Engine Instrumentation — Page 540, Lesson 635

Let’s pick up with the fuel quantity system and how the pointer behaves. I want you to picture the pointer on the fuel quantity indicator. In a system that measures fuel by volume, that pointer is directly related to two things: the change in fuel level, and the ER-1 value. Now, ER-1 is the dielectric value of the fuel — we’ll come back to that in a moment, but hold onto it because it’s central to how this whole system works. The heart of the measurement is an AC capacitance circuit. In that circuit, the current is equal to the voltage divided by the capacitive reactance. So we write I equals V over Xc, where I is current, V is voltage, and Xc is capacitive reactance. Now, capacitive reactance itself is given by 1 over 2π f c — that’s one divided by two pi times the frequency times the capacitance. Here, f is the frequency of the AC supply, and c is the capacitance of the sensing unit. Here’s the key relationship: voltage, frequency, and 2π are all constants in this circuit. So as the fuel level changes, the capacitance changes, and therefore the current in the circuit changes. That changing current is what drives the pointer. So the pointer position is a direct function of capacitance, which is a direct function of fuel level. Now, temperature complicates things. Changes in fuel temperature affect three properties at once: density, volume, and the dielectric value, which we call Er. Let me walk you through what happens when temperature decreases. A decrease in temperature causes a decrease in volume — the fuel contracts. It causes an increase in density — the same mass occupies less space. And it causes an increase in Er, the dielectric value. So temperature shifts all three of these, and that would corrupt a simple volume reading. The circuit is compensated for these temperature changes. Once compensated, the system can indicate the mass of fuel, which is far more valuable to the pilot than a raw volume. Mass is what actually tells you how much energy you have left. Now, for calibrating the gauges 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 constant in the real world, so the system incorporates a Compensating Capacitor circuit. This is fitted to the reference unit. With that compensation, the system senses changes in the Specific Gravity of the fuel — that’s SG — and so it can indicate mass. Let me show you the physical layout. Here’s the capacitance tank unit — the sensing element that sits in the tank and whose capacitance changes with fuel level. Now, the indicating system can also incorporate an additional indicator called the Fuel Totalizer. This totalizer indicates 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 and 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. When you release the switch, the pointer should return to its original position — that’s your check that the system is working. One important failure mode: if water is present in the tanks, it causes errors in the indicating system. The capacitors in the sensing units are effectively shorted, and the indicator is driven beyond the full scale. So a reading pegged past full is a red flag for 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. So the pilot can see at a glance the range where the gauge is reading unusable fuel. Let me tie it together. The pointer tracks fuel level through capacitance. Temperature compensation lets it read mass instead of volume. The totalizer sums all tanks, fails safe to zero, and has a test function. Water shorts the capacitors and pegs the gauge. And unusable fuel gets a red arc when it exceeds that one-gallon-or-5% threshold. That’s the complete fuel quantity picture.