Plumbing and Tubing
In aircraft fuel and oil needs to be routed throughout the fuselage to the engine and fuel tanks. In order to install the correct amount, type and lengths of tubing most builders wait until the fuselage, or sections thereof, is completed.
Tubing and lines are found throughout the aircraft: oil lines to and from the oil cooler, fuel lines from the tanks to the gascolator and carburetor to name but a few. The builder must install and fabricate all of these sometimes expensive tubing.
And after a number of years or flight hours these fuel and oil lines must be replaced due to possible corrosion. On this page we take a look at what is involved.
These are used behind the firewall opposite of the engine where there is no movement of these lines. The engine is hung in rubber shock mounts and can move around quite a bit, so flexible hosing must be used to connect it. Metal lines would break after a short period of time under such harse conditions.
This is the material used for general purpose low pressure tubing in aircraft and there are different alloys used: 1100-0, 3003-0 and 5052-0. They are soft enough to flare and can be formed easily with a tube bender for routing through wings and fuselage. You may also use this sort of tubing for pitot/static, primer, fuel, oil and low pressure hydraulic lines.
When aluminum is subject to vibration and repeated bending it will harden and may become brittle. The solution is to anneal it with heat and let it air cool at its own pace. Forced cooling with a fluid will harden it again!
It is used mainly where corrosion resistance is an important factor and with high pressure hydraulic lines aircraft with retractable gear. Not many homebuilt aircraft will use these steel tubing.
Oil pressure gauges sometimes have copper tubing but one of the major drawbacks is, is that it hardens with age and vibration. You will have to anneal copper tubing after forming and regularly afterwards or else the reliability can not be guaranteed.
When forming and bending metal lines you will need some special tooling: a tube bender, cutter and flaring tool. Aviation tube flaring is set at 37°, keep in mind that automotive flaring tools are set at 45°. Do not use those tools and install aircraft grade fittings, it will not fit properly and will leak after a short period of time!
Before flaring your tubing, make sure that no burrs are present if you have used a fine-tooth hacksaw to cut the tubing. The best tool for the job is still the tube cutter. To prevent corrosion, use the same type of material in the tubing. And use plenty of cushion clamps to install the lines throughout the fuselage.
Whenever there is movement between two sections flexible hosing will make sure that the connection will not break and fuel and or oil will not be spilled. These hoses are not cheap and when obtained from surplus sources the quality and reliability may be questionable. Make sure to verify any markings on the hose.
When selecting hoses make sure you know what kind of fluid at what pressure is needed to carry through the hose. Some hoses are only to be used with low pressure air and are not suitable for fuel or oil or any other chemical fluids. You may also want to check the fire resistance of the hose you have selected.
Hoses under the cowling carrying fuel need protection by a fire sleeve (the orange type) in case of trouble.
These are especially made for aviation purposes. Manufactured in different types, lengths and with steel braided protection if so required by you. They can be made on special order with fittings attached and are even used in the automotive industry.
For fuel and oil lines you may want to select the high temperature high pressure hose, this is the one with the steel braid. Ideal for engine installations!
When installing these hoses make sure that the fitting is clean and properly aligned. Do not twist the hose while tightening the fitting, it won't last. Keep fuel lines away from exhaust stacks and do not bend the hose too sharp. Overtightening will eventually result in a crack and fluid leak.
When storing hoses keep them capped so that any dirt or insects will not be able to get inside the hose, this may result in a failed engine or worse.