Engine Instrumentation — Page 534, Lesson 632

I want to walk you through the fuel quantity gauging systems now, because this is where we move from measuring how fast fuel flows to measuring how much is actually sitting in the tanks. And I want to start with a distinction that matters for every calculation you'll do as a pilot: the difference between measuring fuel by volume and measuring it by mass. The fuel gauge exists for one essential reason — you need to know how much fuel is in the tanks, and when you combine that with the rate at which fuel flows to the engine, it lets you fly the aircraft at maximum efficiency. So the quantity measurement and the flow measurement work together. Now, there are two principal methods of indicating fuel quantity. You either measure the Volume, in gallons, or you measure the Mass, in kilograms or pounds. Here's the key point: volume measurement is now only used on light aircraft, because the mass of the fuel is what interests the pilot. Why? Because mass is what helps you calculate the aircraft's all-up weight — that's the total weight of the aircraft including fuel, payload, everything. And mass also gives you a better indication of the energy the fuel can release. Think about this: one pound of fuel has the same number of energy molecules regardless of temperature and volume. So a pound of fuel is a pound of energy, no matter how much space it takes up. That's why mass is the meaningful quantity for the pilot. Let me show you the simplest form of volume indication — the float system. Early aircraft had a float that sat on the surface of the fuel. Attached to that float was a piece of wire that protruded out the top of the fuel tank. As the fuel level reduced, the wire disappeared from view. That's the most basic version. But the most common variation is where the float moves to reposition a wiper on a variable resistor. That wiper alters the current going to an indicator, which moves a pointer over a scale calibrated in volume. This is a DC powered system. Now, this system has a real disadvantage, and I want you to understand it clearly. Fuel tanks are rarely a symmetrical shape. So the float level is not a true measure of quantity — the float tells you the height of the fuel, but if the tank is an odd shape, height doesn't directly translate to quantity. The gauge is set to be accurate at the low and empty positions. And the system is also subject to errors whenever the aircraft manoeuvres and the attitude changes — bank the aircraft, and the fuel sloshes, and the float reading shifts. That brings us to the capacitance type fuel gauge system, which is the more sophisticated approach. In its basic form, a capacitance system consists of a variable AC capacitor located in the fuel tank — that's the tank unit — plus an amplifier and an indicator. This system will indicate volume without the errors of the float system. And here's the clever part: if you feed a correction voltage into the circuit due to a change in volume or a change in temperature, then the system will indicate the mass of the fuel instead. Let me describe the tank unit construction, because it's very specific. A tank unit consists of two concentric aluminium alloy tubes — that means one tube inside another, sharing the same centre. They're held apart by pairs of insulating pins. The electrical connections are insulated, and the unit itself is insulated from the tank. And co-axial connectors are used throughout — that's the type of connector where one conductor runs inside another, which keeps the signal clean. Now, incorporated in the system are reference units. These improve indication errors that would occur if the permittivity of the fuel changes from its normal value. Permittivity is a property of the fuel that affects how the capacitor behaves — if the fuel type changes, or its properties change, the capacitance reading would drift. The reference unit corrects for that. And here's the critical detail: the reference unit is located on the lower end of a tank unit, and it is always totally submerged in the unusable fuel level in the tank. So it's always sitting in fuel, giving you a constant reference point to correct against. So to tie it together: the float system is simple, DC-powered, and prone to attitude and shape errors. The capacitance system uses an AC capacitor in the tank, an amplifier, and an indicator, and it can give you either volume or, with the correction voltage, mass — and the reference unit keeps it accurate even if the fuel's permittivity changes. That's the heart of modern fuel quantity measurement.