RFT 375: B737 Trim System
The B737 has an interesting stabilizer trim system, which operates at four different speeds.
Main Electric Trim
Flaps Up: .2 units per second
Flaps Down: .4 units per second
Autopilot Trim
Flaps Up: .09 units per second
Flaps Down: .27 units per second
On the -300 and later models, there is a system called Speed Trim, which provides trim inputs during low speed operations with low gross weight, and aft center of gravity. It is most frequently observed during takeoffs and go-arounds. Conditions for the system to operate are:
Airspeed 100-300 KIAS
10 seconds after liftoff
5 seconds following release of trim switches
N1 above 60%
Autopilot not on
Sensing of trim requirement