This document summarizes key concepts related to aircraft hydraulic and pneumatic power systems. It discusses the history and definition of hydraulics, and describes common functions of aircraft hydraulic systems such as operating landing gear, flaps, brakes and flight controls. The document then covers important hydraulic concepts and formulas including Pascal's Law, mechanical advantage, and relationships between pressure, force, area, distance and volume. It also discusses hydraulic fluids used in aircraft, including vegetable-based, mineral-based and synthetic fluids, as well as considerations for fluid selection including viscosity, stability, flash point and compatibility with aircraft materials.

1. Chapter IX
Hydraulic and Pneumatic Power Systems
The word hydraulics is based on the Greek word for
water, and originally meant the study of physical
behavior of water at rest and in motion. Today, the
meaning has been expanded to include the physical
behavior of all liquids including hydraulic fluids.
A. Aircraft Hydraulic Systems
Hydraulic systems are not new to aviation. Some
early aircraft used hydraulic brake systems. As
aircraft became more sophisticated, newer systems
with greater complexity were developed.
Although some aircraft manufacturers make
greater use of hydraulic systems than others, the
hydraulic system of the average modern aircraft
performs many functions. Among the units com-
monly operated by hydraulic systems are landing
gear, wing flaps, speed and wheel brakes, and flight
control surfaces.
Hydraulic systems have many advantages as a
power source for operating various aircraft units.
They combine the advantages of light weight, ease of
installation, simplification of inspection, and mini-
mum maintenance requirements. Hydraulic opera-
tions are almost 100% efficient, with only a negligible
loss due to fluid friction.
Aircraft hydraulic systems belong to that branch
of physics concerned with fluid power/mechanics.
They do their work by moving fluid, and the fluid
they use is incompressible. Pneumatic systems work
in much the same way, obeying many of the same
laws, but the fluid they use (air) is compressible.
To better understand how a hydraulic system
accomplishes its task, a brief review of the physics
involved is necessary.
1. Pascal's Law
This is the basic law we use when we think of
transmitting power by a hydraulic system. The
French mathematician Blaise Pascal observed that
any increase in the pressure on a confined liquid was
transmitted equally and undiminished to all parts of
the container, and acts at right angles to the enclos-
ing walls of the container. This means simply that if
we have an enclosed vessel full of liquid, and we
apply a force to a piston in the vessel to raise the
pressure, this increase in pressure will be the same
anywhere in the system. Each of the gauges attached
to the container shown in figure 9-1 will have the
same reading.
2. The Hydrostatic Paradox
The pressure produced by a column of liquid is directly
proportional to its density and the height of the
column, and in no way depends upon the shape of the
container or the amount of liquid the container holds.
For example, 1 cu. in. of water weighs 0.036 lb. A tube
that is 231" tall with a cross section of 1 sq. in. will
hold 1 gal. of water (1 gal. = 231 cu. in.). If the tube is
Figure 9-1. Pressure exerted on a fluid in an enclosed
container is transmitted equally and
undiminished to all parts of the container
and acts at right angles to the enclosing
walls.
Figure 9-2. The pressure exerted by a column of liquid
is determined by the height of the column
and is independent of its volume.
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2. FORCE AREA x PRESSURE
AREA FORCE / PRESSURE
PRESSURE FORCE / AREA
(A)
(B)
(C)
standing straight up, the 1 gal. of water will exert a
pressure of 8.32 PSI at the bottom of the tube.
If the tube were 231" high and had an area of 100
sq. in., it would hold 100 gal. of water, but the
pressure at the bottom would still be 8.32 PSI. The
force exerted by the column of water is, of course,
equal to the pressure acting on each square inch
times the number of square inches, or 832 lbs.
It makes no difference as to the shape or size of
the vessel that contains the liquid; it is the height of
the column that is the critical factor. In figure 9-3,
the pressure (P) read by the gauges will be the same
in all four instances, since the height (H) is the same.
Naturally, all of the vessels must be filled with the
same liquid.
