Aircraft Fuel Systems — Page 321, Lesson 400

I want to walk you through the fuel system components that go beyond what we saw in the light aircraft. We're now looking at the larger, multi-engine jet aircraft systems, and the components here are the building blocks you'll see on the schematic in Figure 16.3. Let's start with the filters, which we also call screens. Their job is to prevent any debris in the tank from being drawn into the booster pumps. Think of them as the first line of defense—they keep the fuel clean before it ever reaches the pumps. Now, the booster pumps. These are normally fitted in pairs in each tank, and their job is to pump fuel from the tank to the engine. They're a necessity in high-altitude aircraft because they prevent cavitation of the engine-driven pump. Let me explain cavitation: it's when the fuel pressure drops so low that the fuel starts to vaporize, forming bubbles. Those bubbles can damage a pump. The booster pumps prevent that by keeping the fuel pressure up. They're typically centrifugal pumps driven by AC induction motors, and they provide low pressure—20 to 40 psi—but high flow. Here's an important operational point: if you have a double booster pump failure in one main tank, the aircraft's Minimum Equipment List—the MEL—will invariably limit the aircraft to a maximum operating altitude. That's to prevent fuel starvation, because without those pumps, you can't maintain the pressure needed at altitude. Next, we have the collector tank, which is also called the feeder box. The booster pumps are fitted inside this collector tank, and it always holds a measured quantity of fuel—typically 500 kilograms. Why? So the pumps can be continually submerged in fuel. That prevents pump cavitation due to attitude changes of the aircraft, which could otherwise cause the pumps to become uncovered. If the pumps are uncovered, they can cavitate and fail. The collector tank may also have a facility that lets you replace the pumps without draining all the fuel from the main tank—that's a maintenance convenience built into the design. Then we have the cross-feed and shut-off valves. These enable fuel to be fed from any tank to any engine, and they allow you to isolate a tank in the event of a fault or emergency. So if one engine fails, or one tank has a problem, you can shut it off and still feed the other engine from another tank. Now, the high and low level float switches, or level sensors. The high level switches are used to automatically close the refuel valve when the tank is full—that's the automatic top-off during refuelling. The low level switches are used to maintain a required minimum fuel in the main tanks during fuel jettison or dumping. So if you need to dump fuel to reduce weight, the low level switches make sure you don't dump below a safe minimum. We also have fuel drains. Just like in a light aircraft, each fuel tank has a fuel drain at the lowest point in the tank, and its purpose is to allow water to be drained from the tank. Water is heavier than fuel, so it settles at the bottom, and you drain it off during pre-flight checks. Next, baffles. These are fitted in the tanks to dampen rapid movement of fuel—what we call surging or sloshing—during manoeuvring. Without baffles, the fuel would slosh around and could cause control problems or structural stress. Finally, the overpressure relief valve. In the event that the fuel tank is over-pressurized due to a malfunction, this relief valve may be incorporated to prevent structural damage to the tank. It's a safety valve that lets excess pressure escape. Now, let me point out something important about the wing tanks on a typical two-engine jet. The wing tanks are split into two elements: outer and inner sections. This is sometimes incorporated to allow a certain amount of fuel to remain in the outer section until the inner section has reached a pre-determined level. Why do we do this? Keeping fuel outboard in this manner helps to reduce wing bending stress and relieve flutter. So it's a structural design consideration—by keeping fuel in the outer wing, you're actually helping to manage the loads on the wing structure. That's the full set of components you'll see on the schematic in Figure 16.3. Each one has a specific job, and together they form the fuel system that feeds the engines safely and reliably.