Aircraft Fuel Systems — Page 321, Lesson 402

I want to walk you through the fuel system of a twin-engine aircraft — this is the heart of how we manage fuel in the air, and it's a system you'll operate on every single flight. Let's start with the normal sequence of fuel usage after take-off. The design intent is to use the centre tank fuel first, followed by the wing tank fuel. Why? Because this sequence helps to relieve the wing bending stress. Think about it — the centre tank sits right at the aircraft's centre of gravity, so burning that fuel first keeps the heavy mass close to the fuselage, reducing the bending moment on the wings. That's a structural reason, not just a convenience. Now, when the booster pumps can no longer pump fuel from the centre tank — meaning the tank is nearly empty — the residual fuel can be removed to the No.1 tank by use of the centre tank scavenge system. So there's a dedicated scavenge system that transfers that last bit of centre tank fuel into the No.1 tank, so nothing is wasted. Let's look at the cross-feed valve. This is a critical component. It allows both engines to be fed from one side, or one engine to be fed from both sides. So if you have an imbalance or a pump failure, you can use the cross-feed to redistribute fuel flow between the engines. Next, the suction valves. These are in the tanks, and they allow the engine to be fed by gravity or by suction from the engine-driven pump in the event of both booster pumps failing in one tank. So if both electric booster pumps in a tank fail, the engine can still draw fuel through the suction valve — either by gravity feed or by the mechanical engine-driven pump pulling fuel through. That's your redundancy. Now, the control panel. It shows selector switches for each pump, accompanied by low pressure warning lights. These lights indicate pump failure or low fuel level. So if a pump fails or the fuel level drops, you get a low pressure warning. There's also a control switch and indicator light for the cross-feed valve — so you can select it open or closed and see its position. There's a temperature sensor in the No.1 tank which transmits the fuel tank temperature to an indicator on the control panel. This is important for monitoring fuel temperature, particularly for icing considerations. Now, the engine fuel shut-off valve. This is closed by the operation of the fire handle for that particular engine. In some aircraft, it's also operated by the selection of the fuel switch during the normal start or shutdown procedure. So the fire handle is the primary way to shut off fuel to an engine in an emergency, and in some designs, the normal fuel switch also does this during start or shutdown. The APU — the Auxiliary Power Unit — takes its fuel from the No.1 tank from a bypass valve if there are no booster pumps operating. But it could be fed from any tank if a booster pump in that tank was selected on. So the APU has a bypass valve that lets it draw fuel directly from the No.1 tank without a pump, but if you want to feed it from another tank, you'd select that tank's booster pump on. The APU shut-off valve is typically operated by the automatic start or stop sequence — so it opens and closes automatically as part of the APU's start and stop cycle. Finally, fuel imbalance in flight between the No.1 and No.2 tank can be corrected by selective switching of the booster pumps and cross-feed valve. The procedure is: open the cross-feed and switch off the pumps in the tank with less fuel until the correct balance is achieved. So you're using the cross-feed to let the engine draw from the fuller tank, while shutting off the pumps on the lighter side to let it equalize. Let me show you the schematic so you can see how all these components connect. Now, let me walk you through the schematic in detail. You have the No.1 tank and the No.2 tank, one on each side. There's a centre tank in the middle. Each tank has fuel pumps — the centre tank has its own fuel pumps, and there are engine-driven fuel pumps. The fuelling station is where you fill the aircraft, and there's a manual defuelling valve for removing fuel. There's a cross-feed valve that connects the two sides, and there are fuel valves that can be open or closed. You'll see low pressure indicators throughout — these are the warning lights I mentioned. There's a fuel temperature sensor, and there are filter bypass valves — these allow fuel to bypass a clogged filter so the engine still gets fuel. There's a scavenge jet pump in the centre tank for that residual fuel transfer, and there's a fuel scavenge shut-off valve. For the APU, you have an APU fuel shut-off valve and an APU bypass valve. And for the engines, you have engine fuel shut-off valves. There's also a suction valve in the tanks for that gravity or suction feed I mentioned. So the whole system is designed with redundancy — multiple pumps, suction valves, bypass valves — so that even with failures, you can keep fuel flowing to the engines. That's the core of what you need to understand about this system.