Chapter 10 - Hydraulics
Photo by: Anton Heumann
The hydraulic distribution block houses a
hydraulic filter, a pressure regulating valve,
a hydraulic pressure switch, a hydraulic
test solenoid and on certain models a
hydraulic filter clogging indicator.
INTRODUCTION
The elastomeric based rotor system on
the AS350 creates in flight dynamic forces
that causes feedback in the cockpit flight
controls. To counteract these forces the
AS350 is equipped with a single hydraulic
system that assists with the pitch changes
and dampens out dynamic feedback on
the main and tail rotor blades.
The hydraulic filter removes contaminates
from the hydraulic fluid and the clogging
indicator gives a visual warning of a
impending blockage. The pressure
regulating valve keeps the systems
pressure at a constant and the hydraulic
pressure switch activates the red HYD light
on the caution and warning panel and
the horn. The hydraulic test solenoid valve,
when activated, routes pressurized fluid
from the pump back to the reservoir.
GENERAL
The hydraulic system is comprised of
a reservoir, variable delivery pump, a
hydraulic distribution block, 3 servo control
manifolds, 3 main rotor servos, a single tail
rotor servo and on certain models a yaw
load compensator.
Each of the three main rotor hydraulic
servos is fitted with a servo control
manifold. Each servo control manifold
houses a one way check valve, an
accumulator and an isolation solenoid
valve.
The reservoir is mounted on the main
gearbox and gravity feeds to the
hydraulic pump. The pump is belt driven
off of the engine to main gearbox drive
shaft and provides pressurized fluid for the
system.
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Figure 166, Hydraulic Component Locations
The one way check valve traps
pressure created by the accumulator
after a hydraulic pressure failure. The
accumulators provide hydraulic boost
after a hydraulic failure and the isolation
solenoid allows pressurized fluid to bypass
the servo and return to the reservoir.
The helicopter is equipped with either
a hydraulic isolation switch on the
collective head or, on earlier models, with
a hydraulic isolation pushbutton on the
end of the collective arm (Figure 168).
The hydraulic isolation switch/pushbutton
activates the isolation solenoid valve on
the main rotor servo control manifold.
The three main rotor servos are attached
via a rod end bearings on the bottom to
the main gearbox and on the top to the
non rotating swash plate.
The system is equipped with a single red
HYD light located in the warningcaution panel (Figure 167). When the
pressure drops at the hydraulic pressure
switch below 32 bars the HYD light will
illuminate and the aural warning horn will
sound (provided the horn pushbutton or
switch is in the enabled position).
The single yaw servo is located horizontally
in the forward section of the tail boom
under the tail rotor drive shaft.
The AS350B1 & B2 models include a yaw
load compensator which assists in
pedal inputs after a hydraulic failure. The
yaw load compensator consists of a
trim rod, a compensator lever, an
actuator piston, a compensator body,
an accumulator, a solenoid valve, and a
pressure release valve.
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A hydraulic test pushbutton (Figure 169)
or switch (Geneva panel) is located
on the center console which activates
the hydraulic test solenoid valve on the
hydraulic distribution block and on the
AS350 B1 & B2 models also activates
the solenoid valve on the yaw load
compensator .
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Figure 167, Hydraulic Warning Light
Components
Figure 168, Hydraulic Isolation Switch/Button
Reservoir
The hydraulic reservoir is secured by two
clamps to the aft side of the conical
housing of the main gear box and is
constructed of light aluminum alloy
(Figure 170). It has a capacity of 2.1 liters
and is equipped with a sight glass on
the right side of the reservoir to view the
level of the hydraulic fluid. The reservoir
is equipped with a vented cap that can
be easily opened to add hydraulic fluid to
the system and a strainer mounted in the
reservoir opening to filter out debris when
adding fluid to the system. Hydraulic fluid
from the reservoir flows by gravity to the
hydraulic pump.
Figure 169, Hydraulic Test Switch/Button
Return
The reservoir has one feed to the pump
and two return ports.
Hydraulic Pump
The hydraulic pump is located in the
right hand side of the main gearbox
compartment just behind the transmission
(Figures 171 & 172).
Feed
Return
The pump is belt driven by the forward
end of the engine to main gearbox drive
shaft off of the main gear box input pinion
pulley flange. There are two variants of
belts used on the AS350 models, either a
green Filon type (Figure 171) or a black
V Polychloroprene – Polybutadiene belt
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Figure 170, Hydraulic Reservoir
(Figure 172). Each belt utilizes different
input pinion pulley flanges and they are
not interchangeable. The black Poly V
belts are more durable and have a longer
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Figure 171, Hydraulic Pump (Green Belt)
Figure 172, Hydraulic Pump (Black Belt)
life limit then the green belts.
In order to replace a hydraulic belt the
articulated coupling and engine to
main gearbox drive shaft needs to be
disconnected at the forward end in order
to get the belt around the pulley. Since
the articulated coupling is the forward
mount for the engine, the engine needs
to be supported by outside means during
installation.
