Trimmable Horizontal Stabilizer is a fully moving horizontal tail surface that is part of the tailplane and includes a load-bearing structure. It trims the aircraft in the pitch axis by adjusting the entire horizontal tail assembly, thereby maintaining horizontal static equilibrium and relieving control pressure on the pilot. The system compensates for the centre of gravity position prior to takeoff and is manually, electrically, or hydraulically actuated.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a Trimmable Horizontal Stabilizer (THS)?
A Trimmable Horizontal Stabilizer (THS) is a fully moving horizontal tail surface that stabilizes the aircraft in the pitch axis. It combines with an independently-moving set of elevators. The design incorporates a trimmable horizontal stabilizer which provides strong trimming power over the full speed range of the airplane.
The trimmable horizontal stabilizer is mounted by hinges such that it can rotate about the lateral axis of the aircraft. The whole assembly moves in response to the trim system, adjusting the angle of the entire horizontal stabilizer instead of using separate trim tabs. In doing so, THS replaces trim tabs on aircraft like the Airbus A320, Boeing 737 and regional jets, and is used to trim the aircraft for hands-off flight.
The THS is part of the trimming system of the aircraft and it stabilizes the aircraft in the pitch axis and helps maintain center-of-gravity balance throughout flight as fuel is consumed or payload shifts. By altering the incidence angle, the THS exerts a vertical force at a distance, creating a pitch moment about the centre of gravity that maintains horizontal static equilibrium. Trim range examples include +3 to -15, where angles with a positive sign (+3) indicate trimming upwards and angles with a negative sign (-15) indicate trimming downwards.
What is the purpose of a trimmable horizontal stabilizer?
A trimmable horizontal stabilizer prevents the up-and-down pitching motion of the aircraft nose and provides a stable reference point that allows smooth, controlled pitch adjustments. By adjusting the entire horizontal tail-plane angle, the trim system continuously re-positions the whole assembly to maintain equilibrium, so the pilot need not provide continuously variable pitch inputs and the aircraft avoids constant pull or push on the control stick. This optimization suppresses small loops, reduces drag, and thereby optimizes aerodynamic efficiency throughout the flight envelope.
Most aircraft have a trimmable horizontal stabilizer because the centre of pressure is some distance from the aircraft centre of gravity. Small, precise angular corrections are more effective and lighter than re-trimming the elevators alone. The permitted range is typically +3 to -15, and the movable areas can be moved independently of the elevator by separate or common actuators.
The purpose of the trimmable horizontal stabilizer actuator is to impart that angular change. On types like the DC-10, two screw-jacks form the primary drive. On the Boeing 737 and Airbus A320 a single jackscrew, electrically operated by a dedicated motor, performs the task. Torsion shafts couple the drive stations to a central unit inside the fuselage, forming a compact rotary drive that lowers the physical height of the rear fuselage structure compared with older linear-drive layouts.
The purpose of the jackscrew itself is to convert electrical power into precise linear motion, thereby trimming is carried out by spindle drive. Because the load-bearing structure is firmly connectable to the fuselage and the screw-jack is powered electrically, the system remains reliable while allowing the aerodynamic profile to be neutral or to produce positive or negative lift as required.
The complete trimmable horizontal stabilizer system thus integrates the actuator, jackscrew, and movable tail-plane into one closed-loop function: it senses pitch disturbance, commands a small angular correction, and holds that position so that the aircraft remains in smooth, stable flight with minimum drag.
A trimmable horizontal stabilizer keeps the aircraft balanced across a wide range of loads and centers of gravity, so the pilot can fly hands-off without applying constant force on the column. By adjusting the tail's angle, the system stabilizes the airplane, relieves the pilot, and reduces fatigue, allowing attention to remain on guidance, communication, and monitoring subsystems. This continuous automatic assistance lessens drag and improves fuel efficiency.
How does trim affect the horizontal stabilizer?
When the pilot commands pitch trim, the movable surface is the horizontal stabilizer itself, so pitch trim is applied to the horizontal stabilizer and the entire stabilizer pivots through a small arc. The movement is called trimmable horizontal stabilizer movement. It re-positions the whole stabilizer instead of just an elevator tab. Mach trim adds stabilizer increment and the system reduces drag as the stabilizer surface and the elevator are in alignment whenever the aircraft is in trim.
As the stabilizer tilts, a moment of equal magnitude but opposite direction is generated at the horizontal stabilizer to keep the aircraft in trim, so the trimmed angle holds the nose at the desired attitude with no further pressure and allows you to take hand off yoke. Because trim requirements change with speed and power settings, the pilot or the speed trim computer must keep adjusting the stabilizer so that control forces stay near zero. The speed trim returns the airplane towards the reference speed. Careful stabilizer positioning improves landings because trim helps on-speed approaches and allows the pilot to monitor the glide-path.
How does the pilot use electric trim for the horizontal stabilizer?
The pilot uses the electric switches mounted on the yoke to readjust the trim. Manual electric trim motor commands from the pilot travel through these control yoke switches. Trim is applied by electric trim switches, by the autopilot, or with the manual trim wheel that sits by the pilot's knee. Electric trim authority can drive the stabilizer to the nose-down limit of 0.5 units, but pilot trim is limited to about 4 units while flaps are up. If necessary, the pilot can respond by disabling the electric motor that controls the trim system and can readjust the trim manually by turning the trim wheel.
What is the difference between a trimmable horizontal stabilizer and an elevator?
A trimmable horizontal stabilizer changes the entire stabilizer angle while an elevator is the small moving section at the rear of the stabilizer, attached to the fixed sections by hinges. The elevator varies the amount of force generated by the tail surface by altering effective airfoil camber. The stabilizer, when trimmed, repositions the whole surface to achieve the same result without continuous elevator deflection. Elevators respond to pilot or autopilot input to control pitch and are the primary control for varying the pitch attitude of an aircraft. The trimmable stabilizer is actuated by a dedicated trim system to relieve pilot effort, allowing the elevator to return to a neutral position once the desired attitude is set.
What is the difference between a trimmable horizontal stabilizer and a stabilator?A trimmable horizontal stabilizer (THS) is an adjustable stabilizer whose elevators are deflected separately, whereas a stabilator is a fully movable aircraft horizontal stabilizer that has no discrete elevators at all. Because the entire surface pivots, a stabilator combines the functions of a fixed stabilizer and elevator in one large moving surface. The pilot trims the THS mainly to offload sustained loads, then commands the elevator for transient manoeuvres. A stabilator is controlled directly by the pilot's control yoke or stick, so every pitch input rotates the whole tail.
Modern commercial airliners employ THS for its modest deflection range and fine trimming capability, while supersonic aircraft and most high-performance fighter jets rely on a stabilator to gain the strong control response needed at transonic and supersonic speeds. The large angular authority of a stabilator is the reason for the unusual pitch-trim design called anti-servo: an antiservo tab moves in the same direction as the stabilator but with increased travel, producing higher hinge moments that give the pilot appropriate stick force. Adjustable stabilizers are therefore not the same as stabilators; one supplements an elevator while the other replaces it entirely.
