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Modern aircraft rely on hydraulic systems to perform some of the most demanding mechanical tasks, including landing gear operation, braking systems, and flight control movement. In this lesson, we explain how aircraft hydraulic systems work and explore the key differences between open-center and closed-center designs.
You’ll learn:
✈ The basic components of every aircraft hydraulic system
• Pump
• Reservoir
• Selector / Directional Control Valves
• Actuators
• Check Valves
• Pressure Relief Valves
• Filters
✈ How open-center hydraulic systems operate
✈ How closed-center hydraulic systems operate
✈ Why closed-center systems are widely used in modern aircraft
✈ A clear comparison between open and closed-center designs
This video is ideal for:
• Student pilots
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0:03
Modern aircraft depend on hydraulic
0:05
power to perform demanding mechanical
0:07
tasks like operating landing gear,
0:09
brakes, and flight controls. Instead of
0:12
heavy mechanical linkages, hydraulics
0:14
use pressurized fluid to multiply force
0:17
efficiently and reliably. Even a small
0:19
control input can produce powerful
0:21
movement. To understand how this works,
0:24
let's begin with the basic components of
0:26
an aircraft hydraulic system.
0:30
Regardless of its design or purpose,
0:32
every hydraulic system contains a
0:34
minimum set of essential components. The
0:37
pump is the heart of the system. It
0:39
draws hydraulic fluid from the reservoir
0:41
and forces it into the system under
0:43
pressure. The reservoir stores the
0:45
hydraulic fluid. It also allows the
0:48
fluid to cool and gives trapped air a
0:50
chance to escape before the fluid is
0:52
recirculated. To control where the fluid
0:54
goes, selector or directional control
0:57
valves are installed. These valves
0:59
determine which actuator receives
1:01
hydraulic pressure. The actuator is the
1:04
working unit of the system. It converts
1:06
hydraulic pressure into mechanical
1:08
motion. This motion may extend landing
1:10
gear, apply brakes, or move a flight
1:13
control surface. To prevent damage from
1:15
excessive pressure, a relief valve is
1:17
installed. If pressure rises above safe
1:20
limits, the valve opens and returns
1:23
fluid to the reservoir. Check valves
1:25
ensure fluid flows in only one
1:27
direction, preventing backflow. Filters
1:30
remove contaminants from the hydraulic
1:32
fluid to protect system components.
1:34
Together, these parts form the
1:36
foundation of every aircraft hydraulic
1:39
system.
1:40
An open center hydraulic system is
1:43
designed so that fluid flows
1:44
continuously when the system is
1:46
operating. But pressure does not build
1:48
up unless a component is being actuated.
1:51
In this system, the pump circulates
1:53
fluid from the reservoir through each
1:55
selector valve in sequence and back to
1:58
the reservoir. The selector valves are
2:00
connected in series. This means the
2:02
pressure line passes through one valve
2:04
and then the next. As long as no
2:06
selector valve is activated, fluid flows
2:09
freely through the system. There is
2:11
movement of fluid but no significant
2:13
pressure. When a selector valve is moved
2:15
to operate an actuator, the return path
2:18
to the reservoir is blocked. Fluid is
2:21
directed into the actuator and pressure
2:23
begins to build. This pressure moves the
2:26
actuator piston. Fluid from the opposite
2:28
side of the actuator then returns to the
2:30
reservoir. Depending on the design of
2:32
the selector valve, the system may
2:34
require manual return to neutral or it
2:37
may automatically return once a preset
2:39
pressure is reached. One advantage of
2:41
the open center system is that it is not
2:44
continuously pressurized. Pressure
2:46
builds gradually which reduces shock and
2:48
minimizes pressure surges. However,
2:51
because pressure must build after
2:53
activation, the system operates slightly
2:56
slower than other designs.
2:59
A closed center hydraulic system
3:01
operates differently from the open
3:03
center design. In this system, hydraulic
3:06
fluid is under pressure whenever the
3:08
pump is operating. Unlike the open
3:10
center arrangement, the selector valves
3:12
are connected in parallel rather than in
3:14
series. This means each actuator has its
3:17
own separate path from the pressure
3:19
line. Because the system remains
3:21
pressurized at all times. The moment a
3:23
selector valve is moved, hydraulic fluid
3:26
immediately flows to the actuator. There
3:28
is no waiting for pressure to build.
3:30
This allows rapid and precise operation.
3:33
Another important advantage of the
3:35
closed center system is that multiple
3:37
actuators can operate at the same time.
3:39
Since the valves are arranged in
3:41
parallel, one unit can function without
3:43
interrupting flow to another. Pressure
3:46
control in a closed center system
3:48
depends on the type of pump installed.
3:50
If a constant delivery pump is used, a
3:52
pressure regulator maintains the desired
3:55
system pressure. A relief valve is also
3:57
installed as a backup safety device in
4:00
case the regulator fails. If a variable
4:02
displacement pump is used, pressure is
4:05
controlled by an internal compensator
4:06
mechanism. As system pressure approaches
4:09
normal operating limits, the compensator
4:11
reduces the pump's output. When normal
4:14
pressure is reached, the pump output
4:16
decreases to nearly zero flow. This
4:18
condition is known as full compensation.
4:21
Even in this state, a small internal
4:23
circulation continues within the pump to
4:26
provide cooling and lubrication. A
4:28
relief valve remains in the system as an
4:31
additional safety measure. Because
4:33
pressure is always available, closed
4:35
center systems provide fast response and
4:37
are widely used in modern aircraft,
4:40
especially where instantaneous operation
4:42
is required.
4:44
The main difference between open center
4:46
and close center hydraulic systems is
4:49
how pressure is managed. In an open
4:51
center system, fluid flows when idle,
4:54
but pressure builds only after a
4:56
selector valve is activated. This
4:58
provides smooth operation with minimal
5:00
shock, but response is slightly slower.
5:03
In a closed center system, pressure is
5:05
available at all times while the pump is
5:07
operating. Actuators respond instantly
5:10
and multiple units can operate
5:12
simultaneously because aircraft require
5:15
fast and reliable performance. Closed
5:17
center systems are the most commonly
5:19
used in modern aviation.
5:22
In this lesson, we covered the basic
5:24
components of aircraft hydraulic systems
5:27
and the key differences between open
5:29
center and closed center designs. Closed
5:31
center systems dominate modern aircraft
5:33
because of their fast and reliable
5:35
response.
5:37
Thanks for watching.
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