AF Hydraulic and Pneumatic Power Systems

To protect seals from damage when installed over a threaded section, the threaded section should be

coated with a heavy grease.
covered with tape.
covered with a suitable sleeve.

Which of the following is the most commonly used seal to prevent internal and external leakage in both directions of a hydraulic unit?

O-ring.
V-ring.
U-ring.

Which of the following allows fluid to flow unimpeded in one direction but prevents fluid flow in the other direction?

Check valve.
Sequence valve.
Relief valve.

Select the valve used in a hydraulic system that directs pressurized fluid to one end of an actuating cylinder and simultaneously directs return fluid to the reservoir from the other end.

Sequence.
Shuttle.
Selector.

What function does the absolute pressure regulator perform in the pneumatic power system?

Regulates the compressor outlet air pressure to stabilize the system pressure.
Regulates the pneumatic system pressure to protect the moisture separator from internal explosion.
Regulates the compressor inlet air to provide a stabilized source of air for the compressor.

One of the distinguishing characteristics of an open-center selector valve used in a hydraulic system is that

fluid flows through the valve in the OFF position.
fluid flows in three directions in the ON position.
a limited amount of fluid flows in one direction and no fluid flows in the opposite direction.

What type of packings should be used in hydraulic components to be installed in a system containing Skydrol?

AN packings made of natural rubber.
Packing material made for ester-base fluids.
AN packings made of neoprene.

Relief valves are used in pneumatic systems

for one direction flow control.
to reduce the rate of airflow.
as damage-preventing units.

An aircraft pneumatic system, which incorporates an engine-driven multistage reciprocating compressor, also requires

an oil separator.
a surge chamber.
a moisture separator.

The removal of air from an aircraft hydraulic system is generally accomplished

through automatic bleed valves on individual components during system operation.
by operating the various hydraulic components through several cycles.
by allowing the system to remain inoperative for several hours.

Pneumatic systems utilize

thermal valves.
relief valves.
shuttle valves.

The component in the hydraulic system that is used to direct the flow of fluid is the

check valve.
orifice check valve.
selector valve.

What type of selector valve is one of the most commonly used in hydraulic systems to provide for simultaneous flow of fluid into and out of a connected actuating unit?

Four-port, closed-center valve.
Three-port, four-way valve.
Two-port, open-center valve.

What is the purpose of using backup rings with O-rings in hydraulic systems above 1,500 psi?

Prevent internal and external leakage of all moving parts within a hydraulic system.
Provide a seal between two parts of a unit which move in relation to each other.
Prevent high pressure from extruding the seal between the moving and stationary part.

The purpose of the pressure regulator in a hydraulic system is to

maintain system operating pressure within a predetermined range and to unload the pump.
regulate the amount of fluid flow to the actuating cylinders within the system.
prevent failure of components or rupture of hydraulic lines under excessive pressure.

A flexible sealing element subject to motion is a

compound.
packing.
gasket.

Which characteristics apply to aircraft hydraulic systems?

About 80 percent operating efficiency (20 percent loss due to fluid friction).
Minimum maintenance requirements and simple to inspect.
Minimum maintenance requirements, lightweight, and simple to inspect.

If a rigid tube is too short for the flare to reach its seat before tightening, pulling it into place by tightening

is acceptable.
may distort the flare.
may distort the cone.

The installation of a new metal hydraulic line should be made with

a straight tube to withstand the shocks and vibration to which it will be subjected.
a straight tube to permit proper alignment of the fitting and thereby reduce fluid loss through leakage.
enough bends to allow the tube to expand and contract with temperature changes and to absorb vibration.

Extrusion of an O-ring seal is prevented in a high-pressure system by the use of a

backup on the side of the O-ring next to the pressure.
U-ring on the side of the O-ring away from the pressure.
backup ring on the side of the O-ring away from the pressure.

What is one advantage of piston-type hydraulic motors over electric motors?

They are considerably quieter in operation.
There is no fire hazard if the motor is stalled.
They work satisfactorily over a wider temperature range.

Generally, the first step in removing an accumulator from an aircraft is to

relieve system pressure.
discharge the preload.
drain the reservoir.

Refer to Figure 11. The AN flared-tube fitting is referred to in which picture?

1.
2.
3.

Refer to figure 12. Which illustration(s) show(s) the correct spiral for Teflon backup rings?

1 and 2.
3.
1 and 3.

If a hydraulic brake system uses neoprene rubber packing materials, the correct hydraulic fluid to service the system is

mineral-base oil.
synthetic base oil.
phosphate ester-base oil.

