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.
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.
The reservoir is mounted on the main gearbox and gravity feeds to the hydraulic pump. The pump is belt drivenoff of the engine to main gearbox drive shaft and provides pressurized fluid for the system.
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.
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.
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 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 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 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.
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 system is equipped with a single red HYD light located in the warning-caution 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).
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 .
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.
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).
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
Figure 167, Hydraulic Warning Light
Figure 168, Hydraulic Isolation Switch/Button
Figure 169, Hydraulic Test Switch/Button
Figure 170, Hydraulic Reservoir
Return
Feed
Return
(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
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.
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.
The hydraulic pump (Figure 174) produces a rate of flow of 6 liters per minute when the main rotor is turning at a speed ofapproximately 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.
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 BFS155-1), pressure regulating valve, hydraulic pressure switch and the hydraulic test solenoid.
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 BFS155-1 distribution block contains a 3 micron consumable mesh filter.
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
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.
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.
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.
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
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.
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 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
a rod end bearing to the non rotating swashplate of the rotor system (Figure 179).
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.
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.
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.
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.
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)
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.
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
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 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.
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
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.
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.
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.
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
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).
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).
Figure 186, Support No. 6
Figure 188, Tail Rotor Servo Without Yaw Load Compensator
Figure 187, Tail Rotor Servo
Figure 189, Tail Rotor Servo With Yaw Load Compensator
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.
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.
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
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.
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.
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
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.
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.
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
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.
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
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.
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.
