The auxiliary power is supplied from the auxiliary hydraulic system. The function of the auxiliary system is to supply hydraulic power to the lower rudder when one of the main systems does not operate.

The auxiliary system supplies hydraulic power to the rudder during flight, when one of the two engines does not operate. This supplies the pilot with the rudder control necessary to adjust for the asymmetric thrust of the remaining engine. The DC motor-driven pump (DCMP) and a hydraulic accumulator keep the auxiliary system pressurized during flight, but the rudder actuators are isolated from auxiliary power during usual operation through a switching valve.

The switching valve automatically applies the auxiliary hydraulic power to the rudder controls after an engine stops in flight.

The auxiliary system DCMP is controlled through the AUX PUMP selector switch on the HYDRAULIC control panel installed on the center pedestal. The auxiliary hydraulic system is monitored through the hydraulic synoptic page on the engine indicating and crew alerting system (EICAS) display.

A system reservoir is in the aft equipment compartment. The auxiliary system reservoir has a fluid volume of 0.65 U.S gal (2.46 L). Servicing to the auxiliary system is supplied through a ground service quick-disconnect. The ground service quick-disconnect, installed on the auxiliary hydraulic system rack in the aft equipment compartment, is used to fill the hydraulic reservoir.

The auxiliary hydraulic system supplies the lower rudder power control unit (PCU) with secondary hydraulic power during a one-engine flight condition. With a one-engine flight condition, the thrust is not symmetrical, which makes it necessary for the pilot to have more yaw control. The auxiliary system gives more hydraulic power to the rudder controls to adjust for this condition.

The auxiliary hydraulic system has an interface with the right hydraulic system. If the right hydraulic system has a malfunction, a switching valve moves and applies auxiliary hydraulic pressure to the lower rudder PCU. A DC motor-driven pump (DCMP) and a hydraulic accumulator keep the auxiliary system pressurized during flight. During usual flight operation, the rudder actuators are isolated from auxiliary power. In flight, when the left and right engines operate, the switching valve stays closed and isolates the rudder from the auxiliary hydraulic system.

---

The auxiliary DCMP is installed in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00 (WL108.00). The DCMP holds the fluid pressure in the auxiliary system accumulator.

The DCMP has a fixed-displacement hydraulic pump that is attached to a DC electric motor. The DCMP has a pressure (outlet) port and a suction (inlet) port which are directly connected to the aircraft hydraulic system through hoses. The DCMP also has a seal drain port which is connected to the ecology bottle through a plastic tube. Electrical power for the motor is transmitted through a pair of lug type connectors which connect with the aircraft wiring harness. A ground stud is supplied on the side of the motor terminal box for the wire shield.

The auxiliary DCMP is energized through the L ESS BUS through circuit breaker HYDRAULIC AUX DCMP PUMP (CB1-D10).

The DCMP includes the parts that follow:

DC Electric Motor

An air-cooled DC electric motor changes applied voltages (from 16 to 29 VDC) into mechanical energy to operate the hydraulic pump. The DC electric motor has terminal lugs through which electrical power is transmitted. Permanent magnets are included in the motor stator. Four brushes are set radially about the motor axis. When the brushes are in contact with the commutator, an electrical current flows through a part of the armature. Magnetic fields occur when the motor shaft turns while an electrical current flows through the armature.

Hydraulic Pump

A fixed-displacement hydraulic pump changes the mechanical energy supplied by the DC electric motor into hydraulic energy for the auxiliary system. A spline type coupling connects the motor shaft to the internal drive shaft of the pump. Splines connect the internal drive shaft to a cylinder barrel in the pump. The cylinder barrel contains nine chambers radially found about its center. A piston is in each chamber of the cylinder barrel.

One end of the piston is cylindrical while the other end contains a ball on which a piston shoe is swaged. The piston shoes are held through a mechanism against a cam plate which is set at an angle. While the cylinder barrel turns, the piston shoes move against the angle cam plate that result in axial piston movement. While the pistons move in and out of the chamber, fluid is pushed from the outlet port and pulled into the inlet port. The quantity of hydraulic fluid flow changes with the speed of the drive shaft.

