Control Laws — Page 420, Lesson 522

Let’s pick up the idea of autopilot gain adaption, because that’s the heart of this passage. I want you to think of the autopilot as a system that has authority — that is, how much control input it’s allowed to make. That authority isn’t fixed. It changes depending on the situation, and that change is called gain adaption, or gain scheduling. Here’s the key comparison. The Flight Director System uses gain scheduling to reduce its demands when the aircraft is close to the ground. Why? So that the FD system doesn’t demand a manoeuvre that would endanger the aircraft. The autopilot has a comparable system — it does the same kind of thing. So during an autoland, for example, the autopilot’s pitch and roll authority is significantly reduced as the aircraft nears the ground. Let me give you a concrete example from the text. Imagine an aircraft whose autopilot, in manual mode, has its bank angle limited to 45°. That’s the maximum bank it’s allowed to command when you’re flying it manually. Now, during VOR or Localizer tracking — that is, when you’re tracking a VOR radial or the localizer beam of an ILS — that limit may be reduced to 30°, because 30° is deemed all that’s necessary for that task. But here’s the crucial part: during the final phase of an automatic approach, or during an autoland, that bank angle limit may be reduced further, down to 15°. So you see the pattern — as the task gets more critical and the aircraft gets closer to the ground, the autopilot’s authority is progressively dialled back. 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. Think about it — an aircraft behaves differently at high altitude and low speed than it does at low altitude and high speed. The autopilot needs to adapt its authority to match. Here’s where it gets interesting, and I want you to note this carefully. 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 — to the autopilot may be used to reduce the autopilot’s authority in proportion to Q. And Q here is dynamic pressure — the pressure you feel from the airflow as the aircraft moves. So as dynamic pressure increases, the autopilot’s authority is reduced proportionally, protecting the aircraft from overstress. So let me pull that together. Gain adaption, or gain scheduling, is the autopilot’s way of adjusting its own authority based on the situation. It reduces authority near the ground, like during autoland, to keep manoeuvres safe. It reduces authority during specific tracking tasks, like VOR or Localizer tracking, to what’s necessary. And it reduces authority in proportion to dynamic pressure, using input from the Air Data Computer, to prevent overstressing the aircraft. That’s the core of this passage — the autopilot isn’t a fixed-authority system; it’s constantly adapting to keep the aircraft safe.