Autopilot — Page 368, Lesson 437

Let’s pick this up right where the actuator displacement idea leaves off, because that’s the heart of how auto-trim works on a large modern aircraft. We’ve got the actuator sitting at some actual position, and that position tells us two things: the direction of any out-of-trim force, and its magnitude. If the aircraft is trying to pitch nose-up or nose-down and the autopilot has to hold against it, the actuator moves to a specific displacement to generate the control force that holds the aircraft. So the displacement of the actuator is literally a measure of the force it’s fighting. Now here’s the clever part. Large modern aircraft tend to use what we call Trim Tails or all-moving tailplanes. Both of these systems work by physically moving the entire horizontal stabilizer, not just a small tab on the elevator. Because of the way they trim, they reset the control inputs so that the elevator has full movement up and down from the trimmed position. Let me unpack that. Imagine the elevator is sitting at some deflection to hold a nose-up force. The auto-trim system then moves the whole tailplane to take over that force. Once the tailplane is doing the work, the elevator can return to its neutral, trimmed position. So as the auto-trim moves the normal elevator trim, the displacement of the normal control input is reset, and the actuator displacement is removed. The actuator goes back to zero because it no longer has to hold the force — the tailplane is holding it now. That’s the whole point of trimming: you unload the actuator so it has full authority left for manoeuvring. Let me show you this with a diagram. That figure shows trimming by variable incidence tailplane, and the output is applied either to the trim tab actuator or to the horizontal stabilizer. So you can see the two paths: either a trim tab on the elevator, or the whole stabilizer moving. Now, what happens if the trim system fails? The pilots are alerted by warning lights and/or suitable indications on the electronic display unit. At the same time, the autopilot disengages, giving both visual and aural alerts. The aural alert can also sound when there is an excessive trim input — for example, in an actuator runaway situation, where the actuator drives the trim to an extreme position on its own. There’s a critical dependency here. If the auto-trim system is not available, then the autopilot may become inoperative — and for the B737-400 specifically, it will become inoperative. So on that aircraft, no auto-trim means no autopilot. If another autopilot is available, it would be common practice to use the fully operative system. Now, what if there is only one autopilot, or the aircraft is not fitted with auto-trim at all? Then the aircraft must be correctly in trim before the engagement of the autopilot. Why? To minimize the control loading expected on disengagement. If you engage the autopilot while the aircraft is out of trim, the autopilot has to hold that force, and when you disengage it, the control surfaces will snatch — the aircraft will suddenly pitch. So the standard operating procedure for the aircraft will stipulate a time period after which the autopilot must be disengaged, the aircraft re-trimmed, and then the autopilot re-engaged. This minimizes the control snatch on autopilot disengagement. Let me show you the failure warning. That’s the auto-trim failure light. The pilots may have some indication of the trim controls, but in the case of auto-trim there is always a stabilizer trim indicator and an auto-trim failure warning, so the system can be monitored during autopilot operation. And one more important point: the autopilot may not engage at all if there is too great a standing load — that is, an out-of-trim condition already present. So the system protects itself by refusing to engage when the aircraft is too far out of trim. Let me pull the key ideas together. The actuator displacement tells you the direction and magnitude of the out-of-trim force. Trim tails and all-moving tailplanes reset the elevator to neutral by moving the whole stabilizer. Failure of the trim system gives you warning lights, display indications, and autopilot disengagement with visual and aural alerts. The B737-400 will not fly with the autopilot if auto-trim is unavailable. And if you have only one autopilot or no auto-trim, you must be in trim before engagement, and you must follow the SOP time period for disengage, re-trim, and re-engage to avoid control snatch. Finally, the autopilot may refuse to engage if the standing load is too great.