The hydraulic pump (Figure 174) produces
a rate of flow of 6 liters per minute when
the main rotor is turning at a speed of
approximately 390 rpm and can provide
operating pressure for the servos down
to 170 NR. The pump is equipped with a
magnetic chip plug and an internal .8 - 1
mm strainer to help remove debris in the
fluid. With all main gearbox components
intact the hydraulic pump produces
operating pressure, including after an
engine failure, whenever the main rotors
are turning.
During installation the gimbal ring on the
articulated coupling is disconnect in
order to disconnect the engine to MGB
drive shaft. The rear engine mount is then
loosened, and the entire engine is moved
back about 1 inch in order to string the
new belt around the drive shaft.
When equipped with a green belt
system a spare hydraulic belt is secured
around the articulated coupling to
provide for easier installation away from
a maintenance facility after a belt failure
(Figure 173). The articulated coupling
will still need to be disconnected at the
gimbal ring and the engine will need to
be supported but the engine to main
gearbox drive shaft will not need to be
disconnected.
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Figure 173, Spare Hydraulic Belt
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Hydraulic Distribution Block
From Reservoir
The hydraulic distribution block (Figure 175)
is mounted on the right side main gearbox
and receives pressurized fluid from the
hydraulic pump. It houses the hydraulic
filter, hydraulic filter clogging indicator
(only on part numbers BFS-155 and BFS1551), pressure regulating valve, hydraulic
pressure switch and the hydraulic test
solenoid.
Magnetic Chip Plug
Strainer
Splined Shaft
The hydraulic filter is mounted on the
bottom of the hydraulic distribution block.
It further removes contaminates from the
hydraulic fluid.
The BFS150 distribution block contains a 25
micron metallic filter that can be cleaned
between usages. The BFS155 and BFS1551 distribution block contains a 3 micron
consumable mesh filter.
Figure 174, Hydraulic Pump
Hydraulic Test Solenoid
Hydraulic Test Solenoid
Pressure
Regulating
Valve
Pressure
Regulating
Valve
Return To
Reservoir
Return From Tail
Rotor Servo
Return To
Reservoir
Return From Tail
Rotor Servo
Pressure to
Main Rotor
Servos
Pressure to
Main Rotor
Servos
Pressure
Switch
Pressure
Switch
Pressure to Tail Rotor
Servo
Pressure Inlet
From Pump
Pressure Inlet
From Pump
Clogging Indicator
Filter
Pressure to Tail Rotor
Servo
Filter
BFS-155
BFS-150
Figure 175, Hydraulic Distribution Block
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Hydraulic Test Solenoid
Hydraulic Test Solenoid
Pressure
Regulating
Valve
Pressure to
Main Rotor
Servos
Pressure
Regulating
Valve
Pressure to
Main Rotor
Servos
Return From
Tail Rotor
Servo
Return To
Reservoir
Return From Tail
Rotor Servo
Return To
Reservoir
Pressure
Switch
Pressure to
Tail Rotor
Servo
Filter
Clogging
Indicator
Pressure Inlet
From Pump
Pressure to Tail
Rotor Servo
Pressure
Switch
Pressure Inlet
From Pump
Filter
BFS-155
BFS-150
Figure 175, Hydraulic Distribution Block
debris builds up on the filter element
the hydraulic fluid pressure will increase
around the outside of the filter and
decrease after the filter. When the
pressure differential reaches 2.7 ± 0.4 bar
the magnetic piston spring assembly will
be pushed back off of the filter bulkhead.
The difference between the BFS155 and
BFS155-1 is the addition of a restrictor
into the valve body to minimize the risk
of untimely activation of the clogging
indicator.
The filter has no bypass capability, if
the filter becomes clogged the flow
of hydraulic fluid from the hydraulic
distribution block to the servos will cease.
When equipped with either the BFS155 or
BFS155-1 hydraulic distribution block the
unit is equipped with an indicator button
which extends from the bottom of the filter
housing at a 2.7 bar differential pressure
across the filter. This indicator gives a visual
indication that the filter is clogging (Figure
176).
The clogging indicator is held in the
recessed position against the filter
bulkhead by a magnetic piston spring
assembly (Figure 175). When sufficient
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Clogging
Indicator
Figure 176, Clogging Indicator
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Hydraulic Test Solenoid
Without the magnetic attraction of the
piston spring assembly the clogging
indicator will be pushed outward by a
spring under the indicator (Figure 176).
The hydraulic pump provides more
pressure to the hydraulic system then
is needed so that at any operating
rotor speed there is sufficient pressure
to operate the hydraulic servos. The
pressure relief valve, located on top of
the hydraulic distribution block (Figures
174 & 175), opens at approximately 40
bar to allow hydraulic fluid to return to
the reservoir. This allows pressure in the
hydraulic system to remain at a constant.
