Aircraft Fuel Systems — Page 316, Lesson 396

I want to walk you through the fuel system chapter, and we're starting with a phenomenon that can literally stop an engine in flight: boiling. Let's get the physics straight first, because everything else builds on it. The temperature at which a fuel boils isn't fixed — it varies with the pressure on its surface. Think about that. As your aircraft climbs, the ambient pressure drops, and so does the pressure on the surface of the fuel in the tanks. With that reduction in pressure comes an increased likelihood that the fuel will boil and form vapour in the pipelines. That vapour forms what we call vapour locks, and a vapour lock effectively cuts off the fuel supply to the engine. The inevitable result is that the engine stops. So this is a real, serious failure mode, not a theoretical one. Now, how do we overcome it? We fit fuel booster pumps inside the tanks. Their job is to pressurize the fuel in the pipelines from the tank to the engine, pushing fuel towards the engine. The key contrast here is that these pumps push fuel toward the engine, rather than having engine-driven pumps suck fuel from the tanks. Pushing is far more reliable than sucking when you're dealing with vapour formation at altitude. Next, let's talk about specific gravity, which we abbreviate as SG. The specific gravity of a liquid varies inversely with its temperature — so as temperature goes up, specific gravity goes down, and vice versa. On modern aircraft this usually makes little difference, unless full tanks are required, because only the mass of the fuel load is taken into account. The fuel quantity measuring system compensates for changes in fuel specific gravity automatically. However — and this is an important limitation — the maximum governor fitted to some gas turbine engines is sensitive to changes in specific gravity. So if a different specific gravity fuel is uplifted, that governor would require some adjustment. That's a maintenance or operational consideration you need to be aware of. Now let's move into the fuel systems themselves, starting with the aircraft storage system. The fuel is carried in, or on, the aircraft within tanks, and there are three types: integral, rigid, and flexible. First, integral tanks. Here, the inside of the wings and, depending on the aircraft type, the centre section torsion box and horizontal stabilizer, are sealed during manufacture to provide large volume fuel storage. The big advantage is that there's little extra weight added to the aircraft, because the tank structure is formed by structure that's already required. All modern large passenger aircraft will have this type of tank. Second, rigid tanks. These are sealed metal containers mounted in the aircraft wing or fuselage. They're simple, but they do add extra weight and require mounting structure. They're most popular on light aircraft. This type of tank may also be fitted externally — on the wing tip, for example — and can be made of metal or a composite construction. Third, flexible tanks. These are bags made of sealed rubberized fabric, sometimes referred to as fuel bladders or bag tanks. This type requires structure inside the aircraft to attach and support it. They're typically mounted inside the wing or fuselage, and they're more popular on military aircraft because they can be effectively 'self-sealing' in the event of battle damage. Now, inside the tanks themselves, we have baffles. Baffles are fitted within the tank to minimize the large inertial forces generated when the fuel surges during aircraft manoeuvres — for example, acceleration, deceleration, or sideslip. Some large aircraft may be fitted with baffle check valves, which allow the fuel to flow inboard but not outboard towards the wingtips during manoeuvres. That's a directional control on fuel movement. Beyond that, fuel tanks also incorporate vents, water drains, feed pipes, a gauging system, and filler caps. And in larger aircraft, the tanks will also have booster pumps, high and low level float switches, pressure refuelling valves, and filters. So you can see the complexity scales with the size of the aircraft. Let me show you a diagram of a single-engine light aircraft gravity feed fuel system — this is Figure 16.1 — so you can see how these components physically fit together. That's the storage side of the system. We've covered the boiling problem and the booster pump solution, the specific gravity effects and the governor sensitivity, and the three tank types with all their internal components. That's the foundation for everything else in this chapter.