as350_hyd_section_sm.pdf

Chapter 10 - Hydraulics

Photo by: Anton Heumann

The hydraulic distribution block houses a hydraulic lter, a pressure regulating valve, a hydraulic pressure switch, a hydraulic test solenoid and on certain models a hydraulic lter clogging indicator.

INTRODUCTION The elastomeric based rotor system on the AS350 creates in ight dynamic forces that causes feedback in the cockpit ight controls. To counteract these forces the AS350 is equipped with a single hydraulic system that assists with the t he pitch changes and dampens out dynamic feedback on the main and tail rotor blades.

The hydraulic lter removes contaminates from the hydraulic uid 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 pressurized uid from the pump back to the reservoir.

GENERAL The hydraulic system is comprised of a reservoir, 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 tted 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 uid for the system. © 2012 All rights reserved

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EuroSafety International, LLC

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 uid to bypass the servo and return retur n to the reservoir.

The helicopter is equipped with either a hydraulic isolation switch on the collective head or, 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, lever, an actuator piston, a compensator body, an accumulator, accumulator, a solenoid valve, and a pressure release valve.

© 2012 All rights reserved

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 activates the solenoid valve on the yaw load compensator .

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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 uid to bypass the servo and return retur n to the reservoir.

The helicopter is equipped with either a hydraulic isolation switch on the collective head or, 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, lever, an actuator piston, a compensator body, an accumulator, accumulator, a solenoid valve, and a pressure release valve.


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 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 uid. The reservoir is equipped with a vented cap that can be easily opened to add hydraulic uid to the system and a strainer mounted in i n the reservoir opening to lter out debris when adding uid to the system. Hydraulic uid from the reservoir ows 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 ange. 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


Figure 170, Hydraulic Reservoir

(Figure 172). Each belt utilizes different input pinion pulley anges 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 ow 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 uid. 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.


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 uid from the hydraulic pump. It houses the hydraulic lter, hydraulic lter 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 lter is mounted on the bottom of the hydraulic distribution block. It further removes contaminates from the hydraulic uid. The BFS150 distribution block contains a 25 micron metallic lter that can be cleaned between usages. The BFS155 and BFS1551 distribution block contains a 3 micron consumable mesh lter.

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


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

Filter

BFS-155

BFS-150

Figure 175, Hydraulic Distribution Block


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Pressure Regulating Valve Pressure to Main Rotor Servos

Pressure Regulating Valve Pressure to Main Rotor Servos


Return To Reservoir


Return To Reservoir

Pressure Switch


Pressure Switch

Filter

Clogging Indicator




Filter

BFS-155

BFS-150

Figure 175, Hydraulic Distribution Block

debris builds up on the lter element the hydraulic uid pressure will increase around the outside of the lter and decrease after the lter. When the pressure differential reaches 2.7 ± 0.4 bar the magnetic piston spring assembly will be pushed back off of the lter 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 lter has no bypass capability, if the lter becomes clogged the ow of hydraulic uid 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 lter housing at a 2.7 bar differential pressure across the lter. This indicator gives a visual indication that the lter is clogging (Figure 176). The clogging indicator is held in the recessed position against the lter bulkhead by a magnetic piston spring assembly (Figure 175). When sufcient © 2012 All rights reserved

Clogging Indicator

Figure 176, Clogging Indicator

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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 sufcient 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 uid 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 ow to open at 43 ±1 bar. When installed on the helicopter, with pressurized uid owing past the regulating valve at 6 liters per minute, the pressure relief valve will open at approximately 40 bar. This pressure can be modied 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 uid. The pressure at the hydraulic pressure switch will decrease causing the pressure switch to activate the HYD light and the horn. Pressurized hydraulic uid 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 tted 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 specic 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 uid 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


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 uid 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 xed 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 tted 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 uid to ow through them.

When the helicopter ight 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 uid to be routed to one of the two chambers on the piston rod while allowing uid from the other chamber to exit the servo housing back to the reservoir. Deection of the input lever for maximum opening of the slide valve is 2 mm. © 2012 All rights reserved

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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 uid to bypass the servo inlet port (Figure 182) . Hydraulic uid enters the servo through the top of the servo control manifold through the one way check valve (Figure 181). The uid 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 uid to one of the chambers on the piston rod, at the same time allowing uid from the other chamber to be expelled through the return port. The uid 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 uid 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 uid is pumped into the cylinder around the nitrogen lled bladder. When the pressure of the hydraulic system exceeds the charged pressure of the bladder the hydraulic uid compresses the bladder creating pressurized uid.

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 uid entering the manifold will be routed to the return outlet along with any pressurized uid in the accumulator.

