Let’s start a brand-new topic: the Auxiliary Power Unit, or APU — Page 350, Lesson 438

Let’s start a brand-new topic: the Auxiliary Power Unit, or APU. I want you to think about why it exists in the first place. As aircraft became more complex, a need was created for a power source to operate the aircraft’s systems on the ground without having to run the main engines. That became the task of the Auxiliary Power Unit. And there’s a second benefit: with an APU on board, the aircraft is no longer dependent on ground support equipment at an airfield. So it’s self-sufficient. What does the APU actually power? It can provide the necessary power for operation of the aircraft’s Electrical, Hydraulic, and Pneumatic systems. And it should come as no surprise that the power unit selected to do this job is a gas turbine engine. Why a gas turbine? Because the gas turbine produces very high power for a light weight, which makes it ideal for this task. Now, a nice practical point: the APU can use the same fuel system as the main engines, so that reduces the need for additional systems. The type of engine layout normally used is the Free Turbine, Turboshaft Engine. Let me unpack that. A turboshaft engine is both small and lightweight, yet it produces around 600 horsepower. The free turbine arrangement makes the engine very flexible, because the compressor is not affected by changes of load on the free turbine. The free turbine is the one that drives the accessories via a gearbox. So if the load on that turbine changes, the compressor keeps running steadily — that’s the flexibility. The free turbine is usually designed to run at constant speed. That ensures a generator run by the APU maintains a constant frequency without the need for an additional constant speed drive unit. So you get stable electrical output without extra hardware. Now, how does the APU supply pneumatic power? Some aircraft use air bled from the compressor of the APU to power the aircraft’s pneumatic system. But it’s more common for the free turbine to drive a separate Load Compressor to supply those services. So instead of bleeding air off the main compressor, you have a dedicated compressor driven by the free turbine. A typical layout for an APU is shown in Figure 24.1 — that’s the free turbine turboshaft APU. So to tie it together: the APU is a small gas turbine, specifically a free-turbine turboshaft, that gives you electrical, hydraulic, and pneumatic power on the ground without the main engines, and it frees you from ground support equipment. The free turbine runs at constant speed to keep generator frequency stable, and it usually drives a separate load compressor for pneumatic services. That’s the core of the APU.