I was asked to come up with a technical note for our EAA news letter and I want to bounce it off you guys. Thanks in advance for your review comments.

Sometimes when I visit a project the builder will show me how he has installed the fuel pickup in the lowest point of the tank and proudly declares “All my fuel is usable.”

Then I say, “Suppose you get some water in your fuel, where does that go?” That is generally followed by a long uneasy silence.

Composite wing skins frequently have about a half inch of lightweight core material to create a stiff wing skin. Integral wing fuel tanks usually have a drain that is mounted in the bottom skin penetrating this core material. There may be additional plies of reinforcement around the penetration. In some cases the drains are recessed into the skin to be flush. The actual opening where the drain fuel is withdrawn may be as much as ¾ inch above the lower wing surface.

Meanwhile the builder has installed the fuel pickup right next to the inboard rib at the lowest point of the tank where there is no core material.

This is a case where a pilot could drain a large sample of clean gas while the fuel pickup is under water.

Imagine a large coffee can sitting vertically on your work bench. Drill a hole in the side of the can two inches above the bottom and install a fuel tank drain valve in that hole. Now drill another hole four inches above the bottom and install a finger strainer.

Here you have a fuel tank. The fuel above the finger strainer is usable fuel, and the fuel below the finger strainer is unusable fuel. We learned all about those two things when we became pilots.

But the unusable fuel can be divided into two sub volumes. The fuel below the strainer and above the drain is “drainable unusable fuel”, and the fuel below the drain is “undrainable unusable” fuel.

The undrainable unusable volume is of no value to us. It will probably fill with water, which is dead weight we will have to carry with us all the time. It may eventually grow some microbes that will generate waste products that could clog our fine screens.

The drainable unusable space provides a place for water from condensation or contamination to accumulate without being sucked into the engine, and that water can be drained before flight.

How big should the drainable unusable space be? That’s up to you, the manufacturer, it is a tradeoff between lost range and water capacity.

FAR part 23 says;

(c) Each reciprocating engine fuel system must have a sediment bowl or chamber that is accessible for drainage; has a capacity of 1 ounce for every 20 gallons of fuel tank capacity; and each fuel tank outlet is located so that, in the normal flight attitude, water will drain from all parts of the tank except the sump to the sediment bowl or chamber.

[This is confusing, I think water should drain from all parts of the tank to the sump. Perhaps they mean any water that gets into the outlet line  should drain to the sediment bowl]

0.25% feels a bit low, something close to 1%  of tank volume sounds reasonable. So, for a 20 gallon tank that would be 0.2  gallons of drainable unusable fuel.

Tanks that use bladders may hold a great deal of water in wrinkles on the bottom surface. A much more conservative approach may be needed.