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