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Aircraft Fuel System Detail

Fuel System Design

Designing a good, reliable and pilot error proof fuel system is quite an undertaking. Contrary to a car, aircraft fuel systems need to take into account balance problems of having fuel in their wings or even in the horizontal stabilizer (B747) and a lack of certain standardization among aircraft manufacturers doesn't help either.

You will find the tanks usually in both wings and sometimes there is a header or auxiliary tank in the fuselage. To get the fuel to the engine(s) under all circumstances and flight conditions the system is equipped with pumps, filters and valves.

But safety dictates that the system must be simple, safe, easy to operate and not confuse the pilot/ crew. Designing such a system can be a challenge, fun and frustrating all at the same time.

Design options

Most homebuilt aircraft have two tanks, one in each wing. But if it works for you, use one only gravity feed tank and install a fuel shut off valve near the bottom of the tank. For a simple fuel level indication use a cork float with a steel wire running through the fuel cap. This is as basic as you can get and it will always work, even without electricity.

Adding a second tank means more complexity and a possible pilot error with selecting the wrong tank. Unless the tanks are connected through an Y or T fitting. Connect the ON-OFF selector after the Y/T fitting and before any filter. Changing that filter is then possible without any spillage.

Tank venting

With a two tank system you will need a cross-over vent line connecting the tanks, even more so when only one tank has an air vent. This will keep a tank from collapsing when fuel is pumped out of it and the cross-over line keeps air pressure above the fuel at the same level so fuel should be drawn equally from the tanks, as long as you fly the aircraft balanced.

Tank vents are either part of the fuel cap or installed next to the tank facing into the wind creating a positive airflow and increased fuel pressure. Be careful when using vented caps: if the opening faces backwards fuel can be draw out of the tank leading to starvation. Forward facing vents should also have a screen to keep insects and other bugs from clogging the inlet. It is important to maintain a positive pressure in the system.


Fuel Primer

Each tank must have a drain in the lowest part, so that any water or dirt can be drained during pre-flight. Make sure it closes properly after draining as you will loose fuel reasonably fast should this seal fail (personal experience!).

Fuel quantity

Some vintage aircraft use a cork float type, but nowadays the resistive or capacitive types are used in modern aircraft. I prefer the capacitive types as these are more accurate (even with tanks that are oddly shaped) and show a much more stable indication when flying in turbulence. Calibration is also easier: set empty, set full and fill tanks with known quantities and note that in a table with indications from the gauge. Simple!

Fuel lines, valves, fittings & pumps

Ideally each tank should have its own shut-off valve, this is perfect when working on the system and should leaks develop somehow you can close the tank without losing the fuel. Get the pumps as close as possible to the tanks, installation inside the tank would be best of all. Any 90° fitting should be replaced with a 45° fitting for the least amount of fluid resistance.

Fittings may not leak air into the lines and these lines must be installed in such a way that vibration or its harmonics can not cause cracks and leaks. In the engine compartment lines should be isolated so that they are kept away from direct heat generated by the engine, turbo (if any) or exhaust.

Fuel Quick Disconnect

Fuel lines may also be connected with quick disconnects to the fuel filters and the rest of the system. This will greatly help when the time comes to change the filters annually. A nice example is shown in the next video from HomebuiltHELP: Fuel Filter Fanatic. Showing that easy and quick filter replacement is possible.

Selector valves

Helpful with more than one tank but install the valve in a logical way, i.e.: moving the selector to the right should select the right tank and to the left the left tank. Forward should mean both tanks and to the back: fuel is shut off. The valve should be simple to operate and in easy reach with a clear indication too!

Fuel injected engines (diesels too) usually have a return fuel line routing fuel to the tank in use. This complicates the fuel valve installation, especially so with aluminum tubing which needs proper bending without cracking or collapsing the line.

Keep away from the automotive type flaring tools: these are at a 45° angle and aviation tubing and connections are at 37° angle flare. It won't fit properly and will leak eventually caused by vibrations.


Aircraft Fuel System Strainer

There are several types of strainers in the fuel system: finger strainer in the tank exit, gascolator and sometimes a screen in the carburetor. These serve to stop any dirt from reaching the engine preventing an engine failure. Engines with fuel injection require a proper filter to keep the fuel pump from failing, this filter will be installed after the fuel selector valve.

Make certain that you can reach each strainer with ease after the aircraft is completed building, downtime due to maintenance is then kept to a minimum and cost too.

Fuel flow test

Make sure the system delivers the fuel flow required by the engine on takeoff including a healthy safety margin of at least 50%. To do this set the aircraft in a tail low position (climb attitude) and measure the flow from the tanks at the gascolator with the valves open. This will make sure that you always have enough flow and pressure in this nose high attitude.

Written by EAI.

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