3. Relationship Between Pressure, Force,
and Area
Pressure is a measure of the amount of force that
acts on a unit of area. In most American hydraulic
systems, pressure is measured in pounds per square
inch (PSI).
The relationship between force, pressure, and
area may be expressed by the formula:
Force = Pressure x Area
This may be visualized by looking at figure 9-4.
The bottom half represents the product of the area
in square inches and the pressure in PSI. This gives
us the amount of force in pounds, which is repre-
sented by the top half of the circle.
In order to find pressure, divide the force by the
area:
Pressure =Force
Area
In order to find the area, divide the force by the
pressure:
Area = Force
Pressure
4. Relationship Between Area, Distance,
and Volume
Another relationship in hydraulics is between the
area of the piston, the distance it moves, and the
volume of the fluid displaced. We can visualize this
relationship in figure 9-5. One half of the circle
represents the volume in cubic inches, and the other
half of the circle the area in square inches and the
distance the piston moves in inches. Distance is also
known as stroke.
The relationship between volume, area, and dis-
tance may be expressed by the formula:
Volume = Area x Distance
To find the area divide the volume by the distance:
Area =
Volume
Distance
To find the distance divide the volume by the area:
Distance =
Volume
Area
Figure 9-3. Neither the shape nor the volume of a
container affects the pressure.
Figure 9-4. Relationship between area, pressure, and force.
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3. A
AREA DISTANCE
V
VOLUME
VOLUME = AREA x DISTANCE AREA = VOLUME / DISTANCE DISTANCE = VOLUME / AREA
(B) (C)
F 1#
n W = 20#
AREA
20 SO. INCH
llllllll
AREA
. 1 SO. INCH
D 1 INCH
-1D = 1/20 INCH
Figure 9-5. Relationship between volume, area, and distance.
5. Mechanical Advantage in a Hydraulic
System
A hydraulic system has two major advantages over
other types of mechanical systems. One is the ease
with which force can be transmitted over large dis-
tances and into and out of sealed compartments. The
other is the mechanical advantage made possible by
varying the size of pistons.
In figure 9-6, we see the way mechanical ad-
vantage is achieved in a hydraulic system. If we have
a piston whose area is 1 sq. in. pressing down with
a force of 1 lb., it will produce a pressure of 1 PSI,
and for every inch it moves, will displace 1 cu. in. of
fluid.
If the cylinder containing this piston is connected
to one having a piston with an area of 20 sq. in.,
every square inch will be acted on by the same 1 PSI
pressure, and a force of 20 lbs. will be produced. The
1 cu. in. of fluid displaced when the small piston
Figure 9-6. The product of the force times the area of
the large piston is equal to the product of
the weight times the area of the small
piston.
moves down 1 in. spreads out under all 20 sq. in. of
the large piston, and will move up only 1/2o".
This may be expressed as:
A (small) x D (small) = A (large) x D (large)
1 x 1 = 20 x 1/2o
1 = 1
All hydraulic systems are essentially the same,
whatever their function. Regardless of application,
each hydraulic system has a minimum number of
components, and some type of hydraulic fluid.
B. Hydraulic Fluid
While we may not normally think of fluid as being a
component, the fluid used in aircraft hydraulic sys-
tems is most important. This fluid must flow with a
minimum of opposition, and be incompressible. It
must have good lubricating properties to prevent
wear in the pump and valves. It must inhibit cor-
rosion and not chemically attack seals used in the
system. And it must not foam in operation, because
air carried into the components will give them a
spongy action.
Manufacturers of hydraulic devices specify the
type of fluid best suited for use with their equipment.
Working conditions, service, temperatures, pres-
sures, possibilities of corrosion, and other condi-
tions must be considered. Some of the
characteristics that must be considered when select-
ing a satisfactory fluid for a particular system are
discussed in the following paragraphs.
