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All Private Pilots should know the acronym TOMATO FLAMES for the Minimum Equipment List (MEL).

On that list, F means Fuel Gauges.
But what exactly is required of fuel gauges?

Yesterday, my rental plane came with a note that the fuel gauges were swinging wildly, and bouncing to zero occasionally. I planned a 90 minute flight, starting with full fuel, so there was no real risk of running out of gas.

I decided the fuel gauges probably didn't meet the spirit of TOMATO FLAMES, but I admit that I don't know what the exact requirement is.

How accurately do the gauges have to work to be legal?

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The indicator must be calibrated to read zero when that tank is actually empty of all usable fuel (in level flight). Bouncing is fine as long as they do not bounce when the tank is actually empty.

See regs:

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    $\begingroup$ Wow, that is a pretty low bar to clear!! Thank you for the details. $\endgroup$ – abelenky Nov 27 '17 at 17:06
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    $\begingroup$ @abelenky The expectation of the pilot, of course, is to land the plane before the gauge reads empty and to not take off if it does read empty, presumably even if you know there's fuel in the tank. Having fuel when you think you don't is not going to put you in danger, it's being empty when you think there's fuel that's the problem. $\endgroup$ – J... Nov 27 '17 at 20:30
  • $\begingroup$ As long as it's bouncing you're okay! Sounds like the fuel gauges on tugs. It was halfway between the high and low of the bouncing needle $\endgroup$ – TomMcW Nov 27 '17 at 20:54
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    $\begingroup$ A fuel indicator that always reads zero would satisfy this $\endgroup$ – Steve Kuo Nov 28 '17 at 2:40
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    $\begingroup$ @SteveKuo Nope. Then the system would not "indicate to the flightcrew members the quantity of usable fuel". $\endgroup$ – user71659 Nov 28 '17 at 2:46
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The only legal requirement of fuel gauges on normal and transport category airplanes is that they read empty when the fuel tanks are empty (how convenient!). In general, for normal category aircraft, they make use a float type sensor in the tank itself. These floats can drive mechanical linkages attached to fuel gauges in the cockpit or actual rheostats which control solenoids which drive the cockpit fuel gauges.

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Some airplanes eg J-3 Cub, Ercoupe, etc. make use of a fuel tank just forward of the pilot which utilizes a float driving a sight stick.

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Large transport category aircraft use Fuel Quantity Management Systems which make use of capacitive measuring systems. Basically a system which can sense the fuel quantity using the electric capacitance of air and fuel as a means to sense how much fuel is inside of a tank. These tend to be considerably more accurate than mechanical gauges.

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To the best of my knowledge most fuel quantity sensing systems are proprietary to the OEM with little input from the FAA provided they comply with existing FARs.

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    $\begingroup$ 99% of this answer is not relevant to the posted question $\endgroup$ – Steve Kuo Nov 28 '17 at 2:41

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