Autopilot — Page 368, Lesson 437

I want to walk you through the auto-trim side of the autopilot system, because this is where the autopilot actually manages the aircraft's trim to keep the control forces balanced. Let's start with the actuator, because that's the heart of it. The actuator is the mechanical device that physically moves the control surface. Now, here's the key idea: the actual position of the actuator tells you something important. If there is an out-of-trim force to be coped with — meaning the aircraft is trying to pitch or roll away from the trimmed attitude — the actuator will be positioned to input the required control displacement to hold that force. In other words, the actuator moves to a position that produces just enough control deflection to counteract the unwanted force. And here's the clever part: the displacement of the actuator gives you both the direction and the magnitude of that force. So by looking at where the actuator sits, you can tell which way the force is pushing and how strong it is. Now, large modern aircraft tend to use what we call Trim Tails, or all-moving tailplanes. Let me explain what that means. Instead of a fixed tailplane with a separate elevator hinged to it, the entire tailplane moves as one piece. Both of these systems — the trim tab system and the all-moving tailplane — work in a particular way. Because of the way they trim, they reset the control inputs to give full elevator movement up and down from the trimmed position. So here's the sequence: as the auto-trim moves the normal elevator trim, the displacement of the normal control input is reset, which removes the actuator displacement. In plain terms, the trim system takes over the job of holding the force, so the actuator no longer has to hold a deflection — it returns to a neutral position, and the tailplane itself carries the trim load. Let me show you this with a diagram. That figure shows trimming by variable incidence tailplane. The output is applied to the trim tab actuator or the horizontal stabilizer. So the trim system's output goes either to a trim tab actuator or directly to the horizontal stabilizer — that's the all-moving tailplane — and that's how the trim force gets applied. Now, what happens if the trim system fails? This is critical for you as a pilot. In the event of a failure of the trim system, the pilots would be alerted by warning lights and/or suitable indications on the electronic display unit. At the same time, the autopilot would disengage, giving both visual and aural alerts. So you get a light, a display indication, and a sound. The aural alert can also sound when there is an excessive trim input — for example, in an actuator runaway situation. That's when the actuator runs away to an extreme position on its own, and you'd hear the warning. There's an important operational consequence here. If the auto-trim system is not available, then the autopilot may become inoperative. And for the Boeing 737-400 specifically, it will become inoperative — that's a hard requirement, not a maybe. If another autopilot is available, it would be common practice to use the fully operative system. So you'd switch to the working autopilot. Now, let's talk about the case where you don't have auto-trim available. If there is only one autopilot, or the aircraft is not fitted with auto-trim, then the aircraft must be correctly in trim before the engagement of the autopilot. Why? To minimize the control loading expected on disengagement. In other words, if you engage the autopilot while the aircraft is out of trim, the autopilot has to fight that force the whole time, and when you disengage, you suddenly get a big control load dumped on you. 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 will minimize the control snatch on autopilot disengagement. Control snatch — that's the sudden jerk or pull on the controls when the autopilot lets go. So the procedure is: engage, fly for a set time, disengage, re-trim, re-engage. That keeps the control forces manageable. 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 that the system can be monitored during autopilot operation. So you always have a way to see what the stabilizer trim is doing and whether the auto-trim has failed. And one more safeguard: the autopilot may not engage if there is too great a standing load — that is, an out-of-trim condition already present. So the system itself refuses to engage if the aircraft is too far out of trim. That's your protection against engaging into a fight with a heavy control force. So to tie it all together: the actuator position tells you the direction and magnitude of any out-of-trim force; the trim system — whether a trim tab or an all-moving tailplane — resets the control input so the actuator returns to neutral; failures are announced by lights, display indications, and aural alerts, with the autopilot disengaging; and if auto-trim isn't available, you must be correctly trimmed before engagement, follow the SOP's time-limited disengage/re-trim/re-engage cycle, and the autopilot may refuse to engage if the standing load is too great.