Hydraulic Power
The hydraulic system contains, pressurizes, controls, and filters the hydraulic fluid that is supplied to the servo-actuators to help operate the cyclic and collective controls. Without the aid of hydraulic power, pilot effort to manually counteract the control feedback forces of the cyclic and collective flight controls is drastically increased.
The hydraulic system includes the components that follow:
Hydraulic Reservoir Filter Module (HRFM)
Hydraulic pump
Pump adapter assembly
Pressure line (to filter)
Pressure filter assembly
Pressure line (from filter)
Suction line (from HRFM)
Pressure lines (to servo-actuators)
Return lines (from servo-actuators)
HYD SYS switch
Hydraulic Reservoir Filter Module (HRFM)
The HRFM stores hydraulic fluid, regulates system pressure, filters the hydraulic fluid, assists in cooling the hydraulic fluid, and allows for pressure and flow to be directed to the servo actuators or isolated from the servo actuators by the overpressure protection feature. The HRFM is attached to the truss assembly in the upper main rotor drive compartment. The HRFM consists of the components that follow:
Manifold: made from aluminum alloy, it is the mounting base and all the components are installed on it. It has passages that let the hydraulic fluid move in and out of the HRFM and between its components.
Reservoir: made from carbon fiber reinforced plastic, it is installed on top of the manifold. It contains 0.66 quarts (0.62 L) of hydraulic fluid. The port at the top of the reservoir is used to service the hydraulic system. The filler cap gives access to the port and closes it to prevent contamination. The filler cap has an air vent to equalize pressure. A sight glass on the right side of the reservoir gives an indication of the fluid level to the crew and maintenance personnel.
Filter: an aluminum alloy filter bowl installed on the left side of the HRFM contains a 5-micron filter element. The non-bypass filter will clog completely in order to prevent contaminated fluid from being pumped to the servo-actuators. The filter element cannot be cleaned and must be replaced if it is clogged. In case of a total electrical failure, the system is fail-safe ON.
Solenoid valve: a 3-way, 2-position, direct-acting, spool-type, directional valve, it is installed on the left side of the HRFM, downstream of the filter. The valve is an assembly of a cartridge valve and a coil controlled by the HYD SYS switch. When the valve is open, the fluid flows to the servo-actuators. When it is closed, the fluid goes back to the reservoir.
Relief valve: a direct acting, poppet type valve, is installed on the right side of the HRFM, upstream of the filter. The relief valve maintains a continuous system pressure of 600 - 625 psi (4136 - 4309 kPa). Excess fluid not required by the servo-actuators is directed back to the reservoir.
Rupture disc: made from stainless steel, the rupture disc is installed on the left side of the HRFM upstream of all the other components. It will prevent damage to the system in the event that hydraulic pressure exceeds 900 psi (6205 kPa). It then lets the fluid directly back into the reservoir. Maintenance intervention is required in this occurs.
Hydraulic pump
The hydraulic pump is located in the lower main rotor drive compartment and is attached to the pump adapter, on the main case of the transmission. It is a gear type with external teeth and is fixed displacement made from steel and aluminum alloy. It pressurizes the system between 600 and 625 PSI (4137 and 4309 kPa) and supplies a maximum flow of 1.7 gallons per minute (6.44 L/min). The hydraulic pump turns counterclockwise when looking at its mounting flange and engages the oil pump of the transmission through a pump driveshaft assembly. It has one inlet port on the right side that connects to the suction line from the HRFM, and one outlet port on the left side that connects to the pressure line to the HRFM.
Pump adapter assembly
The pump adapter assembly is located in the lower main rotor drive compartment. It consists of the pump adapter and the pump driveshaft assembly.
The pump driveshaft assembly is made from stainless steel and consists of the pump driveshaft and the transmission driveshaft attached together with a shear rivet. The function of the rivet is to break when the torque between the transmission and the pump is 150 to 225 inch-pounds (17 to 25.4 Nm) to protect the transmission if there is a pump failure. The pump driveshaft has cavities for the NR sensor to sense rotation of the transmission.
Pressure line (to HRFM)
The pressure line (to HRFM) is attached to the truss assembly on the left side of the main rotor drive compartment. It consists of a flexible hose made of Teflon, a check valve, and a tube with integrated cooling fins, that connects the hydraulic pump to the HRFM. The check valve opens between 2 and 8 PSI (14 to 55 kPa) and prevents a reverse flow when a Hydraulic Power Unit (HPU) is installed. The finned tube is a rigid stainless steel welded assembly with fins that give more surface area for the air to move around the pipe and absorb heat from the hydraulic fluid. The pressure line also has a port with a removable cap downstream of the finned pipe to connect the pressure line of the HPU.
