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The Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged.

Was the Speed Trim System implemented on the original Boeing 737 or just the Boeing 737 NG series?

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Boeing's patent (US4676460A) for the STS was filed on 1984-11-28, the same day of the 737 Classic (3/4/500 series) entering service.

So since the addition of the bigger engines on the Classic, it's been there.

Below is from a 737 flight manual of a company that had both the Classic and NG at one time, which highlights the differences between the two series.

The 3 (for -300) in a circle is non-EFIS, in case you're wondering how the two 3's differ.

enter image description here

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  • $\begingroup$ Excellent answer! Thank you. $\endgroup$
    – DLH
    Commented Jun 6, 2019 at 21:41
  • $\begingroup$ @DLH: You're very welcome. Not worth an edit, but the missing 4 is because that airline didn't have the -400. $\endgroup$
    – user14897
    Commented Jun 6, 2019 at 22:09
  • $\begingroup$ @ymb1: Which is also, I'm assuming, the reason for the missing 6 (no 737-600s at that airline). $\endgroup$
    – Vikki
    Commented Jun 7, 2019 at 3:37

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