Engine Instrumentation — Page 534, Lesson 632

I want to walk you through the fuel quantity measurement side of engine instrumentation. This is where we answer a very practical question: how much fuel is actually in the tanks, and how do we know it reliably? Let me start with the big picture. Measuring the quantity of fuel in the tanks is an essential requirement. And here's the key point — we don't just measure it for its own sake. In conjunction with measuring the rate at which fuel flows to the engine or engines, it lets the aircraft be flown at maximum efficiency. So quantity plus flow rate together give you the full fuel picture. Now, there are two principal methods of indicating the quantity of fuel carried. Either we measure the Volume — in gallons, for example — or we measure the Mass, in kilograms or pounds. The former, volume, is now only used on light aircraft. Why? Because the Mass of the fuel is what's of more interest to the pilot. Mass helps the pilot calculate the aircraft's all-up-weight — that's the total weight of the aircraft including everything on board. And mass also gives a better indication of the energy that can be released by the fuel. Here's the physical reason: one pound of fuel has the same number of energy molecules regardless of temperature and volume. So mass is the true measure of energy content, not volume. Let me now walk you through the simplest form of volume indication — the float system. Early aircraft had a float that sat on the level of the fuel. Attached to the 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 primitive version. There have been many variations. The most common one is where the float moves to reposition a wiper on a variable resistor. That wiper movement alters the current to an indicator, which moves a pointer over a scale calibrated in volume. This is a DC powered system — direct current. Now, this system has a real disadvantage, and I want you to understand it clearly. Fuel tanks are rarely a symmetrical shape. Therefore, the float level is not a true measure of quantity. Think about it — the float measures height of fuel, but if the tank is an odd shape, height doesn't map linearly to volume. 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 — because the fuel sloshes and the float tilts. That brings us to the more sophisticated approach — the Capacitance Type Fuel Gauge System. 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. AC, alternating current. This system will indicate volume without the errors of the float system. And here's the clever part: if a correction voltage due to change in volume or temperature change is fed to the circuit, then Mass of fuel will be indicated. So the same basic system can give you either volume or mass, depending on whether you feed in that correction voltage. Let me describe the tank unit construction, because it's precise. A tank unit consists of two concentric aluminium alloy tubes — that is, one tube inside another, sharing the same centre. They are held apart by pairs of insulating pins. The electrical connections are insulated, and the unit itself is insulated from the tank. Co-axial connectors are used throughout — that's the type of connector where one conductor surrounds another, sharing the same axis, 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 electrically — if the fuel type or condition changes it, the reading would drift. 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. That's the fuel you can't actually feed to the engine — but keeping the reference unit always submerged in it gives you a stable baseline to correct against. So to tie it together: the float system is simple, DC, and gives you volume — but it's inaccurate in odd-shaped tanks and during manoeuvres. The capacitance system is AC, uses a tank unit with concentric tubes, gives you volume without those errors, and can give you mass if you feed in the correction voltage — with reference units keeping it accurate even if the fuel's permittivity changes. That's the professional picture of fuel quantity measurement.