Aircraft Fuel Systems — Page 321, Lesson 398

Let’s start with the big picture, because the whole job of an aircraft fuel system is one sentence: it stores fuel and delivers it to the engine fuel system. And there’s a hard requirement built into that — the system must be capable of delivering more fuel than the engine can possibly use in its most critical phase of flight, so the engine is never starved. That margin is the whole point. You design for the worst case, not the average. Now, let’s look at the simplest case first: the simple, light aircraft fuel system. Here the tanks are usually rigid tanks fitted in the wings, and they’re filled by the overwing method — that’s an open line through a filler cap in the top of the tank. So you open the cap on top of the wing and pour fuel straight in. These aircraft can use one of two delivery methods: a gravity feed system, or one using a pressure pump. Let’s take the gravity feed system first, because it’s the most intuitive. Many high-wing single-engine aircraft use it. The principle is exactly what the name says — gravity does the work. This works when the fuel tank sits high enough above the carburettor to provide the pressure required at the carburettor float chamber. So the height difference between the tank and the carburettor creates the pressure head that pushes fuel down to the engine. That’s why it’s only practical on high-wing aircraft — the wing tanks are above the engine. If you want to see the layout, look at Figure 16.1, the single-engine light aircraft gravity feed fuel system. Now the other option for light aircraft is the pressure fed system. It’s similar to the gravity system, but the fuel is delivered by a pressure pump instead of by height alone. Here’s the flow path: the fuel is drawn from the tanks by a mechanical or electrical fuel pump, through a tank selector and a filter, before being delivered to the carburettor. So you have a tank selector — that’s the valve you use to choose which tank feeds the engine — and a filter to clean the fuel on its way through. There are two more things in this system worth naming precisely. First, engine priming. That’s achieved by a priming pump, which takes fuel from the filter housing and delivers it to the inlet manifold. So when the engine is cold and you need to get it started, the priming pump pulls fuel from the filter housing and sprays it into the inlet manifold to get the mixture ready. Second, the system is monitored for contents and pressure — you have gauges telling you how much fuel is in the tanks and what pressure the fuel is at. And there are fuel drains, which allow any water to be removed before flight. Water is heavier than fuel, so it settles at the bottom, and you drain it off on the ground before you fly. Figure 16.2 shows this whole single-engine light aircraft fuel system. Now let’s step up to multi-engine aircraft, because the systems get more complex. The reason is that multi-engine aircraft have extra requirements for altitude and engine configuration. The fuel tanks are invariably integral tanks — that means the tank is built into the wing structure itself, not a separate rigid container — and they’re in the wings. Most modern aircraft may also have a centre tank, which is a tank in the centre section torque box between the wings. And there are aircraft fuel systems that include fuel tanks in the empennage — that’s the fin or the stabilizer at the tail. Those tail tanks do two jobs: they increase the fuel capacity, and they can be used to affect the aircraft centre of gravity. So by putting fuel in the tail, you can shift the centre of gravity aft or forward as needed. Finally, let’s look at the vent system, because that’s a critical part of the multi-engine setup. The vent system may include vent valves and a vent surge tank. Its job is to allow the air pressure above the fuel in the tank to equalize with the ambient pressure — the pressure outside the aircraft. It may also provide for ram air to be introduced, which partially pressurizes the tanks in flight. That does two things: it assists the fuel flow, and it helps to reduce fuel boiling at altitude. Remember, at high altitude the air pressure is low, and low pressure makes fuel boil more easily — so a little pressurization helps prevent that. Now, if any fuel overflows into the vent system, it’s collected by the vent surge tank and recycled back to the main tanks. So you don’t lose that fuel. And there’s a specific regulatory number here: the vent space in each fuel tank, as required by CS-23 and CS-25, is 2% of the tank volume. That’s the space left above the fuel to allow for expansion and venting — two percent of the tank’s volume, mandated by the certification standards. So to tie it together: simple aircraft use rigid wing tanks with gravity or a pump; multi-engine aircraft use integral wing tanks, possibly a centre tank and tail tanks, and a vent system that equalizes pressure, can pressurize with ram air, recycles overflow, and keeps a 2% vent space per the regulations.