I have some questions regarding the Boeing 738 speed trim system (STS) which I hope someone out there can answer for me. English is also my second language, so please try not to deduct too many assumptions from apparent language errors.
I am undergoing a type rating for the 738 and cannot find the speed range the STS aims for when it engages. In previous threads I have read:
"Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust - i.e. on most take-offs. Speed trim is a dual channel system. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition."
When "positive speed stability characteristics" is mentioned I guess it means a speed a set value above Vmd. Would anyone know what value it aims for?
I can neither find it in the FCOM nor the Maintenance Manual.
Does anyone know what the Vmd / Max Endurance - speed range is on the 738?
And in which situations does the STS engage?
"Low speed, low weight, aft C of G and high thrust" is a combination of conditions that makes the airplane unstable around the lateral axis, and I thought it may be a stall protection system, but it also states that the STS can trim the stabilizer towards a lower (!) speed. ("The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed ... ")
If anyone could help me I would be very grateful.
