flight
type of selector valve is similar to the valve previously
hydraulic power system and the combined hydraulic
discussed. When the actuating mechanism reaches the
power system. These systems are pressurized by two
end of its cycle, the pressure continues to rise to a
independent engine-driven hydraulic pumps on each
predetermined pressure. The valve automatically
engine. The auxiliary power system also operates on
returns to the neutral position and to open center flow.
3,000-psi pressure. It is pressurized by the hydraulic
Closed-Center System
hand pump and/or the electric motor-driven hydraulic
pump. The auxiliary power system is similar to the
In the closed-center system, the fluid is under
combined hydraulic power system. The primary
pressure whenever the power pump is operating.
difference is that the combined system supplies
Figure 12-2 shows a complex closed center system.
hydraulic pressure to utility hydraulic circuits and the
The power pump may be one used with a separate
flight controls.
pressure regulator control. The power pump may be
The hydraulic control valves and actuators that
used with an integral pressure control valve that
operate the primary flight controls are of the tandem
eliminates the need for a pressure regulator. This
construction type. This design permits operation from
system differs from the open center system in that the
either or both of the two power systems. With this
selector or directional control valves are arranged in
arrangement, either engine can fail or be shut down
parallel and not in series. The means of controlling
without complete loss of hydraulic power to either
pump pressure will vary in the closed center system. If
system. The flight system reservoir supplies fluid to
a constant delivery pump is used, the system pressure
the two engine-driven flight system pumps. The
will be regulated by a pressure regulator. A relief valve
combined system reservoir supplies fluid to the two
acts as a backup safety device in case the regulator
engine-driven combined system pumps and to the
fails. If a variable displacement pump is used, system
auxiliary hydraulic power system. Both reservoirs are
pressure is controlled by the pump's integral pressure
of the pressurized piston type. They are pressurized by
m e c h a n i s m c o m p e n s a t o r. T h e c o m p e n s a t o r
engine bleed air during engine operations and by an
automatically varies the volume output. When
external air (nitrogen) source during maintenance
pressure approaches normal system pressure, the
operations.
compensator begins to reduce the flow output of the
pump. The pump is fully compensated (near zero flow)
Hydraulic system pressure is indicated on the
when normal system pressure is attained. When the
integrated hydraulic pressure indicator. This indicator
pump is in this fully compensated condition, its
displays the output pressure of the flight and combined
internal bypass mechanism provides fluid circulation
hydraulic power systems. The flight hydraulic power
through the pump for cooling and lubrication. A relief
system provides power for the operation of the rudder,
valve is installed in the system as a safety backup.
stabilizer, and flaperons. It also provides power for
operation of the automatic flight control system
An advantage of the open-center system over the
actuators, which are an integral part of the rudder and
closed-center system is that the continuous
stabilizer control surface actuators. The flight
pressurization of the system is eliminated. Since the
hydraulic system also controls the automatic operation
pressure is built up gradually after the selector valve is
of the isolation valve. This valve is a part of the
moved to an operating position, there is very little
combined hydraulic system.
shock from pressure surges. This action provides a
smoother operation of the actuating mechanisms. The
The combined hydraulic power system consists of
operation is slower than the closed center system, in
two parallel circuits--one to power the primary
which the pressure is available the moment the selector
systems and the other to power the secondary systems.
valve is positioned. Since most aircraft applications
The primary system consists of spin recovery, rudder,
require instantaneous operation, closed-center
stabilizer flaperon, speed brakes, and electric ram air
systems are the most widely used.
turbine systems. The secondary system consists of
wing slats, wing flaps, wing fold, landing gear,
Power systems are designed to produce and
arresting gear, wheel brakes, nosewheel steering, and
maintain a given pressure. The pressure output of most
the nose strut locking systems.
of the Navy's high-performance aircraft is 3,000 psi.
The hydraulic system, shown in figure 12-2, is an
The isolation valve shuts off flow to the secondary
example of a representative 3,000-psi hydraulic power
systems during flight and limits the combined system's
12-3
