Measuring fuel levels in an aircraft is an incredibly important task. Running out of fuel for an automobile typically means a ruined afternoon; an aircraft running out of fuel while operating, meanwhile, is potentially disastrous. Therefore, accurate and reliable fuel gauges are essential, with the most commonly chosen type in small aircraft being resistive-type fuel systems. In a resistive-type fuel system, a float level gauge feeds information on the fuel level to magnetic couplings and potentiometers that then relay information to the pilot. As fuel is expended in flight and the levels in the tank drop, the float inside the tank drops as well and slides a moving contact a long a resistor, increasing its resistance. The amount of resistance translates to how much fuel is left in the tank.
There are several advantages to resistive fuel measurement system, with the first being cost. Components in a resistive fuel gauge are simple and therefore inexpensive to manufacture and replace. They are also very reliable, with few moving parts to suffer stress and fail. The third and final advantage is that, while not completely optimal, they do provide a reasonable amount of accuracy. When used on a small aircraft, carrying relatively small quantities of fuel with a limited flight range, the accuracy provided by a resistive fuel gauge is adequate.
The greatest disadvantage of a resistive fuel gauge system is due to the laws of physics. Simply put, the sensor float moves when the aircraft moves, and is affected by gravity and centrifugal forces. When the aircraft banks for a turn, the fuel slopes to one side, and when the aircraft climbs, the fuel flows to the back of the tank. The float within the tank is in a fixed position, and can only respond to the up and down motion of the fuel. Therefore, if all the fuel moves forward and away from the float, then the float will fall down and indicate a lower amount of fuel than what is actually available, and vice versa if the fuel gathers disproportionately where the float is located. Only when the aircraft is flying straight and level can the system provide an accurate report of fuel quantity. These inaccuracies are intolerable in long-range aircraft with thousands of pounds of fuel like commercial airliners, which use more advanced systems to measure their fuel quantity.
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