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Hydraulic and Pneumatic Power Systems
Terms in this set (22)
What are the two basic types of hydraulic fluid that are used in modern aircraft?
Mineral base fluid and phosphate ester base fluid.
What kind of filter is a micronic filter?
A filter with a special paper element.
Does the main hydraulic pump take its fluid from the bottom of the reservoir, or from a standpipe?
The main pump normally takes its fluid from a standpipe, while the emergency pump takes its fluid from the bottom of the reservoir. If a break in the system should allow the main pump to pump all of its fluid overboard, there will still be enough fluid in the reservoir to allow the emergency system to extend the landing gear and actuate the brakes.
Why are some hydraulic reservoirs pressurized?
Pressurization ensures that fluid will be supplied to the inlet of the pumps at high altitude where there is not enough atmospheric pressure to do this.
What is a double-action pump?
A pump that delivers fluid with the movement of the pump handle in both directions.
Why do most engine-driven hydraulic pumps have a shear section in their drive couplings?
If the pump should seize, the shear section will break, disconnecting the pump from the engine and preventing further damage.
What does an unloading valve do in a hydraulic system?
The unloading valve, or pressure regulator, controls system pressure by shifting the pump outlet fluid from the pressurized system back into the reservoir when the system pressure is high enough. The fluid circulates with very little load on the pump until the system pressure drops to the regulator kick-in value. The pump then forces fluid into the system until the pressure builds back up to the regulator kick-out value.
What is the purpose of an accumulator in an aircraft hydraulic system?
The accumulator holds pressure on the hydraulic fluid in the system. The pressure is held by compressed air or nitrogen acting on the fluid through a bladder, a diaphragm, or a piston.
What is the purpose of an orifice check valve in an aircraft hydraulic system?
An orifice check valve allows full flow of fluid in one direction through the valve, but restricts the flow in the opposite direction.
Where are line-disconnect fittings normally located in an aircraft hydraulic system?
Normally in the lines that connect the engine-driven pump to the aircraft hydraulic system.
What is meant by a single-action hydraulic actuating cylinder?
A linear actuating cylinder that uses hydraulic fluid under pressure to move the piston in only one direction. The piston is returned by a spring.
What is the source of the compressed air that is used in a medium-pressure pneumatic system on a turbine-engine powered aircraft?
This air is normally bled from one of the stages of the engine compressor.
What kind of device is used to control the speed of movement of the piston in a pneumatic actuator?
A variable orifice.
Why do most high-pressure pneumatic systems include a moisture separator?
When the pressure of the stored air is reduced to the value that is needed in the system, the temperature drops enough to freeze any water that is in the air. The moisture separators remove this water before it can freeze and block the system.
What is the difference between an open-center selector valve and a closed-center selector valve?
Open-center selector valves are installed in series with each other, and the hydraulic fluid flows through their center when no component is being actuated. The valves act as a pump unloading valve. Closed-center selector valves are installed in parallel with each other. They direct fluid under pressure to one side of the actuator, and fluid from the other side of the actuator to the system return manifold. In their off position, they trap fluid in the lines between the valve and the actuator.
What is used to flush a hydraulic system that uses Skydrol hydraulic fluid?
What is used to flush a hydraulic system that uses mineral base hydraulic fluid?
Naphtha, varsol or Stoddard solvent.
Where can you find the type of hydraulic fluid that is required for a particular aircraft?
In the maintenance manual for the aircraft. This information is also on a placard on the system reservoir.
What is used to remove phosphate-ester base hydraulic fluid from aircraft tires?
Soap and water.
What must be done to the lines that are disconnected when servicing an aircraft hydraulic system?
They must be capped with the correct fluid line cap or plug. Masking tape or other types of adhesive tape should never be used.
What are two ways aircraft hydraulic reservoirs may be pressurized?
By an aspirator in the fluid return line or by bleed air from one of the engine compressors.
Why do some hydraulic pressure gages have a snubber installed between them and the hydraulic pump?
The snubber keeps the gage from fluctuating.
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