nuts & bolts
building basics
Understanding Flexible Hose
A hose shop can help, but making your own isn’t dif ficult
R I C HARD K OEHL ER
S
ome of the most important parts of that aircraft
you are building are the fluid lines. On even the
simplest planes, fuel and hydraulic fluid are transported in rigid lines and flexible hoses. You get to not
only install (and maybe even make) these during the
building process, but also maintain them for the life of
the aircraft.
Fluid lines in the engine compartment must meet all
the requirements of lines installed in the airframe, and in
addition, many of them must also be protected from heat
with fire sleeves. In this article we will look at flexible
hose. Rigid tubing was covered in a previous article.
Flexible lines are used in aircraft locations where a rigid
line must be connected to a component that has motion
Bonnie Bartel-Kratz
Flexible lines are used in aircraft locations where a rigid line must be connected
to a component that has motion relative to the aircraft structure.
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relative to the aircraft structure. They
are also commonly used where a line
must be frequently connected and
disconnected.
A flexible hose may be used in any
part of an aircraft fluid system where
the aircraft manufacturer (homebuilder) has proven that it meets all
of the FAA requirements. Not only
must it pass the required fluid flow
rate, but also it must do it without
any excessive pressure drop. It must
also be able to carry the required
system pressure and withstand the
vibration and heat in the location of
its use in the aircraft.
Flexible hose is sized by comparing
its inside diameter (ID) to the equivalent rigid tube ID and then measuring the outside diameter (OD) of the
tube in 1/16-inch increments. It is
all a matter of equivalent flow characteristics. For example, a –6 ID hose
has flow characteristics equivalent to
a piece of –6 or 3/8-inch (6/16-inch)
OD rigid tubing. The ID of the hose is
about the same as the ID of the tube.
It is smaller than the OD of the tube
by twice the wall thickness of the
tube. The OD of the hose is way bigger than the equivalent tube OD and
can vary depending on the material
the hose is constructed from. Therefore, the OD of the hose is never used
for determining its size.
Flexible hose has a linear stripe,
called a lay line, running along its
length. The lay line helps ensure you
do not twist the hose during installation. If the line spirals around the
hose, the hose has been twisted,
which seriously degrades its ability to
withstand pressure.
There are four general divisions
of hose separated by the amount of
pressure they can operate at. These
divisions are 1) low-pressure hose
that has a maximum working pressure of about 300 pounds per square
inch (psi), 2) medium-pressure hose
that operates up to about 1,500 psi,
3) high-pressure hose that is suitable
for working pressures between 1,500
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Photos by Bonnie Bartel-Kratz
building basics
MIL-H-6000 hose has two layers of braid and is more resistant to
degredation from ozone in the atmosphere.
This low-pressure hose has a seamless inner liner reinforced with
a stainless steel inner braid and a full-coverage stainless steel
outer braid.
Flexible lines are used in aircraft
locations where a rigid line must be
connected to a component that has
motion relative to the aircraft structure.
If the hose has a 45-degree or 90-degree fitting on an end,
measure from the center of the flare cone, parallel to the hose
and not around the corner.
and 3,000 psi, and 4) extra-high-pressure hose for working pressures of 3,000 to 6,000 psi. Note that many aircraft parts supply catalogs list “burst” pressure, which is
usually about two to four times higher than “working”
pressure. Homebuilders usually need only low- or medium-pressure hose, so I won’t bore you with the high and
extra-high stuff. If you have particular questions, check
with your local EAA technical counselor.
Low-pressure hose conforming to MIL-H-5593 specifications is made up of a synthetic rubber liner, cotton
braid, and a synthetic rubber outer cover. It is most commonly used in homebuilts for pitot-static systems and
vacuum systems. It usually has a yellow lay line and the
letters LP along with the manufacturer’s code. Aeroquip
306 and Stratoflex 193 are typical examples of low-pressure hose. Also, low-pressure first cousins are MIL-H-6000
hose and Stratoflex 160 hose. These have two layers of
braid and are more resistant to ozone attack, but they are
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otherwise similar to –5593. A typical price for low-pressure hose is about $3 to $5 per foot, depending on size
(bigger costs more). These hoses are typically good up to
about 250°F.
Another type of low-pressure hose is covered with
stainless steel braid. It has a seamless rubber inner liner
and is reinforced with a stainless steel inner braid and
a full-coverage stainless steel outer braid. It looks good
but costs about three times as much as typical low-pressure rubber hose. Aeroquip 601 is an example of this type.
This hose is good up to about 300°F.