If the ow of hydraulic pressure from the pump ceases, the accumulator will dispel its pressurized uid into the servo. The one way check valve, mounted on top of the servo control manifold at the pressure inlet port, traps hydraulic uid dispelled from the accumulator to the inlet port Pressure

In case of a slide valve seizure the valve could be opened to stop the ow of pressurized uid into the servos. After the loss of pressure to the servos, the solenoid is opened after obtaining the Pressure

Return

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 ight controls after a hydraulic failure. With the input lever at hydraulic zero the slide valve is in a position that will not let hydraulic uid enter or exit the servo. After a hydraulic failure the trapped uid 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 uid in the chambers to ow between them during control inputs after a hydraulic failure. When the servo is rst pressurized, uid 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 uid is routed to chamber A the increased volume of uid in the chamber moves the housing and extends the length of the servo. Compression of chamber B forces uid out of the servo through the return line. © 2012 All rights reserved

When uid is routed to chamber B the increased volume of uid in the chamber moves the housing and shortens the length of the servo. Compression of chamber A forces uid 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 uid 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 ight 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 uid in the chambers to ow between them during control inputs after a hydraulic failure. When the pitch servo is pressurized, uid 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

Piston Rod

Input Lever Chamber B Push Pull Tube

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, uid 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 uid is routed to chamber A the increased volume of uid in the chamber moves the housing and extends the length of the servo. Compression of chamber B forces uid 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 Teon 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 uid is routed to chamber B the increased volume of uid in the chamber moves the housing and shortens the length of the servo. Compression of chamber A forces uid 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 uid 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 ex 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


Figure 183, Dunlop Hydraulic Servos

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Extension of Servo Actuator

Return Locking Pin Chamber A

Piston Rod

Slide Valve

Chamber B

Pressure

Input Lever Input Lever Slot

Push Pull Tube



Figure 184, Dunlop Hydraulic Servos with Locking Pin


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Extension of Servo Actuator

Return Locking Pin Chamber A

Piston Rod

Slide Valve

Chamber B

Pressure

Input Lever Input Lever Slot

Push Pull Tube



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 uid 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



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The uid 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 uid 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 uid is pumped into the cylinder around the nitrogen lled bladder. When the pressure of the hydraulic system exceeds the charged pressure of the bladder the hydraulic uid compresses the bladder creating pressurized uid. 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 ow of pressurized uid 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 uid exits the accumulator


and lls 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 uid out of the actuator and back into the accumulator. Hydraulic Line Routing Hydraulic uid gravity feeds from the reservoir to the pump. The pressurized uid is then routed to the hydraulic distribution block. If the pressure of the uid is above 40 bars the regulating valve opens and allows uid to be routed back to the reservoir until the pressure returns to 40 bars. The uid is routed from the left side of the hydraulic distribution block to a “T” tting on the main gearbox. From the “T” tting the uid is allowed to ow 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 uid from the right roll servo and the pitch servo are routed through the main gearbox to a “T” tting by the right roll servo. The return uid from the right roll servo meets with the return uid from the other servos and then is routed externally to the reservoir. Pressurized hydraulic uid 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 uid 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 uid from the regulating valve. The return uid then exits the hydraulic distribution block and is routed directly to the reservoir. 18


Accumulator Pressure

Neutral Pitch

Compensator Force Accumulator Pressure

Servo Extending

Compensator Force

Accumulator Pressure

Servo Retracting



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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 sufcient pressurized uid 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 ow control lever in the ight gate. Friction of the cyclic is adjusted to the level used in ight. 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.


With the solenoid valve on the hydraulic distribution block open the pressurized uid from the hydraulic pump, by path of least resistance, will return to the reservoir, and the ow of hydraulic uid to the main and tail rotor servos will cease. The hydraulic uids 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 uid in the yaw load compensator will be allowed to exit the unit back to the reservoir. Conrm that the HYD warning light is illuminated and the horn has sounded. After conrmation 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 uid 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 conrming 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. Conrm that the HYD light has extinguished. Hydraulic Isolation Check Following the hydraulic accumulator check the hydraulic pressure isolation check is performed (Figures 197 & 198). Conrm 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 uid entering the manifold will be routed to the return outlet along with any pressurized uid 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.


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 uid. 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 conrming 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 lling 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 lled bladders in the accumulators may be overcharged. With the bladders over charged less hydraulic uid can t into the accumulators. Since the accumulators will ll to their capacity faster the hydraulic system will achieve 38 ±2 bars faster. If the horn sounds for longer then 3 seconds the nitrogen lled bladders in the accumulators may be undercharged. With the bladders under charged more hydraulic uid can t into the accumulators. Since the accumulators will ll to their capacity slower the hydraulic system will achieve 38 ±2 bars slower. 25


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 ight 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 ight 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 lter 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 uid to the servos allowing the main rotor ight 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 uid in them the length of time they will provide pressurized hydraulic uid to the servos depends on the amount of movements made to the main rotor ight controls. In ight, or in a out of ground effect hover, with the system properly serviced and with smooth and limited inputs, there


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 ight 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 ight manual the pilot then is recommended to “land as soon as possible”. Even though the helicopter can be maneuvered without extreme difcultly, extended ight can over fatigue the pilot and make the landing of the helicopter more of a challenge. 28


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


30


Figure 201, Loss of Pressure in Models Not Equipped w ith a Yaw Load Compensator


31


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 eld. 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


switch on the collective. Perform the same landing procedure as for a loss of pressure in forward ight. Slide Valve Seizure The hydraulic servo slide valve routes hydraulic uid to one of the two chambers on the piston rod and allows the uid from the other chamber to exit the servo. If the slide valve seizes in any position besides hydraulic zero, uid 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 uid 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 ight 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 ight and the speed of the helicopter is outside the range of 40 to 60 kts, the helicopter may be difcult 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 ight should be accomplished.

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as350_hyd_section_sm.pdf

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