The Design of most of the modern and transport aircrafts feature a large, slow-moving trimmable tail plane (Trimmable Horizontal Stabilizer (THS) or Stab Trim, that comes combined with an independently - moving set of elevators.
The elevators are controlled by the pilot or autopilot and primarily serve to change the aircraft's attitude, while the whole assembly (THS) is used to trim (maintaining horizontal static equilibrium) and stabilize the aircraft in the pitch axis.
The trimmable stabilizer does not move in response to control column or control stick movement, the entire horizontal tail assembly moves in response to the trim system to stabilize the aircraft in the pitch axis.
Boeing B777 aircraft Trimmable Tail Plane.
The trimmable tail plane's primary advantage is that it provides a trimming advantage over the full speed range of the airplane. The system also reduces drag as the stabilizer surface and the elevator remain in alignment, whenever the aircraft is in a trim config.
Among the primary functions, the stabilizer trim is normally adjusted to compensate for Centre of Gravity (CG) position prior to takeoff to ensure optimum elevator effectiveness.
In most cases, a trimmable stabilizer function is either manually or electrically controlled and hydraulically actuated screw jack . In some Fly- By- wire aircrafts, the Trim Stab automatically adjusts to a one G loading without needing a pilot input.
Collins Aerospace Screw Jack Assembly
► The function Pitch trim is applied to the horizontal stabilizer. Trim can be applied by electric trim switches, autopilot or a manual trim wheel. Electric and autopilot trim may be disengaged by dedicated switches on the control stand in the event of a runaway or other malfunction.
► Defence - Moving the control column in the opposite direction to electric trim will stop the trim function , unless the STAB TRIM switch is set to OVERRIDE. This function could be used to control the pitch of the aircraft in case of a jammed elevator.
► The trim authority varies according to aircraft series and method of trim. For B737 NG aircrafts, the full range is only available with the manual trim wheel, but if at an extreme setting, electric trim can be used to return to the normal range. There are two electric trim switches on each control column, the right is for the direction and the left is an earth return for protection against spurious electrical signals.
► Low Speed trim (B737 NG aircrafts) is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust. 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.
Boeing 737NG horizontal stabilizer (Stab Trim) jack screw in Operation , Video : Anders Cornelius Olesen
Higher Speed Mach trim (B737 NG aircrafts) is automatically applied above M0.615 (B737 Classics onwards), M0.715 (-1/200) to the elevators. This provides speed stability against a Mach Tuck (On Mach increase, the centre of pressure moves aft and the nose tend to drop).
Maneuvering Characteristics Augmentation System (MCAS - B737 MAX only) will apply a nose down stabilizer trim during high AoA flight , when the flaps are up and the A/P is not engaged.
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