The internal resistance of a fluid which tends to prevent it from flowing is called

volatility.
viscosity.
stability.

What is the viscosity of hydraulic fluid?

The increase in volume of a fluid due to temperature change.
The fluid’s ability to resist oxidation and deterioration for long periods.
The internal resistance of a fluid which tends to prevent it from flowing.

Which is a characteristic of petroleum-based hydraulic fluid?

Flammable under normal conditions.
Compatible to natural rubber seals and packings.
Nonflammable under all conditions.

When servicing aircraft hydraulic systems, use the type fluid specified in the

aircraft manufacturer’s maintenance manual.
instruction plate affixed to the unit.
aircraft manufacturer’s maintenance manual or instruction plate affixed to the unit.

Petroleum-based hydraulic fluid is which color?

Purple.
Blue.
Red.

Which of the following is adversely affected by atmospheric humidity if left unprotected?

MIL-H-5606 and Skydrol hydraulic fluid.
MIL-H-5606 hydraulic fluid.
Skydrol hydraulic fluid.

Which is a characteristic of synthetic-based hydraulic fluid?

Low moisture retention.
High flash point.
Low flash point.

Which statement about fluids is correct?

Any fluid will completely fill its container.
All fluids are considered to be highly compressible.
All fluids readily transmit pressure.

Two types of hydraulic fluids currently being used in civil aircraft are

mineral base, and phosphate ester base.
mixed mineral base and phosphate ester base.
petroleum base and mixed mineral base.

Which of the following lists only desirable properties of a good hydraulic fluid that has chemical stability?

High viscosity, low flash point, high fire point.
High flash point, low viscosity, low fire point.
Low viscosity, high flash point, high fire point.

Characteristics of MIL-H-8446 hydraulic fluid are

blue color, phosphate ester base , fire resistant, butyl rubber seals.
light purple color, phosphate ester base, fire resistance, butyl rubber seals.
light green color, phosphate ester base, fire resistance, butyl rubber seals.

Where can information be obtained about the compatibility of fire-resistant hydraulic fluid with aircraft materials?

Fluid manufacturer’s technical bulletins.
Aircraft manufacturer’s specifications.
AC 43.13-1A.

Components containing phosphate ester-base hydraulic fluid may be cleaned with

Stoddard solvent.
Naphtha.
Carbon tetrachloride.

How can the proper hydraulic fluid to be used in an airplane be determined?

By referring to the pilot operating handbook.
By consulting the aircraft’s Type Certificate Data Sheet.
By consulting the aircraft manufacturer’s service manual.

Phosphate ester-base hydraulic fluid is very susceptible to contamination from

rapid changes in temperature.
water in the atmosphere.
icing conditions.

Materials which are Skydrol compatible or resistant include

epoxy paints and most common aircraft metals.
polyurethane and epoxy paints.
common aircraft metals, polyurethane, and epoxy paints.

Characteristics of MIL-H-5606 hydraulic fluid are

light purple color, phosphate ester base, fire resistant, uses butyl rubber seals.
blue color, will burn, natural rubber seals.
red color, petroleum base, will burn, synthetic rubber seals.

Characteristics of MIL-H-7644 hydraulic fluid are

red color, petroleum base, will burn, synthetic rubber seals.
light purple color, phosphate ester base, fire resistant, butyl rubber seals.
blue color, vegetable base, will burn, natural rubber seals.

If an aircraft hydraulic system requires mineral-base hydraulic fluid, but phosphate ester-base hydraulic fluid is used, what will be the effect on the system?

No effect.
System will be contaminated, fluids will not blend, and the seals will fail.
System will be contaminated, fluids will not blend, but there will be no seal problem.

What is used to flush a system normally serviced with MIL-H-5606 hydraulic fluid?

Methyl ethyl ketone or kerosene.
Naphtha or varsol.
Lacquer thinner or trichlorethylene.

The hydraulic component that automatically directs fluid from either the normal source or an emergency source to an actuating cylinder is called a

bypass valve.
shuttle valve.
crossflow valve.

What is commonly used to connect an emergency source of power, and at the same time disconnect the normal hydraulic source from critical parts of a landing gear or wheel braking system for operation (usually when the normal source system fails)?

Selector valve.
Shuttle valve.
Sequence valve.

The primary purpose of a hydraulic actuating unit is to transform

fluid motion into mechanical pressure and back again.
fluid pressure into useful work.
energy from one form to another.

The primary function of the flap overload valve is to

prevent the flaps from being lowered at airspeeds which would impose excessive structural loads.
cause the flap segments located on opposite sides of the aircraft centerline to extend and retract together so that the aircraft will not become aerodynamically unbalanced to the extent that it becomes uncontrollable.
boost normal system pressure to the flaps in order to overcome the air loads acting on the relatively large flap area.