---

The pressure filter manifold is installed in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00. It holds the pressure filter element which is used to remove solid particle contamination from the fluid flow that comes from the auxiliary DCMP.

The hydraulic fluid goes into the pressure filter manifold through the pressure inlet port. The fluid then flows around the shutoff diaphragm to the outer area of the filter element and through the pressure filter element. It then flows through the center of the shutoff diaphragm and comes out of the manifold through the pressure outlet port.

The pressure filter manifold includes the parts that follow:

Automatic Shutoff Device

An automatic shutoff device which is installed in the filter head at the top of the filter bowl. This shutoff device prevents hydraulic fluid leakage when the filter element is replaced. While the filter bowl is removed, a spring pushes the shutoff diaphragm into the closed position. The automatic shutoff device is resistant to 200 psi (1378.95 kPa) without leakage (with the filter bowl and the element removed).

Differential Pressure Indicator

A differential pressure indicator (DPI) gives a visual warning indication (an extended red button) when the pressure filter element must be replaced. The red button on the top of the DPI extends when the differential pressure across the filter element increases to 70±10 psi (482.63±68.95 kPa). When operated, the red button remains extended until pushed in manually. When the DPI is in the pushed in position (usual operation), the indicator can not be seen (fully retracted).

A surge arrester device in the DPI prevents its accidental operation when there is a flow surge that occurs momentarily. A thermal lockout device in the DPI makes sure that it is locked out when the hydraulic fluid temperature is less than 50 °F (10.01 °C). When the hydraulic fluid temperature is more than 80 °F (26.69 °C), the thermal lockout device lets the DPI operate.

The DPI is a cartridge configuration which can be easily removed from and installed on the filter manifold.

---

A pressure filter element is installed in the aft equipment compartment in the filter bowl on the pressure filter manifold. The pressure filter element is used to remove solid particle contamination from the fluid flow that comes from the auxiliary DCMP.

The pressure filter element is a 15-micron, disposable type cartridge which is continuously monitored for dirt by the DPI on the pressure filter manifold. The pressure filter element is of the same type as the left and right hydraulic systems case drain filter elements and is interchangeable.

The filter bowl has standard 0.875-in (22.23-mm) square wrench flats. The filter bowl is torqued on the pressure manifold housing.

---

The accumulator is in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00. The accumulator decreases a hydraulic pressure surge. It also is used with the DCMP to hold auxiliary hydraulic system pressure when the right hydraulic system does not operate.

Thrust is not symmetrical during a one-engine flight condition. The accumulator keeps the lower rudder PCU inlet pressure at a sufficient level to decrease the pilot’s force. This pressure is also available if left and right hydraulic system pressure is not available.

The accumulator contains pressurized Phosphate Ester base hydraulic fluid. The accumulator is precharged with nitrogen and has a nominal volume of 50 in³ (819.35 cm³). A piston in the accumulator moves to keep gas and hydraulic fluid at equal pressure and sealed from each other.

The accumulator precharge pressure changes with the hydraulic fluid temperature to let the accumulator keep hydraulic fluid after the accumulator is discharged. This is necessary for a smooth continuous operation of the lower rudder while the DCMP comes on at 2,500 psi (17,236.89 kPa).

---

The accumulator charge manifold is installed in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00 forward of the accumulator. The accumulator charge manifold is used to monitor the accumulator precharge pressure and to fill the accumulator with nitrogen.

The accumulator charge manifold contains a charging valve (schraeder-type) and a pressure gauge which are installed into a manifold. A short tube connects the charge manifold to the gas end of the accumulator.

A service placard (attached to the charge manifold) gives accumulator precharge instructions. The pressure gauge reads from 0 to 5,000 psi (0 to 34,473.78 kPa) with 200 psi (1,378.95 kPa) increments.

---

The auxiliary reservoir is installed in the aft equipment compartment on the top side of the auxiliary hydraulic system rack at FS788.00. The reservoir has a capacity of 150 in³ (2,458.06 cm³). The auxiliary system reservoir has a fluid volume of 0.65 U.S gal (2.46 L).

The reservoir is of bootstrap-type which uses system pressure fluid on a piston to give control of auxiliary hydraulic system return pressure. The reservoir supplies the necessary fluid volume to the hydraulic system which includes pressurization of the auxiliary DCMP pump inlet.

