Control Laws — Page 420, Lesson 522

Let’s start with the idea of gain scheduling — and I want you to hold onto the Flight Director comparison, because that’s the key to understanding why the autopilot does this. The Flight Director System uses gain scheduling to reduce its demands when the aircraft is close to the ground. The reason is safety: if the Flight Director demanded a manoeuvre near the ground, that manoeuvre could endanger the aircraft. So the system deliberately softens its commands as the aircraft gets low. The autopilot has a comparable system. It’s the same principle — the autopilot’s authority is reduced as the aircraft nears the ground. Authority here means how much control the autopilot is allowed to exert — how much pitch and roll it can command. Let me give you a concrete example. Take an aircraft whose autopilot, in manual mode, may have its bank angle limited to 45°. That’s the maximum bank it can command when you’re flying manually. Now, during VOR or Localizer tracking — that is, when the autopilot is following a VOR radial or the localizer beam of an ILS — that limit may be reduced to 30°, because 30° is deemed all that is necessary for that phase. But during the final phase of an automatic approach, or during an autoland, the limit may be reduced further, to 15°. So you see the pattern: the closer to the ground and the more critical the phase, the less authority the autopilot has. Now, gain adaption isn’t only about proximity to the ground. It can also be used to alter the autopilot’s limits to allow for differing aircraft performance at different altitudes and speeds. The aircraft behaves differently at high altitude versus low altitude, and at high speed versus low speed — so the autopilot’s limits need to adapt to those conditions. Here’s the crucial safety point. Artificial feel is provided to give pilots awareness of control forces — that’s the system that makes the controls feel heavier or lighter so you can sense what the aircraft is doing. But autopilots could easily ignore artificial feel inputs and overstress the aircraft. In other words, the autopilot doesn’t feel the forces the way you do, so it could command something that puts too much load on the structure. To prevent that, an input from the ADC — that’s the Air Data Computer — may be used to reduce the autopilot’s authority in proportion to Q. Q is dynamic pressure — the pressure you feel from the aircraft’s motion through the air, which increases with speed and with air density. So as Q increases, the autopilot’s authority is reduced proportionally. That way, at high speed or high dynamic pressure, where overstressing is more likely, the autopilot is deliberately limited. So the whole picture is this: gain scheduling and gain adaption are the autopilot’s way of managing its own authority — reducing it near the ground, reducing it during critical approach phases, and reducing it in proportion to dynamic pressure — all to keep the aircraft safe and within its structural limits.