As per aviation rules, “...the aircraft fuel gauge must indicate the quantity of usable fuel in each tank during flight.” Let us see what this is achieved.

One would assume that an aircraft fuel gauge would be somewhat identical to that of a car. In some respects you would be right. But there are some fundamental difference between what happens in a car verses what happens in an airplane that makes reading fuel levels in an airplane quite complex and complicated.

Here are the differences:
1. A car has only one fuel tank whereas, an aircraft has a three or more fuel tanks.
2. A car has one engine where as an aircraft has a minimum of one engine.
3. The fuel density in the car remains steady whereas in an aircraft, it can fluctuate wildly depending on the aircraft altitude. Fluctuating fuel density needs to be factored into while displaying fuel quantity.

So, fuel that slouches around the tanks as the plane flies, multiple fuel tanks and changing fuel density all make work of the aircraft fuel gauge much more complicated and difficult. This why as part of their training, pilots are trained how to manually calculate fuel in the tanks. Of late however, after-market manufacturers like J.P. Instruments have been making well designed and accurate digital aircraft fuel gauges.

So how does and airplane Keyspan USB work?

Small aircraft being essentially simple, have a float level gauge fitted in each tank. These in turn feed fuel level information to magnetic couplings and potentiometers which in turn are directly connected to the Aircraft Gauge.

In larger aircraft, things becomes instantly complicated but here are the basics of how it is done:
To obtain a decent level of accuracy, larger aircraft are fitted with a sophisticated system of capacitance probes which are mounted in the fuel tank. Capacitance sensors (also called Dielectric sensors) use capacitance to measure the dielectric permittivity of the fuel. To make a long story short, as the fuel is pumped in, the tank’s capacitance alters, allowing tank levels to be accurately calculated by the on-board fuel computer in the cockpit.

Given that aircraft travelling through storms can make fuel calculation difficult, modern large passenger aircraft have multiple methods of level detection and use more than one technology to calculate residual fuel. This is why digital fuel gauges are more accurate than the analog fuel guages.