Aircraft Fuel Systems — Page 327, Lesson 411

I want to walk you through the fuel quantity measuring systems and the instrumentation that backs them up. We're starting with a critical safety principle: how a capacitive gauging system fails. If a capacitive gauging system fails, it does so in a way that draws your attention. There's a fail-safe circuit built in that drives the gauge pointer slowly towards the empty position. The whole point is to prevent the indicator from showing there's more fuel in the tank than there actually is. That's the cardinal rule — never let the pilot believe there's fuel when there isn't. Some systems also have a test switch that uses this same fail-safe circuit. When you operate the test switch, the indication moves towards empty, and when you release it, the pointer should return to its original position. That's your functional check that the fail-safe is working. Now, what happens if the electronic measuring system itself fails? We still need to determine the quantity of fuel. The simplest method is a dipstick used from the top of the tank. But that exposes you to the dangers of walking on high, slippery surfaces — not ideal. A better method is the 'dripstick'. This is a calibrated hollow tube that you withdraw from the under surface of the tank through a fuel-proof aperture. When the top of the tube becomes lower than the fuel level, fuel drips through the tube — hence the name. You establish the volume of fuel by reading the calibrations on the tube. The disadvantage? Your armpit soon becomes saturated with fuel dripping from the pipe. Not pleasant. A more user-friendly version is the 'dropstick', also called the Magnetic Level Indicator, or MLI. Here's how it works. The tube becomes a rod, calibrated to show the fuel level. The rod sits inside a fuel-proof tube in the tank. Around that tube is a magnet supported on a float. The tip of the rod also has a magnet. When you lower the rod through the tube, the fields of the two magnets interact. The length of rod protruding from the underside of the wing tells you the fuel level. Then, by referencing the aircraft manual and using the density of the fuel, you can establish the mass of fuel in the tanks. Notice the distinction — the rod gives you level, and you convert that to mass using density and the manual. Now let's move to fuel system instrumentation. On a light aircraft, this consists of contents and pressure gauges. But on a large aircraft, you need far more information. Not just quantity and pressure, but also fuel used, and the position of valves — things like cross-feed, inter-engine, and firewall shut-off valves. You also need to know whether pumps are on or off, and the fuel temperature. These indications are usually presented as 'mimic' diagrams — schematic representations of the fuel system — with 'doll's eyes' and lights on the flight engineer's panel, or as electronically presented schematic displays. A typical example is the Airbus Electronic Centralized Aircraft Monitoring system, which we call ECAM. The Boeing equivalent is the Engine Indicating and Crew Alerting System, or EICAS — a similar display. So to tie it together: the fail-safe circuit protects you from false high readings, the dripstick and dropstick give you manual fallbacks when electronics fail, and the instrumentation — from simple gauges to ECAM and EICAS — gives you the full picture of quantity, pressure, valve positions, pump status, and temperature.