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I would like a clarification from the operational point of view: Both the THS and elevator trim tab is used to remove pilot pressure on the yoke / stick.

On the elevator trim similar to the C-172 the yoke deflection is a function of the speed: so in trimmed condition the yoke deflection depends on the IAS we what to fly.

What happens on the THS trimming? Does the yoke deflection always stays in a "center" position after trimming or the behavior is similar to the C-172 ?

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    $\begingroup$ Does this answer your question? What are the advantages of a Trimmable Horizontal Stabilizer? $\endgroup$
    – Manu H
    Jun 24, 2020 at 19:32
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    $\begingroup$ I am talking about the yoke/stick deflection of the two systems. $\endgroup$
    – Ambrogio Riboli
    Jun 24, 2020 at 20:58
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    $\begingroup$ @ManuH Could you show where exactly this question is answered in the linked duplicate. I cannot find it... $\endgroup$
    – Bianfable
    Jun 25, 2020 at 7:14
  • $\begingroup$ @Bianfable the original question was "THS vs elevator trim" (see edition history) $\endgroup$
    – Manu H
    Jun 26, 2020 at 15:37

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When I was type rated in the CRJ200 with hydraulic controls and a THS, directly coming from light aircraft, the differences in technique are significant and it was definitely something new to learn.

With a THS, column neutral is always the same spot. This means trimming to a speed is not a case of moving the control to location X and trim until it stays at location X. You have to apply a control input to achieve a speed/attitude, and trim until you can relax the control input and let it go back to its "permanent" neutral position while the airplane maintains the target trim speed.

So what you do is, say, you want to slow down from 200 kts to 180 kts, so you pull and hold to slow down. While you're holding the input, you start to blip the trim switch with your thumb as, at the same time you let the column move a bit back toward neutral. The objective is to make the trim input as you relax the control input so that the stab incidence change is replacing the elevator deflection and the airplane holds a constant attitude and speed.

You keep doing this; blip-release-a-bit, blip-release-a-bit, blip-release-a-bit, until the column is back at neutral and the plane is holding your target speed with no input. With practice you start to pull and blip the trim right away and it all just flows automatically.

It's murder on the trim screw jack drive systems, because they get many many start/stop cycles a flight from pilots going blip-blip-blip on the switch whenever they are hand flying. The autopilot also does the same thing, sending numerous trim pulses to trim out speed changes to offload the AP drive servo.

Light planes with trimmable stabs, like the Cessna 180-185 family, are very similar, because the elevator just wants to "trail" when hands free, and the neutral position doesn't change the way it does with a tab, although it varies a little compared to a jet with hydraulic controls.

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  • $\begingroup$ so in trimmed flight condition a THS ac the yoke is in the same neutral position disregarding speed and CG. The trimming procedure is then different from the elevator trim tab of the cessna c_172. THS requires to trim to get neutral position of the yoke while the elevator trim tab requires to trim in order to maintain the actual yoke position. $\endgroup$
    – Ambrogio Riboli
    Jun 25, 2020 at 12:44
  • $\begingroup$ Exactly.On a jet with cables and hydraulic control surfaces, the control cable loop's neutral, set by a pitch feel system of some kind (on the RJs it's a box with bungee springs inside, that are the actual force you working against when you move the column) never changes.And elevator neutral is always aligned with the stab chord line. Linkage geometry at fin/stab keeps this neutral relationship as the stab's incidence is varied and is transparent to the pilot. So the only time the elevator is not aligned with the stab chord line is when you or the autopilot makes a temporary maneuvering input $\endgroup$
    – John K
    Jun 25, 2020 at 13:26
  • $\begingroup$ I am only talking about yoke/stick displacement. Jjust to summerize: is my previous statement correct ? $\endgroup$
    – Ambrogio Riboli
    Jun 25, 2020 at 14:11
  • $\begingroup$ Yes. If you make a trim input, the yoke doesn't move. $\endgroup$
    – John K
    Jun 25, 2020 at 14:37

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