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Ultralights under FAR 103 are regulated to a capacity of 5 gallons. I'm wondering how exactly fuel capacity is defined, does it include fuel in the lines, and in the carburetor bowl or elsewhere or just the tank feeding the engine?

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FAA Advisory Circular AC 103-7, Paragraph 19 has this to say:

MAXIMUM FUEL CAPACITY OF A POWERED ULTRALIGHT VEHICLE.

The maximum fuel capacity for a powered ultralight vehicle is 5 U.S. gallons. Any powered ultralight with fuel tank(s) exceeding this capacity is ineligible for operation as an ultralight vehicle.

a. Determination of Fuel Capacity.

The total volume, including all available space for usable and unusable fuel in the fuel tank or tanks on the vehicle is the mtal fuel capacity. The fuel in the lines, pump, strainer, and carburetor is not considered in a calculation of total volume.

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    $\begingroup$ Okay, so you use exhaust piping for your fuel lines, and get another gallon or two of capacity. $\endgroup$ – Mark Mar 26 '18 at 4:23
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    $\begingroup$ @Mark I suspect if you pushed that and got called on it, you'd find the FAA (or your competition director; powered hang/paraglider comps do "maximum distance on a single tank" tasks) will simply consider that a second cylindrical tank! $\endgroup$ – Graham Mar 26 '18 at 11:35
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    $\begingroup$ @Mark - I know of someone who uses really large diameter fuel lines in his track car, just to get a few extra laps w/o refueling. Has passed several tech inspections, but I know that isn't the FAA. $\endgroup$ – ivanivan Mar 26 '18 at 13:00

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