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Inspired by this question, what exactly is the meaning of a stab trim unit? Is it exactly one degree of stabilizer movement?

The 737 NG FCOMv2 (9.20.9 Flight Controls - System Description) only lists the ranges of the units:

Stabilizer Position Indication and Green Band

Stabilizer position is displayed in units on two STAB TRIM indicators located inboard of each stabilizer trim wheel. The STAB TRIM indicators also display the TAKEOFF green band indication.

The trim authority for each mode of trim is limited to:

  • Main Electric Trim

    • flaps extended 0.05 to 14.5 units

    • [737-600] flaps retracted 4.10 to 14.5 units

    • [737-700] flaps retracted 4.30 to 14.5 units

    • [737-800] flaps retracted 3.95 to 14.5 units

    • [737-900] flaps retracted 3.90 to 14.5 units

  • Autopilot Trim 0.05 to 14.5 units

  • Manual Trim -0.20 to 16.9 units.

The green band range of the STAB TRIM indicator shows the takeoff trim range. An intermittent horn sounds if takeoff is attempted with the stabilizer trim outside the takeoff trim range.

The full range of travel can be seen as a black arc in the following image (taken from this answer), which could be 17°, but it is hard to say from the image:

737 Horizontal Stabilizer

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The stab trim unit is an offset to the actual horizontal tailplane angle. It is added by Boeing to make the trim value positive.

1STU = 1 degree

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    $\begingroup$ Do you have a source for that? $\endgroup$ – Bianfable Sep 30 '19 at 9:09
  • $\begingroup$ This post on pprune.org confirms your statement. It also does not list any sources, but I'm going to accept for now... $\endgroup$ – Bianfable Oct 31 '19 at 7:19

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