The pressure relief valve is set at a 0 flow to
open at 43 ±1 bar. When installed on the
helicopter, with pressurized fluid flowing
past the regulating valve at 6 liters per
minute, the pressure relief valve will open
at approximately 40 bar. This pressure
can be modified by maintenance to the
correct pressure by adjusting the spring
calibration of the pressure relief valve.
Pressure
Switch
Figure 177, Pressure Switch/ Solenoid Valve
longer receive pressurized hydraulic fluid.
The pressure at the hydraulic pressure
switch will decrease causing the pressure
switch to activate the HYD light and the
horn. Pressurized hydraulic fluid exits the
hydraulic distribution block and is routed
to the hydraulic servos.
The hydraulic pressure switch (Figure 177),
located on the front of the hydraulic
distribution block, activates the red HYD
light (Figure 167) and the aural warning
horn when the pressure in the hydraulic
system drops below 32 ±1 bar. The switch
will turn off the HYD light and the horn
when pressure increases past 38 ±2 bar @
25° C ±10° C.
Main Rotor Servo Actuators
AS350’s are fitted with either SAMM
or Dunlop servos. These servos are
interchangeable and their mounting
are identical, though it is recommend
to use all the same brand on a specific
airframe. The forward/left servo controls
pitch and the left and right servos control
roll. All three main rotor servos are used for
collective pitch changes.
The hydraulic test solenoid valve is located
on top of the hydraulic distribution block
(Figure 177) and is opened by engaging
the HYD TEST pushbutton (Figure 178) or
switch (Geneva Panel) on the center
console. When the valve is opened
the pressurized fluid from the pump is
allowed, by path of least resistance, to
return directly to the hydraulic reservoir.
The main and tail rotor servos will now no
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The pistons on the bottom are attached
via a rod end bearing to the conical
housing of the main gearbox. The upper
housing of each servo is attached via
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Figure 178, Hydraulic Test Push-Button/ Switch
a rod end bearing to the non rotating
swashplate of the rotor system (Figure 179).
The housing will continue to move until
movement of the control tube ceases and
the input lever is level or at hydraulic zero.
When the input lever is at hydraulic zero
the slider valve is in a position that doesn’t
allow fluid to enter or exit the servo.
The extension or retraction of the servos
is accomplished through the movement
of the servo housing. The steel piston
rod remains fixed to the conical housing
during servo movements. The maximum
useful travel of the servos is 110 mm and
the travel between stops is 135 mm.
Each of the three main rotor hydraulic
servos are fitted with a servo control
manifold (Figure 180). The manifold is
attached to the servo with banjo screws
(Figure 180) which thread into the
pressure and return ports on the servo
body through the servo control manifold.
A banjo screw is a hollow screw with
connecting holes to the hollow middle
near the bolt head. The screws allow
attachment of the servo control manifold
and the servo while allowing hydraulic
fluid to flow through them.
When the helicopter flight controls are
moved, the servo input lever (Figure 182)
is moved in the corresponding direction
via rigid push pull tubes. Displacing the
input lever moves the position of the servo
distribution slide valve allowing hydraulic
fluid to be routed to one of the two
chambers on the piston rod while allowing
fluid from the other chamber to exit
the servo housing back to the reservoir.
Deflection of the input lever for maximum
opening of the slide valve is 2 mm.
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Attached to each manifold is a one way
check valve that traps pressure created
by the accumulator after a hydraulic
failure, an accumulator to provide
hydraulic boost after a hydraulic failure
and an isolation solenoid which allows
pressurized fluid to bypass the servo inlet
port (Figure 182) .
Hydraulic fluid enters the servo through the
top of the servo control manifold through
the one way check valve (Figure 181). The
fluid is routed to the accumulator then
on to the inlet pressure port of the servo
housing through a banjo screw. The slider
valve inside the servo routes the fluid to
one of the chambers on the piston rod,
at the same time allowing fluid from the
other chamber to be expelled through
the return port. The fluid exiting the servo
travels through the banjo screw then out
of the top of the servo control manifold.
Figure 179, Main Rotor Servos (Dunlop)
Rod End Bearing
Rod End Bearing
Accumulator
Accumulator
One Way
Check Valve
Isolation
Solenoid
Valve
Servo Control
Manifold
Banjo Screws
Piston Rod
Rod End Bearing
Rod End Bearing
Piston Rod
Figure 180, Main Rotor Servo (SAMM)
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of the servo. The amount of time the
accumulator will keep the servo boosted is
dependent on the amount of movements
made to the rotor system.
The oleopneumatic accumulator,
mounted on the top of the servo control
manifold, provides pressurized hydraulic
fluid to its respective servo in case of a
hydraulic failure and absorbs surges in
the system. The accumulator consists of
a metal cylinder with a rubber bladder
inside of it which is charged with nitrogen
gas to 15 +1 -0 bar at 20° C. As the
hydraulic system is pressurized, hydraulic
fluid is pumped into the cylinder around
the nitrogen filled bladder. When the
pressure of the hydraulic system exceeds
the charged pressure of the bladder the
hydraulic fluid compresses the bladder
creating pressurized fluid.