1. Viscosity
One of the most important properties of any
hydraulic fluid is its viscosity. Viscosity is a
measure of internal resistance to flow. A liquid
such as gasoline flows easily (has a low viscosity)
while a liquid such as tar flows slowly (has a high
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4. HEATING
UNIT LIQUID
BATH
THERMOMETER
CORKCONTAINER
60
c.c.
OIL
n D. . •
4/ Ar
RESERVOIR
viscosity). Viscosity increases as temperature
decreases.
The viscosity of a liquid is measured with a vis-
cosimeter. There are several types, but the instrument
most often used is the Saybolt universal viscosimeter
(figure 9-7). This instrument measures the number of
seconds it takes for a fixed quantity of liquid (60 cc) to
flow through a small orifice of standard length and
diameter at a specific temperature. This time of flow is
measured in seconds, and the viscosity reading ex-
pressed as SSU (seconds, Saybolt universal).
2. Chemical Stability
Chemical stability is another property which is impor-
tant in selecting a hydraulic fluid. It is the ability of the
liquid to resist oxidation and deterioration for long
periods. Mostl liquids tend to undergo unfavorable
chemical changes during severe operating conditions.
This is the case when a system operates for a consid-
erable period of time at high temperatures.
Excessive temperatures have an adverse effect on
the life of a liquid. The temperature of the liquid in
the reservoir of an operating hydraulic system does
not always represent a true state of operating con-
ditions. Localized hot spots occur on bearings, gear
teeth, or at the point where liquid under pressure is
forced through a small orifice. Continuous passage
of a liquid through these points may produce local
temperatures high enough to carbonize or sludge the
liquid, yet the liquid in the reservoir may not indicate
an excessively high temperature. Liquids with a high
viscosity have a greater resistance to heat than light
Figure 9-7. Saybolt viscosimeter.
or low viscosity liquids which have been derived from
the same source. Fortunately, there is a wide choice
of liquids available for use within the viscosity range
required of hydraulic systems.
Liquids may break down if exposed to air, water,
salt, or other impurities, especially if in constant
motion or subject to heat. Some metals, such as zinc,
lead, brass, and copper have an undesirable chemi-
cal reaction on certain liquids.
These chemical processes result in the formation
of sludge, gums, carbon or other deposits which clog
openings, cause valves and pistons to stick or leak,
and give poor lubrication to moving parts. As soon
as small amounts of sludge or other deposits are
formed, their rate of formation generally increases.
As they are formed, certain changes in the physical
and chemical properties of the liquid take place. The
liquid usually becomes darker in color, higher in
viscosity, and acids are formed.
Flash Point
Flash point is the temperature at which a substance
gives off vapor in sufficient quantity to ignite
momentarily (flash) when a flame is applied. A high
flash point is desirable for hydraulic fluids because
it indicates a good resistance to combustion and a
low degree of evaporation at normal temperatures.
Fire Point
Fire point is the temperature at which a substance
gives off vapor in sufficient quantity to ignite and
continue to burn when exposed to a spark or flame.
Like flash point, a high fire point is required of
desirable hydraulic fluids.
5. Types of Hydraulic Fluid
To assure proper system operation and to avoid
damage to nonmetallic components of the hydraulic
system, the correct fluid must be used.
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. There are three types of hydraulic fluids
currently being used in civil aircraft.
a. Vegetable-base Fluid
MIL-H-7644 fluid has been used in the past when
hydraulic system requirements were not as severe
as they are today. This fluid is essentially castor oil
and alcohol. Although it is similar to automotive
brake fluid it is not interchangeable. This fluid is
used primarily in older type aircraft. It is dyed blue
for identification. Natural rubber seals are used with
vegetable base fluid. If this system is contaminated
with petroleum base or phosphate ester base fluids,
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5. the seals will swell, break down and block the sys-
tem. The system may be flushed with alcohol. This
type of fluid is flammable.
