Autopilot — Page 363, Lesson 435

I want to walk you through the trim system, because it's the part of the autopilot that keeps the aircraft balanced without you fighting the controls. Let's start with the purpose. The trim system exists to relieve you of forces on the aircraft controls while maintaining the aircraft attitude. In plain terms, it takes the physical pressure off your hands and feet so you don't have to hold constant push or pull on the yoke to keep the nose where you want it. In manual control on light aircraft, trim is provided in all three axes — pitch, roll, and yaw — through mechanical linkages to trim tabs on the control surfaces. So you have a trim tab on the elevator, one on the aileron, and one on the rudder, and moving the trim wheel physically moves those tabs through cables or rods. On larger aircraft, this is usually achieved by electrical actuators that bias the Powered Flying Control Unit, or PFCU — that's the hydraulic unit that actually moves the flight controls. This is particularly important for pitch trim. The trim steering signals are sent to the trim tab actuators for elevator, aileron, and rudder, and also to the horizontal stabilizer, which is the big adjustable surface at the tail that sets the overall pitch attitude. Now here's a critical safety point. Manual operation of the pitch trim will, in most systems, automatically disconnect the autopilot. Why? Because the autopilot cannot co-ordinate your manual trim movement with movement of the centre of gravity, or C of G, or with aerodynamic movements. If you're trimming by hand while the autopilot is flying, the two of you are fighting each other, so the system simply kicks the autopilot off. Now let's move to automatic trim, or auto-trim. As the aircraft uses fuel, or changes speed, thrust, or configuration — meaning flaps and gear — the pitch attitude will change. If you're flying manually, you'd trim those forces out by hand to eliminate stick forces. But here's the problem with 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. The servomotor just muscles the control into position and holds it there. That won't pose much of a problem unless the forces overwhelm the servomotor, or until the moment you wish to disconnect the autopilot. And here's why that's dangerous. Not only does this mean the aircraft is producing more drag than necessary — because the control surface is being held against the airflow — but if there's a standing load on the controls when you disconnect the autopilot, you won't know it exists, and you won't know which way the aircraft will pitch. The moment you take over, the aircraft will "snatch" or lurch in response to that out-of-trim condition. That's a nasty surprise on handover. So this is not a very satisfactory situation, and that's why a system of Automatic Pitch Trim was included in most autopilot systems. That's the system that continuously adjusts the trim to keep the control forces at zero, so that when you disconnect, the aircraft is already balanced and there's no lurch.