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I fly a fuel injected Cessna 172SP 200 hp. Tanks were just about full today and every 5 seconds or so in flight, I would get “L fuel low” and the left fuel indicator would drop to 0.

I’ve consulted with a few people and get differing answers. Some say it’s due to the blue dye in the tank? Some say bad sensor? Any thoughts?

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    $\begingroup$ Ask the people that say blue dye in the tank to explain themselves. It’s probably a faulty sensor, but you should find a local A&P you can trust and avoid troubleshooting via random internet strangers. $\endgroup$ Sep 24 '20 at 2:43
  • $\begingroup$ Airplane fuel level sensors are notoriously bad, half the airplanes I fly have them marked as INOP, instead we dip the tanks before flight and keep track of the burn rate. $\endgroup$
    – GdD
    Sep 24 '20 at 7:32
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Fuel dye has nothing to do with it.

The fuel sender in light airplanes is usually an automotive-style float operated potentiometer, a rotary variable resistor, like a volume knob on a radio. Inside is an arm, or wiper, that sweeps around a resistive conductor in an arc, providing a variable output voltage for the fuel gauge. There will be a voltage range for the potentiometer output to the gauge with 28 vdc going in, say, 22 vdc for full, and 4 vdc or less for empty. The low fuel light will be set to come on when the voltage is some margin above that, say, 8 vdc or less or something like that.

Sometimes the resistance element inside the pot is a kind of circular wire coil on a card, sometimes it's a resistive coating on a flat surface. If part of the resistor is worn to the point where there is an open circuit when the wiper passes the worn spot, the voltage going to the fuel gauge drops to zero, until the arm moves off the worn spot and the voltage is picked up again.

The low fuel light coming on plus the gauge dropping to zero suggests that the voltage signal is dropping out upstream of the fuel gauge and sensing circuit for the low fuel light. Could be due to a worn spot in the pot as the float moves when fuel sloshes around in the tank, or it could be an intermittent open in the wiring. If the problem goes away when the fuel level drops to some lower amount where the potentiometer no longer contacts the worn spot, that would tend to confirm that possibility.

If I was the mechanic I'd try to duplicate the fault by sticking something in the tank that can move the float up and down and observe the result. If everything works normally, it's more likely to be a loose connection somewhere giving an intermittent open circuit. Check wiring, and if ok swap out the level sender and see if the problem goes away.

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  • $\begingroup$ A great answer! Some older automotive airflow sensors work on the same principle, and they can develop the same issues, causing rough running at certain throttle levels. There are ways to resurface these plates (a pencil will work in some cases) or work around them by extending the sensor arm contact point to an unworn spot, but that's only really worth it when the part is expensive or hard to find. $\endgroup$
    – GdD
    Sep 24 '20 at 7:30
  • $\begingroup$ Thanks. It tends to affect rental airplanes more because they spend most of their time between 3/4 and full tanks as each renter "fills 'er up" just to be safe and flies around for an hour burning off the top quarter until the next renter fills it again. In the commercial world, where fuel in excess of minimum safety requirements is effectively ballast that replaces revenue, there is way more variability in fuel quantity over time. $\endgroup$
    – John K
    Sep 24 '20 at 13:46

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