Challenger Fuel Line Installation and Maintenance

Fuel system failure is one of the leading causes of sudden engine stoppage on ultralight aircraft. Sudden engine stoppage is one of the most common causes of ultralight aircraft accidents and off-field forced landings. Fuel line failures are one of the leading causes of fuel system failure .

With these facts in mind, it is very important to choose the right fuel line and fuel line fittings and take good care of the aircraft fuel lines as the aircraft ages. Truly, this is a case where a ounce of prevention is worth a pound of cure, or if you use the metric system, a gram of prevention is worth a kilogram of cure.

The fuel lines on most Challenger Ultralights are clear blue flexible tubes made from polyurethane plastic. These fuel lines are connected to the filter, pump, carburetor and any other devices using push-on barb fittings. Many builders use various styles of clamps to ensure that the plastic lines remain installed on the barb fittings. There are a few things to keep in mind when selecting and installing these plastic fuel lines, fittings and clamps and later on after a few years in service on the engine.

Proper Fuel Line Material

There are many varieties of plastics and many of them are not compatible with gasoline or the alcohol that is now often present in gasoline. Many plastics become brittle, crack, and disintegrate when exposed to gasoline for a period of time. A brittle fuel line will crack and leak or even break and fall off a fitting. For a high speed illustration of this potential for gasoline to break down a plastic material, put a small amount of gasoline into a styrofoam cup, count to ten, and then clean up the mess.

With this in mind, use only approved designated fuel lines from an aircraft supply house. Approved aircraft fuel lines will have this approval stated in the ink on the tubing. They are a clear blue colour and are made from high quality polyurethane. They cost a little more but it is worth the extra cost. Do not use cheap clear poly vinyl chloride (PVC) tube from the hardware store. Do not use the cheap stuff made for snow mobiles and ATV’s. Cheap fuel lines are cheap because they are missing key qualities needed in an ultralight aircraft fuel system.

The cheap stuff does not contain UV blockers. Sun light deteriorates plastics over time. Part of the fuel system on a Challenger is exposed to direct sunlight and thus requires the UV protection component. The fuel system in snow mobiles, ATV’s and water craft is entirely buried under the cowl and does not see the light of day. Thus the difference in our fuel line requirement from those other vehicles.

Exposure to heat also causes plastics to degrade over time. PVC plastics are very susceptible to head degradation. They will discolour and become brittle over time. High quality polyurethane plastics are much more tolerant of heat and do not become brittle as quickly as other kinds of plastic lines.

Finally, high quality polyurethane is more scuff resistant and flexible and has a higher tensile strength than less expensive plastics such as PVC. Therefore it will endure rubbing, abrasion, flexing and pulling much better. All of these factors apply to the conditions seen by fuel line tubing installed on an ultralight aircraft where the engine is exposed and not cowled.

This is certainly one instance where you get what you pay for and a little more expense is well worth the cost.

Fuel Line Fittings

The same factors that affect tubing also affect fuel line fittings. Many aircraft builders use plastic fittings in the fuel system where they need to branch off of the main fuel line to go to the manual primer pump and then back to the carburetors. Often the fittings used for this primer line are purchased at the hardware store or automotive supply house and are in fact fittings really meant to be used on windshield washer lines not fuel lines! These cheap plastic fittings may break down over time when exposed to gasoline, sunlight, heat and flexing.

A much superior alternative is to use metal fittings. You will notice that all the fuel fittings used on the carburetors are brass. This is because metal fittings are much stronger and more durable than plastic fittings. Brass or aluminum barb fittings will not deteriorate do to exposure to gasoline, alcohol, water, sunlight, heat, or the flexing encountered by the fuel lines on Challenger aircraft.

Fuel Line Clamps

Many builders insist on clamping fuel lines onto the barbed fittings to ensure that the fuel lines never come off. Often though the clamp is not necessary because the line is already on so tight. We do it for that extra security, the belt and suspenders approach that characterizes the aircraft industry. Sometimes though an improperly installed clamp is actually worse than no clamp at all!