A unit which transforms hydraulic pressure into linear motion is called

an actuating cylinder.
an accumulator.
a hydraulic pump.

If it is necessary to adjust several pressure regulating valves in a hydraulic system, what particular sequence, if any, should be followed?

Units most distant from the hydraulic pump should be adjusted first.
Units with the highest pressure settings are adjusted first.
Units are independent of each other, and therefore, no particular sequence is necessary.

If an aircraft’s constant-pressure hydraulic system cycles more frequently than usual and no fluid leakage can be detected, the most probable cause is

a too high relief valve setting.
pump volume output too high.
low accumulator air preload.

Unloading valves are used with many engine-driven hydraulic pumps to

dampen out pressure surges.
relieve the pump pressure.
relieve system pressure.

What safety device is usually located between the driving unit and hydraulic pump drive shaft?

Thermal relief valve.
Pump motor safety switch.
Pump drive coupling shear section.

Which valve installed in a hydraulic system will have the highest pressure setting?

Pressure regulator valve.
Main relief valve.
Thermal relief valve.

Excluding lines, which components are required to make up a simple hydraulic system?

Actuator, pressure reservoir, accumulator, and selector valve.
Pump, reservoir, selector valve, and actuator.
Pump, reservoir, relief valve, and shuttle valve.

Most variable displacement aircraft hydraulic pumps in use

must be driven at a nearly constant speed in order to be practical for use.
are not practical for use with a closed-center hydraulic system.
contain a built-in means of system pressure regulation.

In a gear-type hydraulic pump,, a mechanical safety device incorporated to protect the pump from overload is the

bypass valve.
check valve.
shear pin.

After installation of a rebuilt hydraulic hand pump, it is found that the handle cannot be moved in the pumping direction (pressure stroke). The most likely cause is an incorrectly installed

hand pump import check valve.
import/outport orifice check valve.
hand pump outport check valve.

Pressure is a term used to indicate the force per unit area. Pressure is usually expressed in

pounds per square inch.
pounds per inch.
pounds per cubic inch.

If two actuating cylinder which have the same cross-sectional area but different lengths of stroke are connected to the same source of hydraulic pressure, they will exert

different amounts of force but will move at the same rate of speed.
equal amounts of force but will move at different rates of speed.
equal amounts of force and will move at the same rate of speed.

Using a hand pump, pressure of 100 psi has been built up in a hydraulic system. The hand pump piston is 1 inch in diameter. A 1/2 inch line connects the hand pump to an actuating cylinder 2 inches in diameter. What is the pressure in the line between the hand pump and the actuator?

100 psi.
150 psi.
200 psi.

Heat exchanger cooling units are required in some aircraft hydraulic systems because of

fluid flammability.
high pressures and high rates of fluid flow.
the high heat generated from braking.

Which is true regarding the ground check of a flap operating mechanism which has just been installed?

If the time required to operate the mechanism increases with successive operations, it indicates the air is being worked out of the system.
If the time required to operate the mechanism decreases with successive operations, it indicates the air is being worked out of the system.
All hydraulic lines and components should be checked for leaks by applying soapy water to all connectors.

A hydraulic system operational check during ground runup of an aircraft indicates that the wing flaps cannot be lowered by using the emergency hand pump. Which is the most likely cause?

The flap selector valve has a severe internal leak.
The pressure accumulator is not supplying pressure to the system.
The fluid level in the reservoir is low.

Many hydraulic reservoirs contain a small quantity of fluid which is not available to the main system pump. This fluid is retained to

prime the main system.
supply fluid to the auxiliary pump.
supply fluid to the pressure accumulator.

The unit in a hydraulic system that requires a certain action to be completed before another action can begin is called a

check valve.
sequence valve.
snubber valve.

The purpose of a hydraulic pressure regulator is to

prevent the system pressure from rising above a predetermined amount due to thermal expansion.
boost the pressure in portions of the system.
relieve the pump of its load when no actuating units are being operated.

Severe kickback of the emergency hydraulic hand pump handle during the normal intake stroke will indicate which of the following?

The hand pump inlet check valve is sticking open.
The main system relief valve is set too high.
The hand pump outlet check valve is sticking open.

What type of valve in an aircraft hydraulic system permits fluid to flow freely in one direction, but restricts the rate at which fluid is allowed to flow in the other direction?

Check valve.
Orifice restriction.
Orifice check valve.

