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 Directional Valve
 Figure 022

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 Bleed Valve
 Figure 023

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 (e) Pressure switch (Ref. Fig. 024) The pressure switch indicates to the ECU that hydraulic circuit is pressurized or not. Pressure switch signal is available to the ECU and can be used for maintenance purpose. Electrical characteristics : - open voltage : 5 to 50VDC

 - min open circuit resistance : 0.1 Megohm

 - max closed circuit resistance : 5 ohms

 - current : 2 to 10 mA When the circuit is closed, the pressure switch indicates to the ECU that the hydraulic circuit is pressurized.

 (f) Check valve restrictor A check valve restrictor is installed in the HCU block, adjacent to the flow limiter. This component protects against the possibility of a blockage in the main hydraulic return line leading to an unsafe pressure build-up within the system. A hydraulic line between the check valve restrictor discharge port and the case drain line provides an additional line to the reservoir. If the main return line is clogged, leading to subsequent increase of pressure within the system (as a result of normal leakage at the pressurizing valve) the check valve restrictor will open (at a differential pressure at 200 psid/13.8 bar) and vent the system to the case drain line, thus maintaining the internal system pressure well below the unlock range.

 (3) Filter and Clogging Indicator
 (Ref. Fig. 025, 026)

 (a) Filter
 - General A filter at the HCU inlet is used to filter the fluid supply from the hydraulic system. The filter is a flow through cartridge type filter.

 - Description/operation The unfiltered fluid arrives through A port which communicates with the internal body filter. The fluid passes through the cartridge and gets out through B port.

 - Removal When removing the cartridge, the shutoff diaphragm (2) is pushed against its lower seat by the spring (3), preventing fluid leakage.

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 Pressure Switch
 Figure 024

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 Filter
 Figure 025

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 Clogging Indicator
 Figure 026

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 (b) Clogging indicator

 - General The clogging indicator monitors the pressure loss through the filter cartridge and features a pop-out indicator to signal when it is necessary to replace the filter element.

 - Description/operation The clogging indicator incorporates two spring loaded magnetic pistons (1 and 4) whose magnetic attraction keep the pop-out indicator (5) in the retracted position. The lower magnetic piston (1) monitors the differential pressure between the filtered and unfiltered fluid across the filter element. As the differential pressure increases, the piston compresses the spring (2) and moves away from the upper magnetic piston (4) at a threshold of 11 bar (159.5 psi). At a preset displacement of approximately 2mm, the upper magnetic piston spring overcomes the magnetic face and drives the pop-out indicator from its retracted position.

 (4) Operation Selection of either stow or deploy from the cockpit sends a signal to the engine ECU which, in turn, supplies two independent signals to the thrust reverser HCU pressurizing and directional control valves. These signals to the HCU are only provided if the ECU has received correct signals e.g. reverser position engine power setting. Solenoid conditions are as follows :
 -------------------------------------------------------------------------------Conditions Pressurizing valve Directional solenoid valve solenoid
 -------------------------------------------------------------------------------Forward thrust 0 0 Deploying 1 1 Reverse thrust 0 1 Stowing 1 0
 -------------------------------------------------------------------------------
NOTE : 1 = solenoid energized
____
 0 = solenoid de-energized
　
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