The isolation solenoid valve is mounted
on the side of the servo control manifold
and is closed during normal operation. The
valve is opened by activating the switch
located on the collective head.
When activated, the pressure inlet and
the return outlet are connected. By path
of least resistance any pressurized fluid
entering the manifold will be routed to the
return outlet along with any pressurized
fluid in the accumulator.
If the flow of hydraulic pressure from the
pump ceases, the accumulator will dispel
its pressurized fluid into the servo. The one
way check valve, mounted on top of the
servo control manifold at the pressure
inlet port, traps hydraulic fluid dispelled
from the accumulator to the inlet port
Pressure
Return
In case of a slide valve seizure the valve
could be opened to stop the flow of
pressurized fluid into the servos.
After the loss of pressure to the servos, the
solenoid is opened after obtaining the
Pressure
Return
Accumulator
Accumulator
One Way
Check
Valve
One Way
Check Valve
Isolation
Solenoid
Valve
Banjo Screws
Banjo Screws
Isolation
Solenoid
Valve
Banjo Screw
Figure 181, Servo Control Manifold
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recommended airspeed to dump the
remaining pressure from the accumulators
and disable the system from coming back
on line.
When the isolation solenoid valve switch
is placed in the hydraulic cut off position
the horn relay for the hydraulic system is
disabled, however the horn will continue
to operate for the NR system.
SAMM Servo Actuators
Prior to start up the servos locking pin is
extended into the servo actuator input
lever slot (Figure 182). This secures the
input lever at hydraulic zero and removes
any input play in the flight controls after
a hydraulic failure. With the input lever
at hydraulic zero the slide valve is in a
position that will not let hydraulic fluid
enter or exit the servo. After a hydraulic
failure the trapped fluid is needed in the
servo for lubrication. The locking pins
upper portion acts as a bypass valve.
When hydraulic pressure to the servo
drops below 14 bars the locking pin drops
and allows chambers A and B to be
interconnected. This allows the fluid in the
chambers to flow between them during
control inputs after a hydraulic failure.
When the servo is first pressurized, fluid is
routed under the locking pin, and above
6 bars lifts the locking pin out of the input
lever slot and compresses the locking
pin spring. This closes the interconnect
between the chambers on the piston rod.
With the locking pin recessed the slide
valve is free to move.
When fluid is routed to chamber A the
increased volume of fluid in the chamber
moves the housing and extends the length
of the servo. Compression of chamber B
forces fluid out of the servo through the
return line.
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When fluid is routed to chamber B the
increased volume of fluid in the chamber
moves the housing and shortens the
length of the servo. Compression of
chamber A forces fluid out of the servo
through the return line.
As long as the pilot inputs control forces,
the servo will continue to travel in the
direction of input. Once the control inputs
are ceased the housing will move until
input lever is at hydraulic zero. In this
position the slider valve does not allow
fluid to enter or exit the servo.
Dunlop Servo Actuators
Unlike the SAMM servos the Dunlop servos
have a locking pin on only the pitch servo.
Prior to start up, on the pitch servo, the
locking pin is extended into the servo
actuator input lever slot. This secures the
input lever at hydraulic zero and removes
any input play in the flight controls after
a hydraulic failure. The locking pins
upper portion acts as a bypass valve.
When hydraulic pressure to the servo
drops below 14 bars the locking pin drops
and allows chambers A and B to be
interconnected. This allows the fluid in the
chambers to flow between them during
control inputs after a hydraulic failure.
When the pitch servo is pressurized, fluid is
routed under the locking pin and above
6 bars lifts the locking pin out of the input
lever slot and compresses the locking
pin spring. This closes the interconnect
between the chambers on the piston rod.
With the locking pin recessed the slide
valve is free to move.
Prior to start up on the roll and tail rotor
servos the bypass valve is positioned to
allow chambers A and B on the piston rod
to be interconnected.
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Extension of
Servo Actuator
Locking Pin
Slide Valve
Chamber A
Return
Pressure
Input Lever
Push Pull Tube
Piston
Rod
Chamber B
Input Lever
Slot
Loss Of Hydraulic
Pressure
Retraction of
Servo Actuator
Figure 182, Samm Hydraulic Servo
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When the roll and tail rotor servos are
pressurized, fluid is routed under the
bypass valve. Above 6 bars the bypass
valve is moved into a position that closes
the interconnect between the chambers
on the piston rod and compresses the
bypass valve spring.
When fluid is routed to chamber A the
increased volume of fluid in the chamber
moves the housing and extends the length
of the servo. Compression of chamber B
forces fluid out of the servo through the
return line.
boom (Figure 186). The servo housing is
secured on the aft end to the tail rotor
pitch change rod via two attaching bolts
that act as guides that slide in a Teflon
track during servo movements (Figures 188
& 189).