The main system pressure relief valve in a simple hydraulic system equipped with a power control valve should be adjusted

with the power control valve held in the CLOSED position.
while one or more actuating units are in operation.
with the power control valve in the OPEN position.

Which of the following safeguards ensures proper system operation and mitigates damage to non-metallic components of the hydraulic system?

The manual bleed valve should be closed prior to servicing hydraulic fluid to prevent fluid loss as the cap is being removed.
Before assembly of any hydraulic components seals and gaskets should be inspected and replaced only if they show signs of wear or leakage.
When adding fluid to a system, use the type specified in the aircraft manufacturer’s maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced.

A hydraulic accumulator is charged with an air preload of 1,000 psi. When a hydraulic system pressure of 3,000 psi is developed, the pressure on the air side of the accumulator will be

1,000 psi.
3,000 psi.
4,000 psi.

How is the air in a hydraulic accumulator prevented from entering the fluid system?

By forcing the oil/air mixture through a centrifugal separating chamber that prevents the air from leaving the accumulator.
By physically separating the air chamber from the oil chamber with a flexible or moveable separator.
By including a valve that automatically closes when the fluid level lowers to a preset amount.

After a hydraulic accumulator has been installed and air chamber charged, the main system hydraulic pressure gauge will not show a hydraulic pressure reading until

at least one selector valve has been actuated to allow fluid to flow into the fluid side of the accumulator.
the air pressure has become equal to the fluid pressure.
the fluid side of the accumulator has been charged.

Which must be done before adjusting the relief valve of a main hydraulic system incorporating a pressure regulator?

Eliminate the action of the unloading valve.
Adjust all other system relief valves which have a lower pressure setting.
Manually unseat all system check valves to allow unrestricted flow in both directions.

Which seals are used with petroleum-based hydraulic fluids?

Polyester.
Butyl rubber.
Buna-N.

Seats used with a mineral-base hydraulic fluid are identified by which color code?

Green dash.
Blue dot or stripe.
Yellow dot or stripe.

This air that is expended and no longer needed when an actuating unit is operated in a pneumatic system is

exhausted or dumped, usually overboard.
returned to the compressor.
charged or pressurized for use during the next operating cycle.

Some hydraulic systems incorporate a device which is designed to remain open to allow a normal fluid flow in the line, but closed if the fluid flow increases above an established rate This device is generally referred to as a

hydraulic fuse.
flow regulator.
metering check valve.

When hydraulic system pressure control and relief units fail to function properly, how are most systems protected against overpressure?

A shear section on the main hydraulic pump drive shaft.
One or more hydraulic fuses installed in the pressure and return lines.
A shuttle valve interconnecting the main and emergency systems.

A worn hydraulic pump shaft seal can normally be detected by

hydraulic fluid flowing from the pump drain line.
evidence of hydraulic fluid combined in the engine oil.
the presence of hydraulic fluid around the pump mounting pad.

If an engine-driven hydraulic pump of the correct capacity fails to maintain normal system pressure during the operation of a cowl flap actuating unit, the probable cause is

mechanical interference to the movement of the cowl flap.
a partial restriction in the import of the selector valve.
restriction in the pump outlet.

Before removing the filler cap of a pressurized hydraulic reservoir, in order to service the system, you must

relieve the hydraulic system pressure
pressurize all components in the system.
relieve the air pressure.

What happens to the output of a constant-displacement hydraulic pump when the hydraulic system pressure regulator diverts the fluid from the system to the reservoir?

The output pressure remains the same, but the volume reduces.
The output pressure reduces, but the volume remains the same.
The output pressure and volume remain the same.

Hydraulic system accumulators serve which of the following functions?

Dampen pressure surges.
Dampen pressure surges and ensure a continuous supply of fluid to the pump.
Supplement the system pump when demand is beyond the pump’s capacity and store power for limited operation of components if the pump is not operating.

Chattering of the hydraulic pump during operation is an indication

of low accumulator preload.
that the main system relief valve is sticking open.
that air is entering the pump.

Quick-disconnect couplings in hydraulic systems provide a means of

easily replacing hydraulic lines in areas where leaks are common.
quickly connecting and disconnecting hydraulic lines and eliminate the possibility of contaminates entering the system.
quickly connecting and disconnecting hydraulic lines without loss of fluid or entrance of air into the system.

Which seal/material is used with phosphate ester-base hydraulic fluids?

Silicone rubber.
Butyl rubber.
Neooprene rubber.

A hydraulic pump is a constant-displacement type if it

produces an unregulated constant pressure.
produces a continuous positive pressure.
delivers a uniform rate of fluid flow.

A hydraulic motor converts fluid pressure to

linear motion.
notary motion.
angular motion.

