Then we get into the heart of the engineering: control loops — Page 352, Lesson 414

This is the start of a brand-new chapter — Chapter 26, the Autopilot. This is a big one, and it's going to tie together a lot of what you've already learned about instruments and flight control. Let me give you the roadmap of what this chapter covers, because the structure itself tells you how a professional thinks about an autopilot system. We begin with an introduction and then go straight into what the autopilot actually is. From there, we look at the fail-safe autopilot concept — that's your safety backbone. Then we get into the heart of the engineering: control loops. You'll see there's a distinction between the inner loop control system and the outer loop control, which is also called flight path modes. That inner/outer loop split is the single most important mental model for understanding how an autopilot works. Then we cover the types of autopilot, followed by the EU-OPS requirements — that's the regulatory framework that dictates what's mandatory on commercial aircraft. After that, we get physical: the types of actuator, which are the muscles that actually move the control surfaces. Then engagement criteria — the conditions that must be met before the system will even switch on. Trim comes next, which is how the aircraft is balanced in steady flight. Then fly-by-wire, which is a whole different philosophy of control. The chapter then moves into the mode annunciator — that's your display telling you what the autopilot is doing — and the aircraft sensor inputs to the autoflight computer. We look at specific examples of outer loop inputs, separately in roll and in pitch. Then we walk through the autopilot in operation, other autopilot features, and finally the autopilot limitations and operational restrictions. The chapter closes with the Flight Management System, which is the brain that sits above the autopilot and plans the whole flight. So the arc is: what it is, how it's made safe, how it's controlled, how it's regulated, how it's physically built, when it's allowed to engage, and how it integrates with the wider flight management. That's the full professional picture. Let's start at the beginning — the introduction and the definition of the autopilot itself.