Trimmable Horizontal Stabilizer

The trimmable horizontal stabilizer (THS) is a key component of modern aircraft tail design, responsible for maintaining longitudinal stability and enabling precise control of the aircraft’s pitch. Unlike fixed stabilizers, a trimmable stabilizer can change its angle of incidence relative to the fuselage. This adjustability allows it to balance the aerodynamic forces acting on the aircraft, reduce pilot workload, and improve efficiency during different phases of flight.

Purpose of a Horizontal Stabilizer

The horizontal stabilizer is located at the tail of the aircraft and serves as a balancing surface to counteract the nose-down pitching moment generated by the wings and engines. Its primary functions are:

Maintaining longitudinal stability.

Assisting in pitch control when combined with the elevator.

Providing trim adjustments to minimize continuous control input by the pilot.

Trimmable vs. Non-Trimmable Stabilizers

Fixed Horizontal Stabilizer with Elevator Trim Tabs: In smaller aircraft, pitch trim is often managed by trim tabs on the elevator. These provide limited trim authority.

Trimmable Horizontal Stabilizer (THS): In larger and faster aircraft, instead of trimming through small tabs, the entire stabilizer surface is adjusted. This provides a much larger trim range, necessary to compensate for wide variations in aerodynamic load.

Design and Operation

The THS can rotate around its span-wise pivot point, typically through actuators driven by electric or hydraulic motors. The system is controlled via:

Trim wheels or switches in the cockpit.

Automatic trim systems, such as autopilot and stability augmentation systems.

When trimmed:

The stabilizer rotates slightly (often within a range of ±10–15°).

This changes the aircraft’s pitching moment without requiring constant elevator deflection.

As a result, aerodynamic drag is reduced, and flight efficiency improves.

Applications

1. Commercial Airliners: Most modern jets (e.g., Airbus A320, Boeing 737, 787) use THS for trim control due to large center-of-gravity (CG) variations from payload and fuel distribution.

2. High-Speed Aircraft: Supersonic aircraft (like the Concorde or fighter jets) rely heavily on stabilizer trimming since aerodynamic centers shift significantly with Mach number.

3. Business Jets and Transport Aircraft: THS allows a wide CG range, improving payload flexibility.

Advantages

Greater trimming authority compared to trim tabs.

Reduced drag and fuel consumption by avoiding continuous elevator deflection.

Improved stability and handling in all phases of flight.

Ability to cope with wide CG variations (forward or aft loading).

Safety Considerations

THS malfunctions can cause serious control issues, as seen in incidents like the MCAS-related Boeing 737 MAX accidents.

Aircraft incorporate redundant systems, mechanical overrides, and cutout switches to ensure safety.

Regular inspection and maintenance of actuators, jackscrews, and control electronics are crucial.

Conclusion

The trimmable horizontal stabilizer is a vital innovation in aircraft stability and control. By allowing the entire stabilizer surface to rotate, it provides greater trim capability, reduces drag, and accommodates the operational demands of modern high-performance and commercial aircraft. Its reliability and safety are paramount, making it one of the most carefully engineered systems in aviation.

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