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
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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 EAA Sport Aviation
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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 108
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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 EAA Sport Aviation
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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 110
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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.