The AS350B1 & B2 models include a yaw
load compensator which assists in pedal
inputs after a hydraulic failure. When
equipped with a yaw load compensator
an output adapter casing is mounted
between the tail rotor servo housing and
the tail rotor pitch change rod (Figure
189).
When fluid is routed to chamber B the
increased volume of fluid in the chamber
moves the housing and shortens the
length of the servo. Compression of
chamber A forces fluid out of the servo
through the return line.
As long as the pilot inputs control forces
the servo will continue to travel in the
direction of input. Once the control inputs
are ceased the housing will move until
input lever is at hydraulic zero. In this
position the slide valve does not allow fluid
to enter or exit the servo.
Tail Rotor Servo Actuators
The AS350 is equipped with either a SAMM
or a Dunlop tail rotor servo. The operating
principles for the servo are the same
as the main rotor servos except the tail
rotor servo does not have a servo control
manifold and the servo input lever is
moved by a ball flex cable.
The servo is mounted in the forward end
of the tail boom just under the tail rotor
drive shaft. The piston rod of the servo is
attached at the forward end via a rod
end bearing to mounting support No. 6
which is fastened to the top of the tail
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Figure 183, Dunlop Hydraulic Servos
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Extension of
Servo Actuator
Locking Pin
Return
Chamber A
Piston
Rod
Slide Valve
Chamber B
Pressure
Input Lever
Input
Lever
Slot
Push Pull Tube
Loss Of Hydraulic
Pressure
Retraction of
Servo Actuator
Figure 184, Dunlop Hydraulic Servos with Locking Pin
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Extension of
Servo Actuator
Locking Pin
Return
Chamber A
Piston
Rod
Slide Valve
Chamber B
Pressure
Input Lever
Input
Lever
Slot
Push Pull Tube
Loss Of Hydraulic
Pressure
Retraction of
Servo Actuator
Figure 185, Dunlop Hydraulic Servos without Locking Pin
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Yaw Load Compensator
Support No. 6
The AS 350 B1 and B2 models are
equipped with a yaw load compensator
on the tail rotor hydraulic servo to assist
in heavy loads during a hydraulic failure
(Figure 190).
Models equipped with a yaw load
compensator have an output adapter
casing (Figures 189 & 191) mounted
between the tail rotor servo housing
and the tail rotor pitch change rod. The
adaptor casing is open in the middle to
allow free movement of the trim rod. The
trim rod is connected on its forward end to
the tail rotor servo housing, and on its aft
end to the compensator lever (Figure 191).
Rod End
Bearing
Input
Lever
Support No. 6
Figure 186, Support No. 6
Guide Track
Piston Rod
Figure 188, Tail Rotor Servo Without Yaw Load
Compensator
Return
Pressure
Figure 187, Tail Rotor Servo
Guide Track
Adaptor Casing
Figure 189, Tail Rotor Servo With Yaw Load Compensator
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The compensator lever pivots between
the two brackets mounted under the
top of the tail boom. The other end of
the compensator lever is connected to
the actuator piston (Figure 190) in the
compensator body.
Actuating
Piston
Compensator
Body
One-Way
Check Valve
The compensator body pivots in between
the same mounting brackets as the
compensator lever.
Pressurized hydraulic fluid enters the
compensator body through a one way
check valve that traps pressure created
by the accumulator in case of a hydraulic
failure.
Solenoid
Valve
Accumulator
Figure 190, Yaw Load Compensator
Output Adapter
Casing
Trim Rod
Compensator
Lever
Actuating
Piston
Solenoid
Valve
Compensator
Body
Accumulator
Figure 191, Yaw Load Compensator
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The fluid entering the compensator body is
routed to the accumulator then on to yaw
load compensator actuator.
The oleopneumatic accumulator,
mounted on compensator body,
provides pressurized hydraulic fluid to the
compensator actuator in the case of a
hydraulic failure and absorbs surges in
the system. The accumulator consists of
a metal cylinder with a rubber bladder
inside of it which is charged with nitrogen
gas to 15 +1 -0 bar at 20° C. As the
hydraulic system is pressurized hydraulic
fluid is pumped into the cylinder around
the nitrogen filled bladder. When the
pressure of the hydraulic system exceeds
the charged pressure of the bladder the
hydraulic fluid compresses the bladder
creating pressurized fluid.
A solenoid valve is mounted to the
compensator body. When opened it
allows the accumulator pressure to be
dumped in case of a loss of control in the
tail rotor system and to depressurize the
compensator after shutdown.
A pressure relief, set at 55 bars, is mounted
at the solenoid valve to prevent hydraulic
locking.
After a hydraulic failure the flow of
pressurized fluid to the tail rotor servo and
yaw load compensator will cease.
