Autopilot — Page 363, Lesson 435

Let’s start with the trim system, because it’s the foundation for everything else here. The purpose of the trim system is to relieve the pilot of forces on the aircraft controls while maintaining aircraft attitude. So, in manual flight, you’re holding the controls, and the trim system takes away the steady pressure you’d otherwise have to keep applying. In manual control on light aircraft, trim is provided in all three axes through mechanical linkages to trim tabs on the control surfaces. So you have elevator trim, aileron trim, and rudder trim, each working through a mechanical connection to a small tab on the control surface itself. On larger aircraft, this is usually achieved by electrical actuators that bias the Powered Flying Control Unit — the PFCU — particularly for pitch trim. So instead of a mechanical linkage, you have an electric actuator doing the work. Trim steering signals are provided to the trim tab actuators for elevator, aileron, and rudder, as well as to the horizontal stabilizer. So the trim system doesn’t just move the tabs; it also moves the horizontal stabilizer for pitch. Now, an important point: manual operation of the pitch trim will, in most systems, automatically disconnect the autopilot. Why? Because the autopilot cannot co-ordinate manual trim movement with movement of the centre of gravity or aerodynamic movements. So if you grab the pitch trim manually while the autopilot is flying, the system can’t keep up with what you’re doing, so it drops out. That leads us to automatic trim — auto-trim. Here’s the problem it solves. As the aircraft uses fuel, or changes speed, thrust, or configuration, the pitch attitude will change. If you’re flying manually, you’d trim those forces out by hand to eliminate stick forces. But our basic autopilot, as it stands, if it’s flying the aircraft and the pitch requirement changes, it will simply hold the stick forces using the brute strength of its servomotor outputs. So the autopilot just muscles the controls to hold attitude, rather than trimming. That’s not a problem unless the forces overwhelm the servomotor, or until you want to disconnect the autopilot. And here’s the real danger: if there’s a standing load on the controls when you disconnect, you won’t know it exists, and you won’t know which way the aircraft will pitch. When you disconnect, the aircraft will “snatch” or lurch in response to the out-of-trim condition. Not only that, but the aircraft is producing more drag than necessary while the autopilot is fighting the controls. That’s not satisfactory, so a system of Automatic Pitch Trim was included in most autopilot systems. Let me show you what the actuator looks like in parallel — this is the arrangement where the autopilot actuator works alongside the pilot’s controls rather than replacing them. And here’s the switching arrangement — though in modern aircraft the actual switching is more likely to be accomplished electronically rather than mechanically. So the key idea: the trim system relieves control forces, and auto-trim lets the autopilot do that trimming automatically, so that when you disconnect, the aircraft is in trim and won’t lurch. That’s the core of what we’re building on.