The reservoir is installed vertically with the bootstrap cylinder at the bottom. There are ports for the bootstrap pressure, suction pressure, and return pressure.

The reservoir includes the parts that follow:

Low-Pressure Storage Chamber

A low-pressure storage chamber that includes a low-pressure piston is operated by a bootstrap piston to give a chamber pressure of 55±3 psi (379.21±20.68 kPa). The chamber pressure is constant to a maximum of 1.20 U.S. gal/min (4.54 L/min) through a fluid temperature range of 20 to 225 °F (–6.67 to 107.22 °C). O-rings are used for all dynamic seals for leakage control. Leakage will not be more than one drop in 50 full travel operations of the piston. The low-pressure storage chamber is opened to the air through the overboard drain/vent.

The low-pressure storage chamber, related fittings, and components are approved for 150 psi (1,034.21 kPa) with a burst pressure of 300 psi (2,068.43 kPa).

Visual Quantity Indicator

A visual quantity indicator is installed at the end of the bootstrap cylinder on the opposite side of the electrical connector receptacle. The visual quantity indicator is a gauge that shows the hydraulic fluid level in the low-pressure storage chamber with marks from 0 to 100% (2.5% increments). The visual quantity indicator senses the position of the low-pressure piston by the volume of the remaining fluid. The quantity indicator also sends electrical output signals to the RDC, which shows fluid quantity data on the HYDRAULIC synoptic page of the EICAS.

Bleed/Relief Valve

A bleed/relief valve has a manual fluid bleed function and a pressure relief function. Maintenance personnel have access to the bleed/relief valve which is manually operated by a lever. With a reservoir pressure of 55 psi (379.21 kPa), the force of 3 to 12 lb (1.36 to 5.44 kg) is necessary to push the lever to operate the valve. When the lever is pushed, the caught air is released and the hydraulic fluid is bled into the ecology bottle. When released, the lever automatically goes back to the closed position.

---

The ecology bottle is installed in the aft equipment compartment on the forward walkway beam at FS756.25. The ecology bottle collects hydraulic fluid leakage from the auxiliary DCMP shaft seal and the auxiliary reservoir bleed/relief valve.

The ecology bottle is made of environmental stress crack-resistant (ESCR) plastic. The plastic is clear which lets the fluid quantity be examined when the bottle is closed and installed. The bottle includes two inlet tubes that let the hydraulic fluid collect from the auxiliary DCMP shaft seal and reservoir bleed/relief valve. An overflow tube is also installed to an overboard vent. The ecology bottle has a fluid capacity of 24 fl oz (709.76 mL).

---

The switching valve is installed in the aft equipment compartment on the right side of the aft fuselage above the auxiliary rack at FS780.36. The switching valve controls the pressurized hydraulic fluid source to the lower rudder PCU. During flight, the switching valve is in the usual mode and sends pressurized hydraulic fluid from the right hydraulic system to the lower rudder PCU. In the event that the pressure from the right hydraulic system is not sufficient, the switching valve goes to an alternative mode. This causes the hydraulic fluid to be supplied from the auxiliary hydraulic system.

The switching valve includes the parts that follow:

• Dump/relief valve
• Check valve
• Position switching valve

Dump/Relief Valve

A button operated dump/relief valve releases pressure manually from the auxiliary hydraulic system. The dump/relief valve also operates automatically as a pressure relief valve when the system pressure is more than 3,700 psi (25,510.60 kPa). With the system pressure of 3,000 psi (20,684.27 kPa) the maximum force of 20 lb (9.07 kg) is necessary to push the button to make the valve operate. When released, the button automatically goes back to the closed position.

The full flow capacity of the dump/relief valve is 0.30 U.S. gal/min (1.14 L/min) at a maximum differential pressure of 3,950 psi (27,234.29 kPa).

Check Valve

A check valve isolates the reservoir bootstrap line to hold pressure in the intensifier. This lets the dump/ relief valve operate as a thermal protection for the reservoir bootstrap pressure line.

