Orville’s Gravity Fuel System

Fuel system problems are the leading cause of experimental airplane accidents. All fuel systems should be carefully evaluated and thoroughly tested prior to flight. This information is provided to builders for their experimental use. Proceed at your own risk.

As soon as requests came in for a drawing of our fuel system, it became clear that my effort would serve more builders if I also included drawings that covered gravity systems with a stand-by boost pump in a parallel line. This provides a “Comfort Button” if your engine starts to sputter for any reason.


In this case it will not be vapor lock that you overcome with a pump, as there is no engine pump to lock up with vapor. Some will tell you that there is no need for a boost pump, since it will help nothing.

I disagree with that assumption.

There are several things that can be helped by a sudden increase in fuel pressure. One pound of fuel pressure is not much, but it is plenty to run most carbureted engines. However, there are some things that can, in special cases, drop that light pressure to an unusable level. A steep climb is the most obvious one.

Another is a partially clogged fuel filter or a partially bug clogged or ice clogged vent line. Remember, with only one pound of pressure it doesn’t take much to reduce it to an unusable level. Suddenly raising the suction or pressure from one pound to five or six might make the difference. There is also another handy feature that many of you may enjoy. That is being able to hook up a belly tank/ferry tank. Of course, you should add an engine-driven fuel pump to the system for this application as a back-up.

The shuttle valve should be pointed with the flow downward or at least steeply slanted.

This way gravity will always help the valve to keep full open and free flowing.

The parts needed for this system can be purchased from Aircraft Spruce.

  • Check valve: ACS free flow, one way check valve part # 10630.
  • Pressure filter: part # 05-25970 + mounting bracket.
  • Fuel pump: Facet 4-6 psi, 12 volt, part #40106 or 40108.
Gravity fuel flow schematic
Gravity fuel flow schematic
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