Medium-pressure rubber hose is usually typified by
MIL-H-8794 specifications, having a seamless synthetic
rubber inner liner, synthetic rubber-impregnated cotton
braid reinforcement, and steel wire braid reinforcement.
All of this is encased in a rough synthetic rubber-impregnated cotton braid. So, the easy way to spot the difference between low- and medium-pressure rubber hose is
that the medium-pressure stuff has a
rough exterior and is magnetic due to
the steel reinforcement.
Typical brands of medium-pressure
hose are Aeroquip 303 and Stratoflex
111. My Mooney came from the factory with Stratoflex 111 hose in the
engine compartment. Medium-pressure rubber hose typically costs about
a dollar more per foot than low-pressure hose, but being rubber it is good
only up to about 250°F.
Another type of medium-pressure
hose is the Teflon type. Tetrafluoroethylene, or TFE, or Teflon, is a material used for the inner liner of hoses
that can carry fluids at temperatures
up to 400°F. Teflon is chemically
inert, and it maintains its strength
at these high temperatures. Mediumpressure Teflon hose has a seamless
Teflon inner liner covered with several layers of spiral-wound stainless
steel wire and one or more layers of
stainless steel braid.
The amount of reinforcement varies with the size of the hose. Teflon
hose is unaffected by any fuel, petroleum or synthetic-base oil, alcohol,
coolant, or solvent commonly used
in aircraft. It is highly resistant to
vibration and fatigue, but one problem is its characteristic of taking a
permanent set after it has been in
service. If such a hose is temporarily
removed from the aircraft, it must
not be straightened out. For this reason, if the hose application involves
large amounts of movement, such as
on a landing gear actuator, it is often
not advisable to use Teflon hose.
Typical examples of medium-pressure Teflon hose are Aeroquip 666
and Stratoflex 156. Again, the Teflon
hose will be two to three times more
expensive than the equivalent rubber
type.
Now we’ll discuss the fittings used
to make a hose and how to get the
correct length. Flexible hoses should
be built approximately 5 percent
to 8 percent longer than the distance between the fittings. The slack
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building basics
To determine the length of the hoses, I usually mock
them up using coat hangar wire. I run it between the
points I want to connect and trim it to the exact length.
It will hold the required bends and can show quickly if
you are trying to use an excessively small bend radius.
allows for contraction of the line as
it expands its diameter and shortens
its length when it is pressurized. The
length of a hose is the length from
the center of the flare cone of the
connector to the same point on the
other end of the hose.
If the hose has a 45-degree or 90degree fitting on an end, measure
from the center of the flare cone, parallel to the hose and not around the
corner. In other words, if you have a
90-degree fitting, measure from the
middle of the fitting nut down the
hose.
You should use the correct elbow
fittings to prevent the hose from
making sharp bends. If the hose is
connected to a movable hydraulic actuator, it must be of sufficient
length and installed in such a way
that it is not crimped in any position
of the actuator.
Flexible hoses may be equipped
with either swaged or replaceable
end fittings. If a hose having swaged
fittings is damaged, the entire hose
must be replaced. On the other
hand, if the hose has replaceable fittings, only the hose portion has to be
replaced if you have the proper tools
to do the work.
We discussed several different
types of hose above. In general, each
type has its own unique fittings that
require specialized tools to install
them. For instance, Aeroquip 306
low-pressure hose uses Aeroquip 471
fittings, Aeroquip 303 medium-pressure hose uses 491 fittings, and Teflon
601 medium-pressure hose uses 816
fittings. Each of these types and each
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size of fitting use a different type of
installation mandrel. The mandrels
are not real expensive, running $20
to $40 each, but you may need several
for the average engine and hydraulic
installation. While we are discussing
price, the average hose fitting runs
about $10 to $20 for a straight piece,
depending on size, but the elbows are
two to three times pricier, so carefully
consider the installation before specifying them. Also, after the fittings are
installed, the hose should be pressure
tested prior to use.
Between the hassle with all the
mandrels and the pressure test, most
homebuilders just have a hose shop
make the hoses for them. While this
is usually the simpler path, it does
require that you precisely measure
the lengths of hose that you will need,
and of course, you must get the sizes
right. If you have a few used fittings
or B-nuts around that you know the
size of, test-mate them to your existing plumbing to help solve the size
issue. To determine the length of the
hoses, I usually mock them up using
coat hangar wire. I run it between the
points I want to connect and trim it
to the exact length. It will hold the
required bends and can show quickly
if you are trying to use an excessively
small bend radius. As I do each layout, I then measure the length of the
wire for the exact length of finished
hose needed, and I label the wire
with masking tape to use as a reference when the new hoses are being
installed. This approach usually minimizes mistakes and speeds the process.