With the exception of 98P, we have fuel totalizers in every airplane in the fleet. I often hear from pilots that they don’t totally understand what a totalizer does versus a traditional fuel gauge, and they don’t really understand what each number on the screen means.
Fuel gauges (like the kind required for VFR day flight by 91.205) measure the quantity of fuel in the each fuel tank. In light airplanes like ours, this is usually done with float sensors. A float, made of cork, foam, or something similar, is mounted on the end of a pivoting arm and floats on top of the liquid fuel. Think of the float in your toilet tank! A sensor measures the angle of the arm to show you a gallon amount on your fuel gauge.
Fuel totalizers, on the other hand, estimate the amount of fuel remaining by taking a pilot’s initial input and tracking fuel flow over time. The totalizer only knows what you told it, minus the fuel flow it measures over time. It does not measure how much fuel is left in the tanks.
There are pros and cons to fuel gauges and fuel totalizers, and they complement each other well. Because a traditional fuel gauge measures liquid level, it can actually tell you how much fuel is in each fuel tank when you are straight and level, if it is calibrated and accurate. But when fuel sloshes around as you accelerate, decelerate, yaw, or roll, even the most accurate float-type fuel gauge can’t get an accurate measurement.
On the other hand, a totalizer can measure fuel flow to the engine very precisely to give you a really useful, accurate estimate for how much fuel you have left, regardless of attitude or acceleration, but it has some important limitations to watch out for.
An accurate totalizer estimate is completely dependent on the starting number being correct. If the pilot inputs the wrong number, or if you trust the last pilot’s input without verifying the fuel in the tanks during your preflight, the totalizer estimate will be wrong.
Another thing to keep in mind is that a totalizer only knows about fuel flow at the place where it is measured; it doesn’t know if fuel is leaving the fuel system (i.e. leaking) anywhere else before that. If you leave the fuel caps open and fuel gets siphoned out of the wing, the totalizer won’t know about it, but the fuel gauges will.
The best understanding of your fuel level in flight comes from using your fuel gauges and a fuel totalizer together, but however you keep track of your fuel in flight, the most important thing you can do is verify the fuel level in each tank during your preflight inspection.
Reach out to me or any of our instructors if you have any questions. Thanks for making safety happen every day!
