Speed Trim SystemThe 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. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction to return the airplane to the trimmed speed. This increases control column forces to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed speed, the STS commanded stabilizer movement is removed.
STS operates most frequently during takeoffs, climb and go-arounds. Conditions for speed trim operation are listed below:
- STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68
- 5 seconds following release of trim switches
- Sensing of trim requirement
I though I had this annoying system nailed, but when I read what the FCOM says, I’m just getting confused. How can a system trim to return the aircraft to the trimmed speed by trimming opposite the speed change? If I increase the speed this means the STS will trim aft. At the same time increased speed will increase the lift. Aft trim will bring the aircraft into a more out of trim state. I always have to trim opposite the STS which is a nuicance, but it looks like the STS is made to do this.
On top of this, the STS is trimming on all my take offs, not only in the high thrust, aft CG, light aircraft configuration, and I’m pretty sure it trims outside the 10 seconds window as well.
My opinoin, we would be safer without this system working agains us when we hand fly. Those of you who engage the auto pilot passing 400 ft will not understand what I’m talking about, but the few of you who hand fly on a regular basis will.
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