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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