Position Switching Valve

A six-way two-position switching valve with a spring-loaded slider moves from its usual position (to set pressure flow from the right hydraulic system) to an alternative position (to set pressure flow from the auxiliary hydraulic system). The switching valve operation is fully mechanical. Thus, there is no electrical interface to control the valve position.

The spring-loaded slider operates with the auxiliary hydraulic pressure (P1) at 2,500 to 3,350 psi (17,236.89 to 23,097.44 kPa). The slider moves with pressure changes at the right hydraulic system (P2). The spring-loaded slider is in the usual position when the right hydraulic system pressure increases to more than 2,550±50 psi (17,581.63±344.74 kPa). With the spring-loaded slider in the usual position, the hydraulic flow from the right hydraulic system is transmitted to the lower rudder PCU. If the right hydraulic system pressure (P2) decreases to less than 2,350±50 psi (16,202.68±344.74 kPa), the spring-loaded slider moves to the alternative position. This sends the pressurized hydraulic flow from the auxiliary hydraulic system to the lower rudder PCU.

The switching valve has two locked positions which are the usual and alternative modes. Thus, the spool is always in the usual or alternative position. While the right hydraulic system pressure changes, the slider stays in the last position until the locked force is cancelled. This lets the slider move to the opposite direction with no middle position. The switching valve also has a controlled slider rate circuit. This circuit prevents high-pressure transients when the valve moves to release pressure from the right or auxiliary hydraulic system to the lower rudder PCU.

Ports

There are ports for three pressure switches, a pressure transducer, and a temperature transducer. The pressure switch C (auxiliary system DCMP pressure switch) is used to control the intermittent operation of the DCMP. The pressure switch A (lower rudder pressure switch), pressure switch B (accumulator pressure switch), pressure transducer, and temperature transducer supply auxiliary system data to the EICAS.

---

The check valve is installed in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00. The check valve prevents opposite flow from the auxiliary system pressure filter manifold to the auxiliary DCMP. In the free-flow direction, the valve lets the fluid over at a rate of 3.5 U.S. gal/min (13.25 L/min). This occurs at a maximum differential pressure of 5 psi (103.42 kPa) and when the temperature of the fluid is between 20 and 275 °F (–6.67 and 135 °C). An arrow on the check valve case shows the direction of flow and helps in correct installation of the valve.

---

The intensifier is installed in the aft equipment compartment on the auxiliary hydraulic system rack at FS788.00. The intensifier is a spring loaded piston that transmits pressure from the right hydraulic system to the bootstrap port of the auxiliary reservoir.

The bootstrap pressure supplies low-pressure hydraulic fluid to the auxiliary DCMP through the suction pressure line. After the auxiliary DCMP operation starts, the intensifier is pressurized and operates independently of the right hydraulic system. The intensifier has a 3,000 psi (20,684.27 kPa) operation pressure. Proof pressure is 4,500 psi (31,026.41 kPa) and burst pressure is 7,500 psi (51,710.68 kPa). The return spring on the piston is used to set the piston when pressure in the right hydraulic system decreases. This prepares the piston for the subsequent cycle when the engines are stopped and the aircraft is parked.

---

The auxiliary hydraulic system supplies the rudder PCU with secondary hydraulic power during a one-engine flight condition. With a one-engine flight condition, the thrust is not symmetrical which makes it necessary for the pilot to have more yaw control. The auxiliary system gives more hydraulic power to the rudder controls to adjust for this condition.

Auxiliary hydraulic power is supplied by the accumulator, which is held pressurized by the auxiliary DCMP. Auxiliary hydraulic power is isolated from the rudder controls by the switching valve. If the left engine is not in operation, the switching valve opens to let hydraulic pressure from the right system energize the auxiliary rudder PCU. If the right engine is not in operation, the switching valve opens to let the auxiliary system accumulator energize the auxiliary rudder PCU. The auxiliary DCMP mode of operation (off or automatic) is set through the AUX PUMP switch, which is found on the HYDRAULIC control panel. The AUX PUMP switch has two positions which are OFF and AUTO.

In usual operation, the AUX PUMP switch is set to AUTO. When the system pressure decreases to 2,500 psi (17,236.89 kPa), the auxiliary DCMP pressure switch (pressure switch C, installed on the switching valve) closes and sets the auxiliary DCMP on until pressure increases to 3,350 psi (23,097.44 kPa). Then, the pressure switch opens and sets the auxiliary DCMP off. When the AUX PUMP switch is set to OFF it stops the auxiliary DCMP.

