I've watched a few videos about the Boeing 737, and in particular because it's a bit of a current topic, about the stabiliser wheels in the cockpit.

On some of these videos, which are often in commercial flight simulators, we see the stabiliser wheels rotating backwards and forwards automatically. On the wheels there are white marks. However these white marks are often not synchronised and level with each other, for example this video, or this GIF below from here:

enter image description here

There are other videos where these marks are sometimes nearly 160 degrees out from each other. From my naive expectations, shouldn't these marks be within a a few degrees of each other to accurately represent that the stabilisers are at the same level?

Or, are stabilisers individually independent such that the left stabiliser may need a more acute angle of attack whereas the other may not?


The rear wing (horizontal stabilizer) of a 737 is effectively one piece. There is no separate left and right trim control. Both sides during assembly are attached as one.

This is just a visual illusion because the white marks are not aligned. If you manually move one wheel, both wheels will move.

It's worth noting that the white marks do not represent the position. For the full stabilizer travel, the wheels would make many, many revolutions.

The stab trim position indicators would show the position, not the wheels:

enter image description here

  • $\begingroup$ Thanks for your answer. Are the stab wheels deliberately installed such that the movement markers(?) are offset (perhaps as a better visual aid?), or are they factory installed where these marks align but over time go out of alignment due to use. $\endgroup$
    – Kev
    Mar 24 '19 at 18:20
  • $\begingroup$ @Kev: I'm not sure. I checked the flight manual, but they're not mentioned. And in the image in the answer, there are no white markings (click source for full res image). $\endgroup$
    – ymb1
    Mar 24 '19 at 19:48
  • $\begingroup$ They're likely hidden from view, but they are there in virtually every 737 cockpit video I've watched. $\endgroup$
    – Kev
    Mar 24 '19 at 22:30
  • $\begingroup$ The white mark is only to signal movement to the person siting next to it, nothing more. I imagine it is either the wheels are splined to their axle rather than with a single key-slot, as such they could be installed in many positions. Or, both are are a single part number and when it is flipped from left to right side the mark is in a different position. Possibly both. $\endgroup$
    – Max Power
    Mar 29 '19 at 5:14
  • $\begingroup$ @Kev, yes deliberately to increase the probability to notice a runaway $\endgroup$
    – user40476
    Jun 3 '19 at 16:14

They are in fact moving at the completely same time. It would be catastrophic if they didn't ^^. The way the white parts are marked on the trim wheels makes us think they move delayed.


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