The pressure trapped in the compensator
will assist in applying pitch to the tail rotor
blades (Figure 192).
When the tail rotor pedals are displaced
the pressure created by the accumulator
is transferred to the actuator piston and
will help in applying pitch to the tail
rotor blades. As the pedals are moved
hydraulic fluid exits the accumulator
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and fills the actuator. When the pedals
are displaced in the opposite direction
the force of the twist in the tail rotor spar
will assist in pushing the fluid out of the
actuator and back into the accumulator.
Hydraulic Line Routing
Hydraulic fluid gravity feeds from the
reservoir to the pump. The pressurized fluid
is then routed to the hydraulic distribution
block. If the pressure of the fluid is above
40 bars the regulating valve opens and
allows fluid to be routed back to the
reservoir until the pressure returns to 40
bars.
The fluid is routed from the left side of the
hydraulic distribution block to a “T” fitting
on the main gearbox. From the “T” fitting
the fluid is allowed to flow to the right roll
servo and into the main gearbox housing.
The hydraulic lines in the main gearbox
housing are routed to the left forward
pitch servo and the left roll servo.
The return fluid from the right roll servo and
the pitch servo are routed through the
main gearbox to a “T” fitting by the right
roll servo. The return fluid from the right roll
servo meets with the return fluid from the
other servos and then is routed externally
to the reservoir.
Pressurized hydraulic fluid for the tail rotor
servo exits the right side of the hydraulic
distribution block . The hydraulic line is
routed directly to the tail rotor servo and if
equipped, the yaw load compensator.
The return fluid from the tail rotor servo and
yaw load compensator is routed back
to the hydraulic distribution block and is
fed into the same return as the fluid from
the regulating valve. The return fluid then
exits the hydraulic distribution block and is
routed directly to the reservoir.
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Accumulator
Pressure
Neutral Pitch
Compensator
Force
Accumulator
Pressure
Servo Extending
Compensator
Force
Accumulator
Pressure
Servo Retracting
Figure 192, Yaw Load Compensator
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Hydraulic System Checks
General
After every start there are two hydraulic
system checks performed to ensure that
the system is serviced and operating
properly.
The hydraulic accumulator check tests
that there is sufficient pressurized fluid in
the accumulators after a hydraulic failure.
For models equipped with a yaw load
compensator the check also tests that the
solenoid valve on the compensator body
operates correctly.
The hydraulic pressure isolation check
tests the proper operation of the isolation
solenoid valves mounted on the servo
control manifold of each of the main
rotor servos. If equipped with a yaw load
compensator the check also tests the
proper operation of the compensator
system.
Hydraulic Accumulator Check
The hydraulic accumulator check (Figures
194 & 196) is performed with the engine
running and the fuel flow control lever in
the flight gate.
Friction of the cyclic is adjusted to the level
used in flight. Check that the collective is
securely locked down by the locking strip.
Cut off the hydraulic pressure by actuating
the HYD TEST push button or switch (if
helicopter is equipped with a Geneva
Panel) on the center console.
When the HYD TEST switch is depressed the
solenoid valve on the hydraulic distribution
block opens and if equipped, the solenoid
valve on the compensator body opens.
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With the solenoid valve on the hydraulic
distribution block open the pressurized fluid
from the hydraulic pump, by path of least
resistance, will return to the reservoir, and
the flow of hydraulic fluid to the main and
tail rotor servos will cease. The hydraulic
fluids pressure at the hydraulic pressure
switch will decrease causing the pressure
switch to activate the HYD light and the
horn.
With the solenoid valve on the yaw load
compensator open the pressurized fluid in
the yaw load compensator will be allowed
to exit the unit back to the reservoir.
Confirm that the HYD warning light is
illuminated and the horn has sounded.
After confirmation of the horn sounding,
it may be disabled by deactivating the
HORN push button or switch on the center
console.
The cyclic is then moved forward and aft
for a total travel of 4 inches, 2 to 3 times.
The cyclic is then moved left and right for
a total travel of 4 inches, 2 to 3 times.
During these movements the
accumulators are discharging their
pressurized fluid by the expansion of the
nitrogen bag. By moving the cyclic fore
and aft the single pitch servo accumulator
system is checked. When moving the
cyclic left and right the two roll servos
accumulator systems are checked. The
cyclic should be fully boosted during all
movements.
If any control feed back is felt during the
movements the accumulator system will
need to be checked by maintenance.
The tail rotor pedals on models equipped
with a yaw load compensator should then
be displaced left and right to check that
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Figure 193, Fully Charged Hydraulic System on Models Equipped with a Yaw Load Compensator
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Figure 194, Hydraulic Test on Models Equipped with a Yaw Load Compensator
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Figure 195, Fully Charged Hydraulic System on Models Not Equipped with a Yaw Load
Compensator
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Figure 196, Hydraulic Test on Models Not Equipped with a Yaw Load Compensator
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the solenoid valve on the compensator
body has dumped the pressure out of the
unit by confirming that the pedals have
become stiff.
