Let’s pick this up right where the autopilot leaves off — Page 363, Lesson 436

Let’s pick this up right where the autopilot leaves off. We’ve got a problem: when the autopilot is holding the aircraft in a condition that isn’t trimmed, it’s fighting a constant force — an out-of-trim situation. That’s not a very satisfactory state to stay in, so most autopilot systems include a system called Automatic Pitch Trim. Here’s the key point: when the autopilot is engaged, automatic trim is available only in pitch. That’s why it’s called Automatic Pitch Trim, or simply Auto-trim. And note the condition — auto-trim is active only when the autopilot is engaged. If the autopilot is off, auto-trim does nothing. Fly-by-wire arrangements are a separate topic we’ll get to later. Now, how is auto-trim actually achieved? Typically by a separate trim servo actuator. That’s a dedicated motor that operates either the normal trim tab, or — and this is more common on modern jet transports — the variable incidence horizontal stabilizer. Let me explain that second one, because it’s important. The variable incidence horizontal stabilizer is the whole tailplane that can change its angle. When the autopilot trims by moving the stabilizer, it allows the elevator to always be in a neutral position with respect to the horizontal stabilizer. Why does that matter? Because it gives the autopilot full elevator control authority on both sides of the trimmed position. In other words, the elevator isn’t stuck holding a deflection — it’s free to move up or down from neutral, so the autopilot has the full range of pitch control available. There’s another important safety consideration here. In the event of an autopilot disconnect, the aircraft will already be in a trimmed condition. So when the autopilot lets go, the aircraft will not suddenly pitch up or down. That’s a real safety benefit — no surprise pitch excursion when the autopilot disengages. Now, how does the system know the aircraft is out of trim? The out-of-trim condition is sensed most commonly by one of two methods. Let’s look at the first method, which is the one described here. It’s a standing load being sensed on an electrical actuator. Here’s the logic: because of the out-of-trim situation, the autopilot is having to hold a force against that condition — exactly the same way a pilot would have to hold force on the controls. The magnitude of the load on the actuator is going to be directly proportional to the force being held. So the bigger the force the autopilot is fighting, the bigger the load the actuator senses. And the direction is known from which way the actuator is having to apply that load — that tells you which way the trim needs to move. That information — the magnitude and direction of the standing load — is then used to move the normal trimming system of the elevators to reduce the standing load to zero. When the load reaches zero, the aircraft is now in trim. So the loop is: sense the load, move the trim to cancel the load, and when the load is zero, you’re trimmed. That’s the first sensing method. The second method is mentioned as an alternative, but it’s not detailed in this passage — we’ll cover it when we get to it. For now, hold onto the core idea: auto-trim is pitch-only, active only when the autopilot is engaged, and it works by sensing the standing load on an actuator and driving the normal elevator trim system to bring that load to zero.