Aircraft Fuel Systems — Page 327, Lesson 408

I want to walk you through the heart of how a modern jet aircraft measures its fuel — because this is one of those systems where the difference between "how much fuel is in the tank" and "how much fuel is actually available to fly" is a matter of physics, not just plumbing. First, let's set the scene with the system layout. On a representative jet aircraft engine fuel system, you've got a centre tank, and then numbered tanks — No. 3, No. 4, and No. 4A — feeding engines No. 3 and No. 4. There's also a stabilizer tank. Now, notice the valves. There's an inter-engine valve, a cross-feed valve, and a fuel transfer cross-feed valve. These let you move fuel between tanks or feed an engine from a tank that isn't its own. Then there's the fuel shut-off valve — that's the one that stops fuel reaching the engine. And for emergencies, you have a jettison master valve and a jettison valve, which let you dump fuel overboard through a refuel and jettison gallery. The refuel connection and refuel valve handle the ground fuelling. You've got a float-operated vent valve and a float switch, a diffuser, a non-return valve, and booster/transfer pumps. There's also a suction valve, and a surge tank with a vent. And there's a spar valve — that's the low-pressure valve that shuts off the fuel as it leaves the tank system, before the engine fuel shut-off. So the spar valve is the first line of isolation at the tank, and the engine fuel shut-off is the second, right at the engine. Now, the part I really want you to understand is fuel quantity measurement. There are two methods, and they're fundamentally different in what they measure. The first method measures volume by varying a resistance with a float. This is normally restricted to light aircraft. It's simple, but it has two serious drawbacks: it's subject to manoeuvring error — meaning if you bank or pitch, the float moves and gives a wrong reading — and it cannot compensate for variations of density. So if the fuel gets warmer and expands, the float reads the same volume, but the mass of fuel is actually less. That's dangerous if you're planning on weight. The second method measures weight or mass by varying capacitance. This is essential on modern passenger aircraft. It does not suffer from manoeuvring error, and it can compensate for variations of density. This is the one we use on transport-category jets, so let's dig into how it works. The capacitive method works by supplying the two plates of a capacitor with AC — alternating current. Now, the current that flows in the circuit depends on four factors: the level of voltage applied, the frequency of the supply, the size of the plates, and the dielectric constant of the material separating the plates. In our circuit, three of these factors are fixed — voltage, frequency, and plate size. The fourth, the dielectric constant, is variable, because the dielectric — the material between the plates — consists of fuel and air. Here's the key relationship: the higher the level of fuel in the tank, the more fuel and less air will be in the capacitor probe, and vice versa. Fuel has a different dielectric constant than air, so the capacitance changes with the fuel level. The amount of current flowing in the circuit therefore depends on the amount of fuel and air between the plates. And by measuring this current, we get an accurate indication of the mass of fuel in the tanks. That's the crucial point — we're measuring mass, not volume. Now here's the elegant part. The system can be made sensitive to the specific gravity — that is, the density — of the fuel. So although the volume of a quantity of fuel may increase with a temperature rise, the resulting decrease in the specific gravity will ensure that the indicated mass — the weight — remains the same. In other words, when fuel heats up and expands, its volume goes up but its density goes down, and the capacitance reading tracks the density change so the mass reading stays correct. That's the density compensation. And finally, attitude compensation. To compensate for changes in aircraft attitude, the capacitive system may have many capacitor probes in the tank connected in parallel, to "average" the measurement of the fuel in the tank. If the aircraft banks, one probe might see more fuel and another less, but because they're averaged in parallel, the system gives an accurate indication irrespective of the aircraft attitude. So to tie it together: the float-and-resistance method gives you volume and lies to you in manoeuvres and with temperature. The capacitance method gives you mass, corrects for density changes with temperature, and averages multiple probes to stay accurate in any attitude. That's why modern passenger aircraft rely on it.