A crossflow valve which is designed to bypass fluid from one side of an actuating cylinder to the other side, under certain conditions, may be found in some aircraft installed in the

flap overload system.
engine cowl flap system.
landing gear system.

Hydraulic fluid filtering elements constructed of porous paper are normally

cleaned and reused.
discarded at regular initervals and replaced with new filtering elements.
not approved for use in certificated aircraft..

A pilot reports that when the hydraulic pump is running, the pressure is normal. However, when the pump is stopped, no hydraulic pressure is available. This is an indication of a

leaking selector valve.
low accumulator fluid preload.
leaking accumulator air valve.

If fluid is added to a reservoir in a constant pressure hydraulic system while the system is pressurized, what will result?

Fluid will spray violently out of the reservoir when the filler neck cap is removed.
The fluid level will increase when system pressure is reduced.
Air will be drawn into the system, when the filler neck cap is removed.

In a hydraulic system that has a reservoir pressurized with turbine-engine compressor bleed air, which unit reduces the air pressure between the engine and reservoir?

Relief valve.
Air bleed relief valve.
Air pressure regulator.

What is the main purpose of a pressurized reservoir in a hydraulic system?

Prevent tank collapse at altitude.
Prevent hydraulic pump cavitation.
Prevent hydraulic fluid from foaming.

One of the main advantages of Skydrol is its

wide operating temperature.
high operating pressure.
inability to mix with water.

Hydraulic fluid reservoirs are sometimes designed with a standpipe in one of the outlet ports in order to assure emergency supply of fluid. The outlet port with the standpipe in it furnishes fluid to the

emergency pump when the fluid supply to the normal system has been depleted.
emergency pump at any time it is required.
normal system power pump.

An emergency supply of fluid is often retained in the main hydraulic system reservoir by the use of a standpipe. The supply line is connected to the

inlet of the main hydraulic system.
inlet of the emergency pump.
outlet of the main system pump.

To check the air charge in a hydraulic accumulator.

reduce all hydraulic pressure, then observe the reading on the accumulator air gauge.
observe the first reading on the hydraulic system gauge while operating a component in the system.
read it directly from the auxiliary pressure gauge.

How would the air pressure charge in the accumulator be determined if the engine is inoperative, but the system still has hydraulic pressure?

Read it directly from the main system pressure gauge with all actuators inoperative.
Build up system pressure with the emergency pump and then read the pressure on a gauge attached to the air side of the accumulator.
Operate a hydraulic unit slowly and note the pressure at which a rapid pressure drop beings as it goes toward zero.

How many of these seals are used with petroleum-based hydraulic fluids?

Natural rubber, Ethylene-propylene.
Neoprene, Buna-N.
Natural rubber, Butyl rubber.

Hydraulic system thermal relief valves are set to open at a

lower pressure than the system relief valve.
higher pressure than the system relief valve.
lower pressure than the system pressure regulator.

Chatter in a hydraulic system is caused by

excessive system pressure.
insufficient system pressure.
air in the system.

If hydraulic fluid is released when the air valve core of the accumulator is depressed. It is evidence of

excessive accumulator air pressure.
a leaking check valve.
a ruptured diaphragm or leaking seals.

Although dents in the heel of a bend are not permissible, they are acceptable in the remainder of the hydraulic tube providing they are less than what percent of the tube diameter?

5 percent.
10 percent.
20 percent.

If the hydraulic system pressure is normal while the engine-driven pump is running, but there is no pressure after the engine has been shut off, it indicates

the system relief valve setting is too high.
no air pressure in the accumulator.
the pressure regulator is set too high.

The purpose of restrictors in hydraulic systems is to

control the rate of movement of hydraulically operated mechanisms.
allow the flow of fluid in one direction only.
lower the operating pressure of selected components.

A common cause of slow actuation of hydraulic components is

cold fluid.
restricted orifices.
internal leakage in the actuating unit.

A loud hammering noise in a hydraulic system that has an accumulator usually indicates

air in the fluid.
too much preload in the accumulator.
too low or no preload in the accumulator.

Teflon hose that has developed a permanent set from being exposed to high pressure or temperature should

not be straightened or bent further.
not be reinstalled once removed.
be immediately replaced.

In a typical high-pressure pneumatic system, if the moisture separator does not vent accumulated water when the compressor shuts down, a likely cause is a

saturated chemical dryer.
malfunctioning pressure transmitter.
malfunctioning solenoid dump valve.

AF Hydraulic and Pneumatic Power Systems

AF Hydraulic and Pneumatic Power Systems

Jon K

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