The auxiliary DCMP holds its accumulator pressure until the right system pressure decreases to 2,200 psi (15,168.47 kPa) minimum. If the right hydraulic system has a malfunction, a switching valve moves and applies auxiliary hydraulic pressure to the lower rudder. This causes the right system pressure to be isolated from the lower rudder PCU.

If a two generator malfunction occurs, the pilots must manually set the AUX PUMP switch to OFF to keep battery power. If this condition occurs, the AUX PUMP switch is set to AUTO for a minimum of 30 seconds before flaps deployment.

The AUX HYD TEMP HIGH caution message will come on when the temperature is >132.00 °C (>269.60 °F).

The AUX HYD SYS FAIL advisory message will come on when the auxiliary DCMP does not operate when commanded to operate.

The AUX HYD PUMP FAIL ON advisory message will come on when the auxiliary DCMP operates continually when commanded to stop.

The EICAS messages that follow are related to the auxiliary hydraulic system:

EICAS MESSAGE(S)	LEVEL (COLOR)
AUX HYD TEMP HIGH	CAUTION (amber)
AUX HYD SYS FAIL	ADVISORY (cyan)
AUX HYD PUMP FAIL ON	ADVISORY (cyan)

The status of the auxiliary DCMP is shown on the HYDRAULIC synoptic page. The auxiliary DCMP is shown in a circle with the letter P in it. The contour color is usually the same as the adjacent line. The lines color can be one of those that follow:

• Red to show an unserviceable status
• Amber to show a caution
• White to show a usual status with no hydraulic pressure
• Green to show a usual status with hydraulic pressure
• Magenta to show invalid or unknown status.

The auxiliary hydraulic pressure, quantity, and temperature display is shown on the HYDRAULIC synoptic page in color.

The white color displays a correct hydraulic pressure signal >3,500 psi. The green color displays a hydraulic pressure correct >1,800 and ≤3500 psi. The amber color displays a correct hydraulic pressure signal ≤1800 psi.

The white color displays a correct hydraulic fluid quantity signal 85%. The green color displays a correct hydraulic fluid quantity signal between 20 and 85%. The fluid quantity displays as a percentage of full. The percentage display is accurate to ±2% of the reservoir fluid volume. If there is an incorrect hydraulic fluid quantity signal, two magenta color dashes are shown instead of the percentage display. The green color displays that the hydraulic temperature is correct and that the AUX HYD TEMP HIGH caution is not set. The amber color shows that the AUX HYD TEMP HIGH caution is set.

---

The auxiliary hydraulic system has interfaces with the systems/components that follow:

• DC Power Center (DCPC)
• Flap Control Unit (FCU)
• HYDRAULIC Control Panel
• Left and Right Hydraulic Systems
• Quick Disconnect
• Indicating
• Cockpit Circuit Breaker Panel (CCBP)
• Engine Indication and Crew Alerting System (EICAS)
• Remote Data Concentrator (RDC)

  ---

Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A192	AUXILIARY SYSTEM DC MOTOR-DRIVEN PUMP	ZONE(S) 312	29-21-01
-	AUXILIARY SYSTEM PRESSURE FILTER MANIFOLD	ZONE(S) 312	29-21-05
-	AUXILIARY SYSTEM PRESSURE FILTER ELEMENT	ZONE(S) 312	29-21-09
-	ACCUMULATOR	ZONE(S) 312	29-21-13
-	ACCUMULATOR CHARGING-GAUGE MANIFOLD	ZONE(S) 312	29-21-17
-	AUXILIARY SYSTEM RESERVOIR	ZONE(S) 312	29-21-21
-	AUXILIARY SYSTEM ECOLOGY BOTTLE	ZONE(S) 312	29-21-25
-	SWITCHING VALVE	ZONE(S) 312	29-21-29
-	CHECK VALVE	ZONE(S) 312	29-21-33
-	AUXILIARY SYSTEM ECOLOGY BOTTLE	ZONE(S) 312	29-21-37

---