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Why are the angles displayed on the fuselage?

I understand having markings with depth on the hull of boats, but I don't understand the use case on a plane.

enter image description here
Photo of the root of the trimmable horizontal stabilizer on an Embraer ERJ-170 (wikimedia.org).

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  • $\begingroup$ To ease some step in the maintenance procedures? Also, this is not al all universal, quite a few aircraft do not have such detailed markings. $\endgroup$ Dec 18 '19 at 23:52
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    $\begingroup$ That looks like a 737 and you usually only see markings at neutral, full up and full down. That's probably a customer request to allow for checking it during walkaround. It made me think there might be an MEL that allows dispatch with an inoperative trim indication, requiring the crew to check the position outside before departing, but that seems kind of far fetched. $\endgroup$
    – John K
    Dec 19 '19 at 1:37
  • $\begingroup$ I don't know from what airline the picture comes, but I've seen some of Ryanair's planes having similar markings, but just the the 3 notches $\endgroup$
    – Thomas
    Dec 19 '19 at 15:26
  • $\begingroup$ Now I wonder if those marking are also present on aircraft without THS. $\endgroup$
    – Manu H
    Dec 22 '19 at 17:08
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I don't have the reference but it's showing the trim setting. The picture looks like is an Embraer 170 on which the entire horizontal stabilizer moves. The gauge gives a quick visual setting to confirm the the trim set in the cockpit matches the actual trim.

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The markers are for the ground crew member to report to the flight crew the actual position of the stabilizer trim, which the flight crew checks against the cockpit reading. This is just to ensure that the stab trim sensors are functioning properly. It is part of the launch checklist, usually. (the photo above shows 4° nose up trim.)

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I believe it is to ensure proper alignment during installation. I have seen attachment shafts that are splined, and when sliding the control surface onto this shaft you would want to ensure that the leading edge mark is properly aligned with the fuselage mark appropriate to the trim setting.

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    $\begingroup$ I suppose you're referring to re-installation after a maintenance of some sort. If it were the initial installation in the factory, the body would most likely still be unpainted and there's probably a jig for that. $\endgroup$
    – FreeMan
    Dec 19 '19 at 17:22
  • $\begingroup$ Yes, good point... $\endgroup$ Dec 19 '19 at 21:13

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