When was the Speed Trim System implemented on the Boeing 737? 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?
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.