Pressure filter assembly
The pressure filter assembly removes the dangerous particles (particles larger than 15 microns). The fluid continues to flow to the check valve.
Pressure line (to filter)
The pressure line (to filter) consists of a hose that connects the pump to the pressure filter assembly.
Pressure line (from filter)
The pressure line (from filter) is attached to the truss assembly on the left side of the main rotor drive compartment. It consists of a pressure tube, a check valve, and a tube with integrated cooling fins, that connects the pressure filter assembly to the HRFM. The check valve opens between 2 and 8 PSI (14 to 55 kPa) and prevents a reverse flow when a Hydraulic Power Unit (HPU) is installed. The finned tube is a rigid stainless steel welded assembly with fins that give more surface area for the air to move around the pipe and absorb heat from the hydraulic fluid.
Suction line (from HRFM)
The suction line (from HRFM) is attached to the truss assembly on the right side of the main rotor drive compartment. It is an assembly of a flexible hose made of Teflon and a tube with integrated cooling fins, that connects the HRFM to the hydraulic pump. The finned tube is a rigid stainless steel welded assembly with fins that give more surface area for the air to move around the pipe and absorb heat from the hydraulic fluid.
Pressure lines (to servo-actuators)
The pressure lines (to servo-actuators) are located in the forward main rotor drive compartment. These are three separate lines that connect the HRFM to the servo-actuators. Each line is a flexible hose made of Teflon.
Return lines (from servo-actuators)
The return lines (from servo-actuators) are located in the forward main rotor drive compartment. These are three separate lines that connect the servo-actuators to the HRFM. Each line is a flexible hose made of Teflon. A check valve that opens between 2 and 8 PSI (14 to 55 kPa) is installed on the HRFM, downstream of the return line to prevent a depletion of the reservoir when a HPU is installed.
HYD SYS switch
The HYD SYS switch is located on the miscellaneous control panel. It is a Single Pole, Double Throw (SPDT) toggle switch that controls the solenoid valve of the HRFM.
Operation
The hydraulic fluid in the reservoir moves out by suction to the hydraulic pump through the suction port (TO PUMP) of the HRFM. The fins on the suction line (from HRFM) cool the hydraulic fluid before it goes into the suction port of the hydraulic pump.
When the rotors turn, the oil pump of the transmission drives the hydraulic pump through the connecting driveshaft assembly. This pressurizes the hydraulic fluid which moves out through the pressure port of the hydraulic pump.
If installed, the pressure filter assembly filters all the oil in the hydraulic circuit and has a blockage indicator that will go out when the filter become clogged.
The fins on the pressure line (to HRFM) cool the hydraulic fluid before it goes into the pressure port (FROM PUMP) of the HRFM.
Inside the HRFM, the pressurized hydraulic fluid enters a manifold that is connected to the filter, relief valve, and rupture disc.
The relief valve regulates the pressure of the hydraulic fluid. It opens when the pressure reaches a threshold and directs some of the hydraulic fluid to the reservoir, letting the pressure drop below the threshold. It then closes and the cycle repeats to stabilize the pressure when the output from the pump and the demand from the servo-actuators changes.
In the case that the relief valve failed or could not compensate for an increase of pressure, the rupture disc would break to let all of the fluid go back directly to the reservoir. It is the last safety measure to protect the hydraulic system. When it breaks, it must be replaced.
In the filter bowl, the fluid moves inward through the pleats of the filter element where contamination is removed.
From the filter, the fluid then moves to the solenoid valve. The solenoid valve is energized with 28 VDC from the MAIN bus and can be isolated with the HYD circuit breaker. When the HYD SYS switch is set to HYD SYS, the contacts are open and no ground is provided to the solenoid valve, de-energizing it, letting the hydraulic fluid to the servo-actuators through the pressure port (TO ACT) of the HRFM. Thus, the main hydraulic system will stay operational after a full electrical failure. When the HYD SYS switch is set to the OFF position, the contacts are closed and provide the ground to the solenoid valve, energizing it and stopping the flow to the servo-actuators, letting the hydraulic fluid back in the reservoir. Thus, it is possible for the pilot to isolate the servo-controls after a failure of the main hydraulic system and operate the cyclic and collective controls directly.
From the return port (FROM ACT) of the HRFM, the hydraulic fluid moves to the reservoir for storage at atmospheric pressure, ready to repeat the cycle.
When the system is operated with an HPU, the pressure line of the unit supplies a pressurized fluid at the port upstream of the pressure port (FROM PUMP) of the HRFM. A check valve is installed on the pressure line (to HRFM) to prevent a reverse flow to the hydraulic pump. The fluid then moves back to the unit through a connection downstream of the longitudinal servo-actuator. A check valve is installed at the return port (FROM ACT) of the HRFM to prevent depletion of the reservoir when the return line is disconnected.
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