Utility Hydraulic System Overview

A component that measures the temperature of the hydraulic fluid in the reservoir/cooler and transmits an electrical signal to the maintenance panel.

A safety device that controls the operation of the cooler fan based on the temperature of the hydraulic fluid.

The part of the hydraulic system that cools the fluid.

Draws air through the cooler assembly to cool the hydraulic fluid.

A relay that allows 115VAC to the cooler fan when it receives 28VDC from the thermal switch.

The core of the hydraulic system, it stores and releases fluid under pressure.

The 115VAC circuit breaker that controls power to the cooler fan.

The 28VDC circuit breaker that controls power to the thermal switch.

The system provides hydraulic power for critical flight systems, including brakes, steering, cargo operations, and engine starting.

The reservoir stores hydraulic fluid for the system, while the cooler helps regulate temperature preventing overheating.

This accumulator stores pressurized hydraulic fluid used to start the Auxiliary Power Unit (APU).

This unit provides hydraulic power for the APU, while the Utility Pump provides power for the main system.

Part of the main system, it provides hydraulic power in the event of engine failure or for pre-flight operations.

A crucial control unit, it monitors and adjusts hydraulic pressure throughout the system.

This module manages hydraulic fluid return back to the reservoir, ensuring a continuous flow.

Seven key subsystems rely on the utility hydraulic system: brakes, steering, swivel locks, cargo ramp/door, cargo hook, power transfer units, engine starters and APU start.

A type of valve that allows fluid flow to the swivel lock housing and centering cams on both aft landing gear assemblies. It's normally closed and electrically opened using 28 VDC.

A valve designed to prevent backflow of hydraulic fluid between the two solenoid valves in the swivel lock system.

A reservoir that absorbs surges in pressure and helps maintain a constant pressure when the system is under heavy demands. It also provides pressure to keep the swivel locks engaged even if the utility system fails.

This component directs hydraulic fluid to either extend or retract the piston rod, which turns the landing gear for steering.

This potentiometer, part of the piston rod assembly, sends signals to the power steering control box, indicating the position of the steering wheel.

This switch is triggered when the steering wheel exceeds certain turning angles, helping prevent over-steering and damage to the landing gear.

Hydraulic pressure is applied to the upper port of the actuator, engaging the swivel lock, which locks the landing gear in place.

Hydraulic pressure is shifted from the upper port to the lower port, disengaging the swivel lock, allowing the landing gear to turn.

The filter bowl assembly separates hydraulic fluid into three sections: the filter bowl which holds the 5-micron filter, the visual contamination indicator alarm, and the electrical transmitter for filter change indication. This assembly is essential for maintaining clean hydraulic fluid and preventing contamination.

The DPI button extends when the pressure differential exceeds 75 PSID + 10, indicating potential filter blockage. At 130 PSID, the filter bypasses, allowing fluid flow despite the blockage. The bypass is deactivated when the pressure differential drops below 115 PSID, ensuring the filter is back in operation.

The differential pressure switch, mounted on the filter bowl, alerts the maintenance panel when the DPI button extends, indicating a potential problem with the filter.

The utility return module has two indicators: one for the APU motor/pump and one for the utility boost pump. These indicators extend when flow rate through case drains exceeds 1.0 GPM +0.25 GPM, with 22 PSI differential pressure. This signals a potential pump failure.

The ramp transfer cylinder supports ramp operation by compensating for the increased fluid flow needed by the ramp actuating cylinders. This ensures smooth ramp operation.

The utility hydraulic boost pump, mounted on the aft transmission, provides a constant pressure of 3000 PSI with a variable flow rate up to 16 GPM, supplying hydraulic fluid to the system. It operates similarly to the flight control hydraulic system pumps.

This port supplies pressurized hydraulic fluid (60 PSI) from the reservoir/cooler to the utility hydraulic boost pump.

The brake master cylinder acts as a pump for the hydraulic system, creating a pressurized fluid that applies braking force to the wheel assemblies. Two independent master cylinders, one for each side, enable individual braking control for the pilot and co-pilot.

The master cylinder has three ports, allowing the flow of pressurized hydraulic fluid: The pressure port receives high pressure, the brake pressure port releases a variable amount of pressure according to brake pedal input, and the return port allows fluid back into the system when brakes are released.

When the brake pedal is pressed, a check valve opens, allowing 1390 PSI into the master cylinder. This prevents the pressurized fluid from flowing into the return system during braking.

While applying the brakes, the cylinder rod compresses and activates a valve, sending high pressure to a reservoir. This pressure is then transmitted to the wheel assemblies to apply braking force.

When the brake pedal is released, the springs in the master cylinder decompress, allowing fluid to return back to the system through the return port. This action removes the pressure from the wheels, releasing the brakes.

The transfer valves are located behind the console and are separate for the left and right sides. These valves ensure that only one master cylinder applies pressure to a specific side, preventing over-pressurization.

The parking brake valve is a mechanical system that engages the brakes using a lever or button. This allows the aircraft to be securely parked without reliance on the hydraulic system.

The braking system relies on hydraulic pressure for efficient control. The master cylinder, transfer valves, and parking brake valve work together to manage this pressure, ensuring safe and effective braking operation.

A valve that controls the flow of hydraulic fluid to the ramp actuating cylinders, determining whether the ramp is lowered (DN), raised (UP), or locked in position (STOP).

The cylinders responsible for the up and down movement of the ramp. They contain a locking mechanism that prevents the ramp from moving when fully extended.

The topmost port on the ramp actuating cylinder, allowing pressurized fluid to unlock the ramp's locking mechanism.

The second port from the top on the ramp actuating cylinder, allowing return fluid to enter from the ramp control valve when lowering the ramp.

The position of the ramp control valve where the ramp is fully extended and locked in place.

The position of the ramp control valve that allows pressurized fluid to the actuating cylinders, unlocking the ramp and allowing it to descend.

The position of the ramp control valve that routes pressurized fluid to the actuating cylinders, raising the ramp.