Autopilot — Page 368, Lesson 440

I want to walk you through the final stretch of the autopilot chapter, and we're starting with something that operates independently of the autopilot itself — the Mach trim system. Let me set the scene. In aeroplanes that fly at high subsonic speeds, there's a phenomenon called Mach tuck. As the aeroplane approaches its critical Mach number — that's the speed at which airflow over some part of the aircraft first reaches the speed of sound — the centre of pressure moves aft. Now, centre of pressure is the point where the aerodynamic forces act, and when it shifts backwards, it creates a nose-down attitude. That nose-down tendency is Mach tuck. It's a serious condition because it wants to push the aeroplane into a dive. The Mach trim system automatically trims out this condition. It's armed at all stages of flight, but it only actually activates at high subsonic speeds — so it's sitting there ready throughout the flight, but it only does its work when the speed gets high enough for Mach tuck to become a threat. And crucially, it operates independently of the autopilot. That's the key point I want you to hold onto: Mach trim is its own system, not a function of the autopilot. Now, yaw dampers — I'll mention them only briefly because they're covered in detail in another chapter. They're another example of an inner loop system. That phrase, inner loop, is going to matter in a moment, so keep it in mind. Let's move to fly-by-wire, because this is where the architecture changes fundamentally. In a conventional aeroplane, the pilot's control inputs move the control surfaces directly through cables or hydraulics. In a fly-by-wire aeroplane, the pilot's input — or the autopilot's input — goes to a flight control computer. The computer translates that input into an aircraft attitude, using all three axes — pitch, roll, and yaw. And here's the remarkable part: it will maintain that attitude until another input is made. Even if configuration changes, or thrust changes, or speed changes — any of which would normally affect the pitch attitude — the flight control computer holds the selected attitude regardless. Now, here's the trimming logic. Any load being maintained by the flight control computer gets trimmed on a regular basis, so the aeroplane is always in trim. Let me give you the pitch example, because it's the clearest. 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 any future use. So the elevator does the immediate work, and the stabilizer takes over the sustained load — that's the trim function. But there's a safety net. There are mechanical trim backups in pitch and yaw that override the automatic flight control computer trim. In pitch, you operate this by moving the pitch trim wheel. 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 always has a mechanical way to take control of trim, even if the computer is doing something the pilot doesn't want. Now let's look at the big picture with outer loop control, also known as flight path modes. The primary function of an Automatic Flight Control System — the AFCS — is stabilization. That's the inner loop. But the AFCS can be developed further to control the path of the aircraft, horizontally or vertically, to predetermined conditions. For example, holding a selected airspeed, or a selected altitude, or a selected magnetic heading, or intercepting and tracking radio beams from ground-based aids. The number of inputs available is 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. And here's the clever engineering trick. The outer loop inputs are applied to the inner loop in such a way as to fool the auto-stability control into believing the aircraft is being disturbed. So instead of the outer loop directly commanding the surfaces, it feeds a disturbance into the inner loop, and the inner loop reacts to correct it — which is exactly the correction the outer loop wanted. That's how the flight path modes ride on top of the stabilization system. Let me tie this together. You have three layers. The inner loop does stabilization — that's where the yaw damper lives. The outer loop does flight path control — airspeed, altitude, heading, radio beams. And the Mach trim system sits independently, guarding against Mach tuck at high subsonic speeds. In a fly-by-wire aeroplane, the flight control computer handles the attitude and trims it out via the stabilizer, with mechanical backups in pitch and yaw as the final authority. That figure shows you the auto-trim failure light — the cockpit indication you'd see if this trimming system has a problem. And the earlier figure, the trimming by variable incidence tailplane, shows how the output is applied to the trim tab actuator or the horizontal stabilizer. So the whole story is about layers of control and layers of backup — the computer trims, the mechanical wheel overrides, and the Mach trim system watches over the high-speed regime independently. That's the complete picture of how the autopilot and its supporting systems work together.