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Tổng quan

The Trim System adjusts the baseline aerodynamic equilibrium of the aircraft so that it will fly straight and level — or at the desired attitude — without the pilot applying sustained force on the controls. On a well-trimmed aircraft, releasing the controls produces no tendency to pitch, roll, or yaw. Trimming is essential because aircraft balance changes continuously with fuel burn, passenger movement, flap configuration, and speed.

Nguyên lý Hoạt động

The most powerful trim mechanism on large jets is the Trimmable Horizontal Stabiliser (THS): the entire horizontal tail pivots to adjust the neutral pitch position. This is far more effective than small trim tabs because the full stabiliser generates a large moment. The THS is driven by electric jackscrew motors and can also be moved manually via the stabiliser trim wheels in the cockpit. Airbus FBW aircraft trim the stabiliser automatically to keep the elevator near neutral so that full elevator authority is always available. Roll trim uses small trim tabs or differential stabiliser positioning, while rudder trim adjusts the rudder neutral for engine-out or crosswind corrections. The centre of gravity position profoundly affects trim: an aft CG requires nose-down stabiliser trim, reducing drag and improving fuel efficiency on long sectors.

Các Thành phần Chính

• Trimmable Horizontal Stabiliser (THS): Full-moving horizontal tail driven by dual electric (and often hydraulic backup) jackscrews; primary pitch trim surface.
• Stabiliser Trim Switches: Located on the control columns or yokes; pilots apply pitch trim during all phases of flight.
• Stabiliser Trim Wheels: Manual trim wheels in the cockpit centre pedestal, providing a tactile backup to electric trim.
• Rudder Trim: Adjusts the rudder neutral position; critical for single-engine operations where asymmetric thrust would otherwise require constant rudder pedal force.
• Roll Trim Tabs: Small adjustable surfaces on the ailerons; remove residual roll forces in cruise.

Ứng dụng trên Máy bay

• Boeing 737-800: Electric stabiliser trim via yoke-mounted switches; manual trim wheels remain fully functional. MCAS (Maneuvering Characteristics Augmentation System) controversy centred on automated stabiliser trim commands in certain conditions.
• Airbus A320: Stabiliser automatically trimmed by FBW computers to keep elevator near neutral; crews can also manually adjust via dedicated trim switches on the FCU.
• Boeing 777: Large-authority THS; automatic trim from autopilot keeps stabiliser positioned for optimal efficiency across the long-range mission profile.
• Boeing 787: Composite stabiliser with electric jackscrew driven by the primary flight computers; fuel transfer between fore and aft tanks also used as a CG management trim tool.

Ưu điểm và Hạn chế

Proper trim eliminates sustained pilot workload and improves fuel efficiency by minimising trim drag — the induced drag from deflected control surfaces fighting the basic aerodynamic balance. On long routes, precise CG management via trim can save hundreds of kilograms of fuel. The principal hazard of the trim system is runaway trim: if the electric motor commands continuous stabiliser movement without pilot awareness, control forces can rapidly become too large to overcome manually. Runaway stabiliser drills are a critical part of all airline type-rating training.