Think about a pilot flying straight and level — Page 358, Lesson 420

I want to walk you through the heart of what an autopilot actually does, and I'll start with the pilot's own actions, because the autopilot is really just an imitation of them. Think about a pilot flying straight and level. A disturbance hits the aircraft — a gust of wind, say. The pilot senses the change in attitude, works out what correction is needed, and applies it with the controls. The aircraft then manoeuvres about its centre of gravity back towards its original position. The pilot senses that the corrective action has taken place, and moves the flight controls back to remove the correcting input. So the loop is: sense, compute, apply, verify, and re-centre. Now, the autopilot is capable of producing the same actions as the pilot to maintain aircraft stability, but in a shorter reaction time. It must detect the disturbance, work out the appropriate corrective action, and then apply that correction using its "muscles" — that's the key word. Those muscles cause the aircraft to manoeuvre about its centre of gravity back towards its original position. Finally, it must detect that the correction has taken place and re-centre the controls. This whole process is known as inner loop control. You'll also hear it called closed loop, or auto-stabilization. Let me give you the list form, because it's the exact sequence of what the system does: First, it senses changes in attitude. Second, it computes the amount and direction of control required. Third, it provides the muscle to move the control surfaces — and that muscle comes from servomotors. Fourth, it detects that the control has been applied and that the aircraft has responded. And fifth, it returns the control surfaces back to the neutral condition when the disturbance has been corrected. Now, you might wonder — and this is a fair question — why bother installing this basic autopilot at all, since aircraft are naturally stable anyway? Yes, they are naturally stable, but here's the subtlety: there are different types of stability, and stability can change with respect to the ambient conditions. So the basic autopilot augments and fine-tunes that natural stability, providing enhanced stability over a greater range of ambient conditions. And I want to emphasize one thing clearly: the most basic autopilot available will be of a type that only provides auto-stabilization. That's it — nothing more. It's the foundation, and everything more advanced builds on this inner loop concept. Let me make sure the terminology is solid. Inner loop control is the full sense-compute-apply-verify-recentre cycle. Auto-stabilization is the same thing by another name — it's the autopilot maintaining stability. And servomotors are the muscles — the devices that physically move the control surfaces. Those are the three terms you need to hold onto from this passage.