Autopilot — Page 368, Lesson 440

I want to walk you through the autopilot chapter now, and we're picking up right where the systems get interesting. We've got three distinct topics here: independent systems, fly-by-wire, and outer loop control. Let's take them one at a time. First, independent systems. There are two systems that work on their own, separate from the autopilot itself. The first is Mach trim. Now, Mach trim is required on aeroplanes that fly at high subsonic speeds and are susceptible to something called Mach tuck. Let me explain that. As the aeroplane approaches its critical Mach number, the centre of pressure moves aft. That movement causes a nose-down attitude, and that nose-down condition is what we call Mach tuck. The Mach trim system automatically trims that condition out. Here's the key detail: the Mach trim system is armed at all stages of flight, but it only actually activates at high subsonic speeds. So it's always ready, but it only does its job when the speed gets high enough for Mach tuck to become a threat. The second independent system is the yaw damper. I'll note that yaw dampers get covered in detail in another chapter, but I want you to know they're another example of an inner loop system. That's a term we'll come back to in a moment. Now let's move to fly-by-wire, because this is a fundamental difference in how the aeroplane is controlled. In a fly-by-wire aeroplane, the pilot or autopilot input goes through a flight control computer. That computer translates the input into an aircraft attitude using all three axes — pitch, roll, and yaw — and then it maintains that attitude until another input is made. Here's the crucial part: even if configuration, thrust, or speed changes are made — changes that would normally affect the pitch attitude — the flight control computer will still maintain the attitude that was selected. It holds what you asked for, regardless of those disturbances. Now, any load that the flight control computer is maintaining gets trimmed on a regular basis, so the aeroplane is always in trim. Let me give you the pitch example. In pitch, the flight control computer deflects the elevators using an electric servomotor to reach the correct aircraft attitude selected by the pilot or autopilot. After a short period of time, that deflection gets trimmed using the horizontal stabilizer. That allows the elevator to return to its neutral position, ready for future use. So the elevator does the initial work, and then the stabilizer takes over the load so the elevator is free again. There's an important safety feature here: mechanical trim backups exist in pitch and yaw, and they will override the automatic flight control computer trim. Let's look at the pitch trim backup specifically. It's operated by moving the pitch trim wheel. Moving that wheel causes the horizontal stabilizer to move, the autopilot to disconnect, and it overrides the flight control computer's pitch attitude. So the pilot has a direct mechanical way to take control of pitch, and it forcibly kicks the autopilot off. Finally, let's talk about outer loop control, which is also known as flight path modes. Beyond the primary job of stabilization, an Automatic Flight Control System — that's the AFCS — can be developed to control the path of the aircraft horizontally or vertically to predetermined conditions. For example, it can hold a selected airspeed, a selected altitude, a magnetic heading, or intercept and track radio beams from ground-based aids. The number of inputs available is actually an indication of how far automatic flight has progressed — from the basic single-axis wing-levelling autopilot all the way up to the sophisticated flight guidance systems in modern transport aircraft. Here's how the outer loop actually works with the inner loop. The outer loop inputs are applied to the inner loop in such a way as to fool the auto-stability control into believing that the aircraft is being disturbed. The inner-loop system then reacts to that perceived disturbance. So the outer loop doesn't directly command the control surfaces — it feeds the inner loop a signal that looks like a disturbance, and the inner loop does the actual stabilising work. That's the clever part of the architecture. Let me show you the trim mechanism we just discussed, because the variable incidence tailplane is exactly what's doing that stabilizer work. And here's the auto-trim failure light, which is what you'd see in the cockpit if that trim system has a problem. So to tie it together: you've got independent systems like Mach trim and yaw dampers that work on their own, a fly-by-wire architecture where the computer holds attitude and trims continuously with mechanical backups, and then the outer loop that shapes the flight path by fooling the inner loop. That's the full picture of how these systems layer on top of each other.