Mineral-base Fluid
MIL-H-5606 is the most widely used hydraulic fluid
in general aviation aircraft today. It is basically a
kerosene-type petroleum product, having good
lubricating properties and additives to inhibit foam-
ing and prevent corrosion. It is quite stable chemi-
cally and has very little viscosity change with
temperature. MIL-H-5606 fluid is dyed red for iden-
tification, and systems using this fluid may be
flushed with naphtha, varsol, or Stoddard solvent.
Neoprene seals and hoses may be used with MIL-H-
5606 fluid. This type of fluid is flammable.
Synthetic Fluid
Non-petroleum base hydraulic fluids were intro-
duced in 1948 to provide a fire-resistant hydraulic
fluid for use in high performance piston engine and
turbine powered aircraft.
The most commonly used fluid of this type is
MIL-H-8446 or, Skydrol® (a registered trade name
of the Monsanto Chemical Co.). This fluid is colored
light purple, is slightly heavier than water, and has
a wide range of operating temperatures from around
-65°F to over 225°F for sustained operation. Cur-
rently there are two grades of Skydrol in use, Skydrol
500B4, and Skydrol LD. Skydrol LD has a lower
density and offers some advantage in jumbo jet
transport aircraft where weight is a prime factor.
Skydrol is not without its problems however, as it
is quite susceptible to contamination by water from
the atmosphere and must be kept tightly sealed.
When servicing a system using Skydrol, be extreme-
ly careful to use only seals and hoses having the
proper part number. Skydrol systems may be
flushed out with trichlorethylene.
Intermixing of Fluids
Due to the difference in composition, vegetable base,
petroleum base and phosphate ester base fluids will
not mix. Neither are the type of seals for any one fluid
usable with or tolerant of any of the other fluids.
Should an aircraft hydraulic system be serviced with
the wrong type of fluid, immediately drain and flush
the system and maintain the seals according to the
manufacturer's specifications.
Compatibility with Aircraft Materials
Aircraft hydraulic systems designed for Skydrol
fluids should be virtually trouble-free if properly
serviced. Skydrol does not appreciably affect com-
mon aircraft metals as long as the fluid is kept free
of contamination.
Due to the phosphate ester base of synthetic
hydraulic fluids, thermoplastic resins, including
vinyl compositions, nitrocellulose lacquers, oil base
paints, linoleum and asphalt may be softened
chemically by these fluids. Skydrol will attack
polyvinyl chloride, and must not be allowed to drip
on to electrical wiring, as it will break down the
insulation. However, this chemical reaction usually
requires longer than just momentary exposure; and
spills that are wiped up with soap and water do not
harm most of these materials.
Skydrol is compatible with natural fibers and with
a number of synthetics, including nylon and
polyester, which are used extensively in many
aircraft.
Petroleum oil hydraulic seals of neoprene or
Buna-N are not compatible with Skydrol and must
be replaced with seals of butyl rubber or ethylene-
propylene elastomers for units that are intended for
use in systems utilizing phosphate ester base
hydraulic fluid. These seals are readily available
from suppliers.
8. Health and Handling
Skydrol fluid does not present any particular health
hazard in its recommended use. Skydrol has a very
low order of toxicity when taken orally or applied to
the skin in liquid form. It causes pain on contact
with eye tissue, but animal studies and human
experience indicate that it causes no permanent
damage. First aid treatment for eye contact includes
flushing the eyes immediately with large volumes of
water and the application of an anesthetic eye solu-
tion. If pain persists, the individual should be
referred to a physician.
In mist or fog form, Skydrol is quite irritating to
nasal or respiratory passages and generally
produces coughing and sneezing. Such irritation
does not persist following cessation of exposure.
Silicone ointments, rubber gloves, and careful
washing procedures should be utilized to avoid ex-
cessive repeated contact with Skydrol in order to
avoid solvent effect on skin.
C. Basic Hydraulic Systems
A hydraulic system is much like an electrical system.
It must have a source of power, a means of transmit-
ting this power, and finally some type of device to
use the power.
1. Open Hydraulic Systems
The most basic form of an open hydraulic system is
that used by hydroelectric power plants. Large dams
block streams of water to form lakes that store
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