There are lots of methods of clamping tube and hoses onto barbed fittings. One common method is to use a worm drive hose clamp such as is often used in plumbing. These clamps have two problems. First, they are easy to overtighten so that they deform the tube and weaken it or bite into the tube and cause a break. Secondly, fuel lines are so small, 3/8″OD for the main fuel line and 1/4″OD for the primer line, that worm drive clamps are usually way out of round by the time they get down to that size. The result is an uneven clamp force that again will bite into the tube or deform it and cause a leak.

Some installers use wire to clamp fuel lines. Wire will bite into fuel lines. It is very hard to judge how tight the wire is biting the tube.

Others use dog ear clamps. These do not clamp the whole 360 degree OD of the tube so you risk deforming the tube because the clamp force is not even all around. These should not be clamped to tight so as to deform the tube.

So the clamps used should evenly fit the size of the tube OD (outside diameter). The clamps should be snug but not so tight that the tube is deformed. The clamp should not cut into the outside of the tube. Some clamps are available with a rubber sleeve that protects the tube being clamped. Another idea is to put a short section of a larger tube over the tube being clamped to protect it.

Another solution is to use tube to clamp tube. For example, to clamp the primer line which has an outside diameter of 1/4″, use a short section of the 3/8″ OD by 1/4″ ID main fuel line over top of the primer line. The larger tube is itself the clamp.

Running the Fuel Lines Through the Plane

A final consideration when installing fuel lines in your ultralight aircraft is to protect them from abrasion, sharp edges and and kinks as you run them through the airframe.

If the line goes through a hole in a wall or gusset, line the tube with a protective sleeve or insert a rubber grommet in the hole first. This will protect the tube from being cut by the sharp edges or from being rubbed by the vibrations.

Do not go around a corner that is tighter than the minimum bending radius of the tube. You tube may kink over time. If your tube needs to go through a tight 90 degree corner, install a 90 degree barbed elbow fitting.

Restrain your tubes as you pass them through your airframe so they will not flop around an interfere with any controls. A particular area of concern in a Challenger ultralight is the area behind the rear seat where the fuel tank is usually located. That area also houses the aileron control cables along the sides and the rear firewall and below it is the torque tube for the elevators. Make sure that at no time will the fuel lines rub against these controls or inhibit their movement.

Often plastic tie wraps are used to restrain the fuel lines. Make sure that these are not so tight as to deform the fuel line. They should be snug not tight. The edge of the tie wrap should not be able to cut into the fuel line.

Fuel Line Tubing Gets Old

Even quality plastic fuel lines will degrade over time with exposure to gasoline, heat and sunlight. Expect to replace all of your fuel lines every few years. This time will vary with each aircraft. If the plane is stored outside, the fuel lines will require more frequent replacement. Make sure that your entire fuel system installation facilitates an easy fuel line change out.

How can you determine when your fuel lines need to be replaced? The main indicator is is the flexibility of the hose. It will get brittle and weak eventually. To determine whether it needs replacing, remove a clamp and pull on the tube. If it readily pulls off of the fitting, it likely needs to be replaced. It should be flexible and rubbery. The tube should have a memory. It should return to its shape when flexed or squeezed. If it is stiff, badly discoloured, or has cracks in it, it needs to be replaced.

Fuel Line Summary

By carefully installing your fuel lines using the right materials and following these guidelines, you will eliminate one of the leading causes of engine failure and forced landings in ultralights. This is a inexpensive and easy way to ensure safe and happy flying.

• Use only proper high quality polyurethane aircraft grade fuel lines.
• Use metal fuel line fittings.
• Use clamps that will not damage the fuel lines.
• Protect the fuel lines from sharp edges using grommets and sleeves.
• Restrain the fuel lines so they do not interfere with controls.
• Replace the fuel lines every few years when they get brittle.