Engine Instrumentation — Page 540, Lesson 635

I want to walk you through the fuel quantity system now, and specifically how it measures fuel by volume and then converts that into a mass reading for you. This is the capacitance tank unit system, and it's the heart of how we tell the pilot how much fuel is actually on board. Let's start with the pointer. In a fuel quantity system that measures fuel by volume, the pointer's position is directly related to two things: the change in fuel level, and the ER-1 value. Now, ER-1 is the dielectric constant of the fuel — I'll come back to that in a moment, but for now, just know that the pointer isn't just reading a float; it's reading an electrical property of the fuel itself. Here's the physics. In an AC capacitance circuit, the current is equal to the voltage divided by the capacitive reactance. So we write that as I equals V over Xc. I is the current, V is the voltage, and Xc is the capacitive reactance. Now, capacitive reactance, Xc, is itself equal to 1 divided by 2π f c — where f is the frequency and c is the capacitance. Since the voltage, the frequency, and 2π are all constants in this circuit, the only things that can change are the fuel level and the capacitance. As the fuel level changes, the capacitance changes, and therefore the current in the circuit changes. That changing current is what drives the pointer. Now here's where it gets interesting, and this is the part that matters for you as a pilot. Changes in the temperature of the fuel will affect its density, its volume, and its dielectric value — that's the Er value. Let me be precise about the relationships. A decrease in temperature causes a decrease in volume, an increase in density, and an increase in Er. So the fuel gets denser, it shrinks, and its dielectric constant goes up. The circuit is compensated for these temperature changes, and once it is, the system can now indicate the mass of the fuel — which is of far more value to you than a volume reading, because mass is what actually determines your range and your weight and balance. For the gauges to be calibrated in terms of mass, we make an assumption: there is a constant relationship between the Er value and the density, ρ, for a given sample of fuel at a given temperature. But temperature is not a constant, so the system incorporates a Compensating Capacitor circuit. This is fitted to the reference unit. With that in place, the system senses changes in the Specific Gravity — the SG — of the fuel, and so it can indicate mass. Now, the indicating system can also incorporate an additional indicator called the Fuel Totalizer. This will indicate the sum of all the tank gauges — so instead of reading each tank separately, you get one total figure. In the event of a failure, the system fails safe: it drives the indicator slowly to the zero position. There's also a test circuit. When you select it, it simulates the emptying of the tank, and when you release the switch, the pointer should return to its original position. That's your pre-flight check that the system is working. One more failure condition you need to know. If water is present in the tanks, it will cause errors in the indicating system. The capacitors in the sensing units are effectively shorted, and the indicator is driven beyond the full scale. So if you ever see a fuel gauge pegged past full, suspect water contamination. Finally, there's a regulatory marking requirement. If the unusable fuel supply for any tank exceeds one gallon, or 5% of the tank capacity — whichever is greater — then a red arc must be marked on that tank's indicator. That red arc extends from the calibrated zero reading down to the lowest reading obtainable in flight. That tells you the region where you can't rely on the fuel being usable. Let me just tie that together. The system measures volume through capacitance, compensates for temperature so it can show you mass, uses a totalizer to sum the tanks, fails safe to zero, and warns you with a red arc if there's a significant unusable amount. That's the complete picture of how your fuel quantity indication works.