Gas Turbines - Fuel Systems — Page 376, Lesson 470

Let’s start with the big picture. The engine fuel system is the part of the aircraft that takes fuel from the airframe’s tanks and delivers it to the engine’s fuel spray nozzles at the correct rate, in proportion to throttle position. Along the way, it filters and monitors the fuel flow. Every component in that chain has a job, and I want to walk you through them in the order the fuel actually travels. First, the booster pumps. These sit in the fuel tank and push fuel out of the tank toward the engine. The fuel leaves the airframe fuel system and passes through non-return valves — those are one-way valves that stop fuel from flowing back into the tank. From there it reaches the engine fuel shut off valve, also called the pylon shut off valve. That valve’s job is to shut off the supply of fuel to the engine when you need to remove a component. It can also be closed by the fire handle if there’s an engine fire warning — that isolates the fuel from the engine. And in an emergency, you can use it to stop the engine, but the engine will take longer to run down because you’re cutting fuel rather than shutting it off instantly. Now, the fuel enters the engine fuel system proper. The next component is the low pressure pump, or LP pump, sometimes called the backing pump. It’s driven by the engine gearbox, and its job is to supply fuel to the high pressure pump. Here’s the important safety feature: if the tank booster pumps fail completely, the LP pump can actually suck fuel from the tank, keeping the engine running. But there’s a catch — in that situation, the aircraft’s MEL, the Minimum Equipment List, may require you to reduce altitude. Why? Because at altitude the fuel can vaporise, and the LP pump would suffer cavitation — that’s when the pump tries to move vapour instead of liquid, which damages it and stops it working properly. So lowering altitude keeps the fuel liquid enough for the pump to keep sucking. Next comes the cooler. In most gas turbine installations you’ll find a fuel-cooled oil cooler, abbreviated FCOC. It does two jobs at once. It cools the oil, and it heats the fuel. Why heat the fuel? To eliminate the formation of ice crystals. Water in the fuel can freeze at low temperatures, and those ice crystals could block components further downstream. So the FCOC warms the fuel enough to stop that from happening. Let me show you the layout. That figure shows the whole system from the fuel tank right through to the spray nozzles. You can see the booster pumps, the LP shut off valve, the LP pump, the cooler, the FCOC, the heater, the filter, the flow meter, the HP pump, the FCU, the HP cock, and finally the fuel spray nozzles. There’s also a drains tank, and a drain valve that opens on shutdown. And you can see the inputs the system uses — P1 intake pressure, P3 HP compressor outlet pressure, HP shaft speed, LP shaft speed, EGT, N1, N2, fuel temperature, and thermocouples. We’ll get into what each of those does as we go further, but for now, just see how the fuel path flows from tank to nozzles. Now, the excerpt continues with the low pressure pump and the cooler, and that’s where we are. The LP pump feeds the HP pump, and the FCOC sits between them in the line, warming the fuel and cooling the oil. That’s the foundation. Next we’ll move on to the filter, the flow meter, and the high pressure pump.