Let me walk you through automatic pitch trim — Page 363, Lesson 436

Let me walk you through automatic pitch trim. We just saw the problem: when an autopilot holds an out-of-trim condition, it's an unsatisfactory situation. 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 this carefully — auto-trim is active only when the autopilot is engaged. Fly-by-wire arrangements are a separate topic we'll cover later. Now, how is it achieved? Typically by a separate trim servo actuator. That actuator operates either the normal trim tab, or — more commonly on modern jet transports — the variable incidence horizontal stabilizer. Let me explain that second option, because it's important. The variable incidence horizontal stabilizer lets the elevator always be in neutral position with respect to the horizontal stabilizer. That means the autopilot has 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 either way. There's another important consideration here. In the event of an autopilot disconnect, the aircraft will already be in a trimmed condition. So it will not suddenly pitch up or down when the autopilot lets go. That's a safety benefit. Now, how does the system sense an out-of-trim condition? There are two common methods, and I'll give you the first one now. It's a standing load being sensed on an electrical actuator. Think about what's happening: because of the out-of-trim situation, the autopilot has 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 directly proportional to the force being held. And the direction is known from which way the actuator has to apply that load. So that information — magnitude and direction — is used to move the normal trimming system of the elevators to reduce the standing load to zero. Once that load is zero, the aircraft is now in trim. So the whole loop is: sense the standing load, use its magnitude and direction to drive the normal elevator trim, reduce the load to zero, and the aircraft is trimmed. That's the first sensing method.