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On a A320, there are white patterns spread regularly on the trim wheel (highlighted on the right of the picture). First I though it was to have visual feedback about THS (trimable horizontal plane) position. But looking closely, this role seems to be played by the graduations (highlighted on the left of the picture).

A320 trim wheel, www.airliners.net

(photo #0393544 from airliners.net)

Why are there those white patterns on the trim wheel? What is their role?

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The white marks make it easy to see if the trim wheel is moving, which would be tricky if it was entirely black.

But wouldn't the pilot always know if they was spinning the trim wheel? Remember that the autopilot can also adjust the trim, which might not be obvious to the pilot. The visual marks make it easy for the pilots to see what the autopilot is doing.

Example video (not an Airbus, but very similar):

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    $\begingroup$ Not just the autopilot, but the electric trim on the yoke. The pilot can verify the trim is moving when the button is pushed. $\endgroup$
    – Ron Beyer
    Sep 21, 2019 at 13:16
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    $\begingroup$ @ManuH It is a different scale. The actual trim wheel will spin many times to go from one end of the scale to the other. So smaller changes would not be obvious just by looking at the scale. $\endgroup$
    – 60levelchange
    Sep 21, 2019 at 18:47
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    $\begingroup$ @ManuH: As you said, “ looking closely”, but the pilot can't always look closely at that specific graduation. $\endgroup$
    – DaG
    Sep 21, 2019 at 21:13
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    $\begingroup$ Wait a minute? The A320 trim wheel automatically moves through autopilot control? I always thought they were simply a manual trimming device, basically for takeoff trim and degraded systems situations. Can someone confirm that the A320 trim wheel moves due to autopilot commands? $\endgroup$
    – Simon Opit
    Sep 22, 2019 at 8:51
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    $\begingroup$ @SimonOpit: Yes the AP moves them, since they're mechanically linked. You can watch it here. (Worth noting that there are no trim switches on the stick.) $\endgroup$
    – user14897
    Sep 22, 2019 at 15:05
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The White patterns on the A320 family Pedestal are provided to give the pilot an indication that the Wheel is moving, especially during a flight where your CG (center of gravity) is constantly moving.

secondly, as part of ROT (rule of thumb), each white pattern corresponds to a change of 0.2 degrees on the THS. there will always be 5 white patterns visible to pilots to give a change of 1.0 degrees on the THS (trimmable horizontal stabilizer) if it was moved by one complete cycle

Picture ref:https://www.airliners.net/photo/BMI-British-Midland/Airbus-A321-231/727825/L

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