The HYD TEST push button is then
disengaged and if the HORN has been
disabled, the HORN push button or switch
should be re-engaged.
Confirm that the HYD light has
extinguished.
Hydraulic Isolation Check
Following the hydraulic accumulator
check the hydraulic pressure isolation
check is performed (Figures 197 & 198).
Confirm that the collective is securely
locked down by the locking strip.
Place the hydraulic isolation switch or
push button located on the collective
in the cut-off position. This will open the
isolation solenoid valve on each of main
servos control manifold. With the isolation
solenoid valve open, the pressure inlet
and the return outlet of the servo control
manifolds are connected. By path of least
resistance the pressurized fluid entering the
manifold will be routed to the return outlet
along with any pressurized fluid in the
accumulator.
With the solenoid valves on the servo
control manifolds open the hydraulic
system can not build up pressure.
The pressure switch on the hydraulic
distribution block senses this loss of pressure
and illuminates the HYD light. Since the
activation of isolation switch or push
button disables the horn relay for the
hydraulic system the horn does not sound.
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After activation the cyclic will almost
immediately get stiff. The cyclic should
be slightly displaced fore and aft and left
and right to ensure control response of the
main rotor system.
With the loss of pressure in the system the
tail rotor servo will no longer be receiving
pressurized fluid.
The tail rotor pedals on models equipped
with a yaw load compensator should then
be displaced left and right to check that
the yaw load compensator has held its
charge by confirming that the pedals are
partially boosted.
The isolation switch or push button is
then returned to its normal position, this
activates the isolation solenoid valves to
close. With the valves closed the pressure
will build in the system including filling of
the accumulators.
With the isolation switch or push button in
the normal position the horn is no longer
disabled and will sound until the system is
charged to above 38 ±2 bars.
The horn should sound for 2 to 3 seconds
if the accumulator’s nitrogen bladders
are properly charged. If the horn sounds
for less then 2 seconds the nitrogen filled
bladders in the accumulators may be
overcharged. With the bladders over
charged less hydraulic fluid can fit into the
accumulators. Since the accumulators will
fill to their capacity faster the hydraulic
system will achieve 38 ±2 bars faster.
If the horn sounds for longer then 3
seconds the nitrogen filled bladders in the
accumulators may be undercharged.
With the bladders under charged
more hydraulic fluid can fit into the
accumulators. Since the accumulators will
fill to their capacity slower the hydraulic
system will achieve 38 ±2 bars slower.
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Figure 197, Hydraulic Isolation Check on Models Equipped with a Yaw Load Compensator
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Figure 198, Hydraulic Isolation Check on Models Not Equipped with a Yaw Load Compensator
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Hydraulic System Malfunctions
General
The loss of pressure to the hydraulic system
or the seizure of the slide valve requires
immediate attention by the pilot. The
proper emergency procedure can be
found in section 3 of the aircrafts flight
manual, the expanded explanation of
these procedures written herein are meant
for the purpose of standardization. They
may not be applicable in all situations
and never supersede the FAA approved
manufactures flight manual or common
sense.
The loss of pressure to the hydraulic system
can result from various causes. These
include a failure of the hydraulic pump
or belt, the filter becoming clogged, or a
break in a hydraulic line.
After a loss of pressure in the system the
pressure switch located on the hydraulic
distribution block will activate the HYD light
and the horn. With the system properly
serviced the accumulators on the main
rotor servos will provide pressurized fluid
to the servos allowing the main rotor
flight controls to be normally boosted.
The yaw pedals on the AS350B and BA
will become stiff and the pedals on the
AS350B1 and B2 will feel partially boosted
due the assistance from the yaw load
compensator.
Since the accumulators on the main
rotor servos have a set amount of fluid in
them the length of time they will provide
pressurized hydraulic fluid to the servos
depends on the amount of movements
made to the main rotor flight controls. In
flight, or in a out of ground effect hover,
with the system properly serviced and
with smooth and limited inputs, there
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should be enough pressure stored in the
accumulators to adjust the helicopters
speed to 40 to 60 kts. During an in ground
effect hover there should be enough
pressure stored in the accumulators to
land the helicopter safely.
Loss of Pressure
When the hydraulic system experiences a
loss of pressure (Figures 199 & 200) the red
HYD light on the warning caution panel
will illuminate and the aural warning horn
activates. With the accumulators properly
charged the cyclic and collective controls
will feel normal. The pedals will feel stiff in
the AS350 B & BA and the pedals will feel
partially boosted on the AS 350 B1 & B2.
After the loss of pressure in forward flight
adjust the airspeed to 40 to 60 kts without
haste. After reaching the target airspeed
range engage the isolation switch on the
collective. This will cause the horn to be
silenced and the remaining pressure in the
accumulators to be dumped. The cyclic
and collective will immediately become
stiff (Figures 200 & 202).
