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Admiral Beez wrote:

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I was looking at a fuel gauge on a motorcycle, and realized that whenever the bike leaned, the gauge would misread the fuel level, as the float in the tank would be affected by the leaning motion. This got me thinking that on a fighter aircraft, measuring the fuel in the internal and external tanks by either a float or weight system would be even more flawed due to the changes in gravity-forces. If the fighter is upside down or turning sharply, does the tank or gauge compensate? Or, is the fuel measured purely by amount used minus the amount loaded prior to takeoff? Perhaps the pilot inputs the fuel load into his flight systems prior to engine start-up, and then the fuel pump measures the number of rpms or fuel used in order to give an accurate reading to the pilot?

Most modern jet engines use digital electronic controls, which include either mass-flow or volume-flow measurement transducers, so (in theory) the fuel remaining is just initial fuel minus the integral of the fuel consumption over time. Of course, that can get broken as combat damage may make the quantity of fuel in the tank drop rather quickly.

Since most aircraft turns are coordinated, e.g., there is no "lean," a float (or something analogous) would be perfectly good for the most steady-state conditions. How the remaining volume in a tank is measured varies, including, iirc, floats, ultrasonics, and measuring the pressure at the bottom of the tank (that last would need to be compensated for accelerated flight conditions). I would expect that most aircraft use a combination of integrating fuel consumption over time, to get one measure of expected fuel remaining, and a fuel gauge.

Note that as long as "g" is positive, the volume displaced by a float is constant, as the weight of the float is equal to the weight of the fluid displaced (Archimede's principle), and acceleration has exactly the same effect on the float and the fuel in which it's floating. When "g" is negative, the fuel will fall to what is normally the top of the tank, and a float valve will be confused.

From my (limited) light aircraft experience, trusting fuel gauges is not a good idea. They don't work worth a darn.