Gas Turbines - Fuel Systems — Page 385, Lesson 478

I want to walk you through the fuel system of a gas turbine, starting with a small but important component: the drains tank. A drains tank is a small tank that collects unburnt fuel from the fuel manifold and the combustion chamber after the engine is shut down, or after a failed start. Think about what happens when you shut an engine down — there's still fuel sitting in the lines and in the combustion chamber that didn't get burned. That fuel has to go somewhere, so it drains into this tank. Now, here's the key detail: when the engine is running, a pressure-operated non-return valve isolates the drains tank. That means the pressure inside the engine, when it's operating, closes that valve so the tank is sealed off. It only opens to collect fuel when the engine is off or after a failed start. Now let's move to the bigger picture: electronic engine control. I want you to understand the goal first, because everything else hangs off it. The goal of any engine control system is to allow the engine to perform at maximum efficiency, within the design safety limits and operating parameters, for any given condition. And here's a crucial relationship: the complexity of this task is proportional to the complexity of the engine. A simple engine needs a simple control system; a complex engine needs a complex one. Let's look at the history, because it explains why we have the systems we have today. In the early days of engine design, the pilot had direct, full control of the engine from start to shutdown. That's a big statement — the pilot personally did everything. He or she had the task of starting the engines; deciding and controlling the power requirement for the stage of flight; monitoring the performance and condition indicators; and shutting the engine down if safety parameters were exceeded. What did the pilot have to work with? A bank of gauges, and a simple mechanical linkage between the throttle lever in the cockpit and the fuel control unit on the engine. That was it. Let me unpack that. The throttle linkage fed pilot inputs to a fuel control unit — abbreviated FCU — which was mounted on the engine. The FCU regulated the fuel flow according to acceleration, deceleration, and altitude requirements. So as you moved the throttle, that mechanical linkage told the FCU what you wanted, and the FCU adjusted fuel flow based on whether the engine was accelerating, decelerating, or at a given altitude. In addition, the FCU had an inbuilt rpm sensor that prevented the engine from overspeeding. So even in the early mechanical days, there was a built-in protection against the engine spinning too fast. Then, in the 1960s, the analogue electronic engine control came into being. Here's the key change: the mechanical inputs to the FCU, which were used to communicate the desired engine settings, were replaced by electrical inputs instead. So instead of rods and linkages carrying your throttle command, it was now an electrical signal. This system was an improvement over the mechanical control system, but it had its own drawbacks — including electronic noise interference. That's a real problem: stray electrical noise can corrupt the signal. And here's a nice historical anchor: this analogue electronic engine control was first introduced as a component of the Rolls Royce Olympus 593 engine — that's the engine that powered Concorde. So where are we now? We've got the drains tank handling residual fuel, and we've traced engine control from full manual mechanical control, to electrical inputs replacing mechanical ones in the 1960s. That sets us up perfectly for what comes next — the move toward supervisory electronic engine control and then full-authority digital engine control, which is FADEC.