Since the elastomeric rotor system is set
with positive pitch in the rotor blades, the
collective will sit close to a 40 to 60 kt pitch
setting. Because of the aerodynamic rotor
force the cyclic will want to travel aft and
to the right. To help over come the extra
needed forces to push the cyclic forward
and left the pilot can position his or her
right leg against the cyclic.
As stated in the rotorcraft flight manual
the pilot then is recommended to “land
as soon as possible”. Even though the
helicopter can be maneuvered without
extreme difficultly, extended flight can
over fatigue the pilot and make the
landing of the helicopter more of a
challenge.
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Figure 199, Loss of Pressure in Models Equipped with a Yaw Load Compensator
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Figure 200, System After Loss of Pressure Emergency Procedure
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Figure 201, Loss of Pressure in Models Not Equipped with a Yaw Load Compensator
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Figure 202, System After Loss of Pressure Emergency Procedure
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The preferred landing area after a
hydraulic failure would be a long smooth
area like a runway or a field. After
selecting the landing area make a shallow
approach. As the helicopter is slowed the
forces required to hold the cyclic and
pedal in position will increase, the cyclic
will want to travel aft and to the right and
additional right pedal will be needed
as collective is increased. If these forces
are not overcome the helicopter will
pitch up and rotate left. The suggested
method of landing is with slight forward
speed on the touch down. After landing
the helicopter, lower the collective and
lock the collective down with the locking
strip. While guarding the cyclic perform a
normal shutdown.
After the loss of pressure in an in ground
effect hover control any tendency of the
helicopter to rotate around the yaw axis.
In models not equipped with a yaw load
compensator the forces required to hold
the nose of the helicopter straight with a
high power setting may be severe. Land
the helicopter normally without delay and
after the collective is fully down engage
the collective locking strip. Tighten the
friction on the cyclic and continue to
guard it until the rotor has fully stopped.
Place the hydraulic cutoff switch on the
collective in the cutoff position. This will
cause the cyclic to become stiff and the
horn to be silenced. After which the pilot
can perform a normal shut down.
After the loss of pressure in an out of
ground effect hover control any tendency
of the helicopter to rotate around the yaw
axis. In models not equipped with a yaw
load compensator the forces required to
hold the nose of the helicopter straight
with a high power setting may be severe.
Immediately adjust forward speed to 40 to
60 kts and engage the isolation solenoid
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switch on the collective. Perform the same
landing procedure as for a loss of pressure
in forward flight.
Slide Valve Seizure
The hydraulic servo slide valve routes
hydraulic fluid to one of the two chambers
on the piston rod and allows the fluid
from the other chamber to exit the servo.
If the slide valve seizes in any position
besides hydraulic zero, fluid will continue
to be routed to a chamber extending
or retracting the servo to full stop (Figure
203).
If one of the main rotor hydraulic servos
experiences a slide valve seizure the
cyclic will move without pilot input. The
forces may be impossible for the pilot
to over come and if the hydraulic fluid
pressure to the servo is not shut off the
helicopter may get into an unrecoverable
attitude. Upon uninitiated movement
of the cyclic the pilot should, prior to
reaching an unrecoverable attitude, cut
off the hydraulic pressure by engaging the
isolation solenoid switch on the collective.
Upon engaging the isolation switch the
red HYD light will illuminate and the flight
controls will immediately get stiff. If in a
hover, after the isolation switch is placed
in the cutoff position, the helicopter should
be landed without delay.
If in flight and the speed of the helicopter
is outside the range of 40 to 60 kts, the
helicopter may be difficult to control
after the isolation switch is placed in the
cutoff position. The pilot should then adjust
the speed as quickly as possible to the
proper speed range. Once the proper
speed is reached and the helicopter is
brought under control the procedure for a
hydraulic failure in forward flight should be
accomplished.
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If one of the yaw hydraulic servo
experiences a slide valve seizure the
pedals will move without pilot input. The
forces may be impossible for the pilot
to over come and if the hydraulic fluid
pressure to the servo is not shut off the
helicopter may get into an unrecoverable
spin. Upon uninitiated movement of
the pedals the pilot should, prior to
reaching an unrecoverable spin, cut off
the hydraulic pressure by engaging the
isolation solenoid switch on the collective.
Upon engaging the isolation switch the
red HYD light will illuminate and the flight
controls will immediately get stiff. If in a
hover, after the isolation switch is placed
in the cutoff position, the helicopter should
be landed without delay.
If in flight and the speed of the helicopter
is outside the range of 40 to 60 kts, the
helicopter may be difficult to control
after the isolation switch is placed in the
cutoff position. The pilot should then adjust
the speed as quickly as possible to the
proper speed range. Once the proper
speed is reached and the helicopter is
brought under control the procedure for a
hydraulic failure in forward flight should be
accomplished.
Slide Valve
Figure 203, Slide Valve Seizure
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