The Main Reservoir
The pressurized main hydraulic reservoir supplies fluid to the electric pump. The main reservoir is pressurized by the emergency accumulator circuit and backed up by a spring-loaded piston within the reservoir to prevent pump cavitation. Reservoir fluid is monitored for overheating by a temperature probe located within the reservoir. Reservoir quantity is indicated by a mechanical quantity indicator on the reservoir and by the HYD QTY indicator in the cockpit. High temperature triggers a master caution alert with flashing amber HYDR on the CWP. The mechanical quantity indicator displays main reservoir quantity on two scales: PRESS and DEPRESS. The PRESS scale is accurate when main accumulator pressure displayed on the hydraulic panel is within the green arc. Reservoir quantity is displayed on the DEPRESS scale when the hydraulic system is fully depressurized.
The Hand Pump Reservoir
The unpressurized hand pump reservoir supplies fluid only to the hand pump. Overflow fluid from the main reservoir is routed to the hand pump reservoir. Reservoir quantity is determined by a sight glass located on the left side of the nose wheel well.
The Electric Hyd Pump
The electric hydraulic pump is the normal pressure source for all hydraulically operated systems. Pump operation is automatic or manually controlled according to the selected mode on the HYD PUMP switch. Pump control power is supplied from the Left Main Bus in automatic mode and from the Left BAT Bus (modified a/c) in override mode. (L ESS buss in unmod a/c.) The Left Battery Bus supplies override control power in later aircraft. The pump is automatically activated when:
- Pressure in the main accumulator is <2100 psi. The pump runs until the pressure increases to 2900 psi
- The landing gear is operated. Pump operation continues until the left main gear downlocks or both main gear uplock
Hand Pump and Selector Valve
The hand pump and selector valve are located in the cockpit floor on the right side of the center pedestal. The valve facilitates selection of individual the accumulators. Hand pump operation requires electrical control power from the Left Battery Bus to move the valve that directs fluid flow for operation of the landing gear and flaps. When extending or retracting the flaps, the FLAP handle must first be set to the desired setting before using the hand pump. This allows the electrical flap control unit to open control valves in order to operate the flaps. The flaps move about 1 degree per pump. When extending the landing gear, the gear lever must first be selected DOWN. This action operates the electrical gear control valve so that extend pressure is directed to the gear actuators. It takes about 30 pumps to get the gear down.
Four identical nitrogen-charged accumulators, located in the right side of the nose wheel well, absorb pump pulsations and surges caused by large fluid demands. Each accumulator incorporates a floating piston which separates the fluid from the nitrogen precharge. Nitrogen is installed (precharged) in each accumulator at approximately 1650 psi. Pressure gages visible from within the nose wheel well are installed on each accumulator for preflight checks. The displayed pressure is the sum of precharge pressure plus the pressure developed by a pump, if any. If the electric pump fails and accumulator pressure subsequently decreases to 1650 psi, the hydraulic pressure in that accumulator circuit decreases to zero as the floating piston reaches its full travel limit. The pressure in each accumulator circuit is also indicated on the related pressure indicator on the hydraulic panel. Low pressure (<1850 psi) in the main or emergency accumulators triggers master caution alerting with flashing amber HYDR on the CWP. The emergency and brake accumulator circuits are isolated by check valves so that each circuit only supplies the following:
• Each brake accumulator supplies the respective wheel brake pair: outboard or inboard
• The emergency accumulator supplies the emergency gear extension system for uplock release
The main accumulator does not have a check valve and is able to supply the flaps, landing gear, both brake systems and nosewheel steering.
There are two filters located on the left side of the nose wheel well to remove fluid contaminations from the pressure and return fluid lines. Filter housings incorporate a clog indicator button which pops up when the related filter becomes clogged. Glass viewer in the wheel well enable visual inspection of the filter clog indicator buttons.