An airliner does not remain perfectly balanced in pitch throughout a flight. Weight changes as fuel burns, the centre of gravity varies with loading and fuel transfer, wing pitching moment changes with flap configuration and Mach number, and thrust changes can alter the aircraft’s pitch tendency. If the pilot had to hold continuous elevator force against every one of those changes, long flights would be exhausting and the elevator would operate inefficiently. A trimmable horizontal stabiliser solves the problem by rotating the entire horizontal tail to create the steady balancing force the aircraft needs. FAA technical material describes the horizontal stabiliser as an aerodynamic surface that balances wing lift, pitching moments and aircraft weight, with a trimmable stabiliser and elevator working as distinct parts of the pitch-control system. [1]
The elevator remains the primary fast-acting pitch control. The stabiliser moves much more slowly and establishes the trimmed equilibrium around which the elevator operates. On many conventional transport aircraft, electric or hydraulic motors drive a jackscrew that changes stabiliser angle. Modern fly-by-wire aircraft may automate much of the trimming process, but the aerodynamic purpose remains the same: remove sustained elevator load and balance the aircraft efficiently across a wide flight envelope.
Why the tail produces a balancing force
The wing’s lift does not normally act through exactly the same point as aircraft weight, and the wing itself produces a pitching moment. The horizontal tail supplies a force at a long distance behind the centre of gravity, creating a moment that balances the aircraft.
On a conventional transport, that tail force is often downward in much of the flight envelope, although its exact direction and magnitude depend on configuration and design.
Why elevator alone would be inefficient
An elevator changes the camber of the tail and therefore changes tail force. Holding the elevator deflected continuously can trim the aircraft in principle, but it creates hinge load, control force and aerodynamic drag.
Moving the whole stabiliser lets the tail generate the required steady force with the elevator closer to neutral. The elevator then retains greater authority for manoeuvring and disturbances.
What “trim” means
Trim is the condition in which the aircraft maintains the desired flight state without a sustained pilot control force. If the aircraft tends to pitch nose-up and the pilot must continually push forward, it is out of trim.
Adjusting stabiliser angle changes the tail’s baseline aerodynamic force until the unwanted control force disappears.
The trimmable horizontal stabiliser
Instead of a fixed stabiliser with only a movable elevator, many transport aircraft mount the whole horizontal stabiliser so its incidence angle can change. The leading edge or trailing edge moves vertically through a small angular range.
FAA lessons-learned material on early jet transports describes the trimmable stabiliser as a major development needed to cover the wide range of speed, Mach number and loading conditions encountered by swept-wing airliners. [1]
The jackscrew
A common mechanical solution uses a large screw and travelling nut. Rotating the jackscrew moves one attachment point of the horizontal stabiliser up or down, changing its incidence.
Because the screw mechanism provides high mechanical advantage, it can move the stabiliser against large aerodynamic loads while resisting back-driving. Motors and gearboxes provide the power.
Why the jackscrew moves slowly
The stabiliser has enormous pitch authority because the entire tail surface changes angle. Rapid movement could create a severe aircraft response. The trim drive is therefore intentionally slower than primary elevator actuation and is protected by limits, monitoring and cutout systems.
This speed difference reflects the division of labour: elevator for rapid control, stabiliser for steady trim.
Electric trim
On many conventional aircraft, switches on the control wheel or yoke command electric stabiliser trim. Holding the switch drives a motor in the nose-up or nose-down direction.
The stabiliser position is displayed on a trim indicator so pilots know the current setting and can set the required take-off trim before departure. FAA descriptions of Boeing 707/720 systems show this classic arrangement. [1]
Manual trim wheels
Some airliners retain large manual trim wheels connected mechanically to the stabiliser drive. They provide a direct manual means of moving or observing the trim system and can serve as a backup if electric control is lost.
The wheel can move rapidly during automatic trimming, which is why pilots are trained to keep hands, clothing and objects clear.
Hydraulic stabiliser actuation
Other aircraft use hydraulic motors or actuators to move the stabiliser, controlled by electrical or flight-control commands. The power source changes but the geometric principle remains the same.
Redundant hydraulic systems and control channels can provide fault tolerance for a surface whose incorrect movement has major pitch consequences.
Fly-by-wire automatic trim
In many fly-by-wire designs the flight-control computers automatically trim the stabiliser as part of the normal control law. The pilot commands a flight-path or load-factor response through the sidestick, and the system moves elevators and stabiliser as needed.
This can make stabiliser movement almost invisible to the pilot during normal flight, but position limits and abnormal procedures remain important.
Take-off trim
Before departure, the stabiliser must be set to a position appropriate for aircraft centre of gravity and take-off configuration. Too much nose-up trim can make rotation overly sensitive; too much nose-down trim can require excessive elevator force and lengthen rotation.
Airline load-control data and aircraft systems calculate the required trim range. Crews verify that the setting is within the approved take-off band.
Why centre of gravity changes trim
If the centre of gravity moves forward, the weight acts farther ahead of the aerodynamic balance point, increasing the nose-down moment that the tail must counter. More tail force or a different stabiliser angle is required.
An aft centre of gravity generally requires less balancing tail load. This can reduce trim drag, which is why some long-range aircraft actively manage fuel to optimise CG within approved limits.
Fuel burn changes the balance
Fuel is stored in different tanks at different locations. As it is consumed or transferred, the aircraft centre of gravity can move. The stabiliser is retrimmed throughout the flight to maintain equilibrium.
On aircraft with trim tanks in the tail, automated fuel transfer can intentionally move CG to reduce stabiliser load and cruise drag. FAA technical material describes such automatic CG optimisation on the A330 family. [2]
Flaps change the pitching moment
Extending flaps changes wing camber, pressure distribution and downwash at the tail. The aircraft may pitch nose-up or nose-down depending on design.
The crew or automatic trim system adjusts stabiliser position to remove the sustained elevator force created by the new configuration.
Thrust changes can require trim
Engines are mounted above or below the aircraft centre of gravity, so changing thrust can create a pitch moment. Wing aerodynamics also change as the aircraft accelerates or decelerates.
Trim is therefore adjusted after major power and speed changes even if centre of gravity has not moved.
Mach trim
As a swept-wing aircraft approaches higher Mach numbers, the aerodynamic centre can shift aft and produce a nose-down tendency known as Mach tuck. Some aircraft use automatic Mach-trim systems to command stabiliser or elevator corrections.
This is a specific automatic trim function designed to preserve longitudinal stability and handling at high speed.
Autopilot trim
An autopilot can initially use elevator to hold altitude or flight path, but if sustained elevator deflection is needed, the system trims the stabiliser to unload the elevator.
This keeps the autopilot from continuously fighting a large steady force and preserves elevator authority for turbulence and manoeuvres.
Why the stabiliser has so much authority
Moving the whole tail changes force over a large aerodynamic surface at a long lever arm from the centre of gravity. Even a small stabiliser-angle change can therefore create a substantial pitch moment.
This high authority is useful for covering the full loading and speed envelope but means unintended movement must be prevented and limited carefully.
Mechanical stops
The stabiliser drive includes physical stops that define absolute travel limits. Electrical or software limits normally stop movement before the mechanical stop is reached.
FAA historical airworthiness material shows how limit-switch locations and allowable trim range have been modified when required to preserve controllability. [1]
Trim limit switches
Limit switches or equivalent electronic position limits stop the trim motor at the authorised ends of travel. Redundant logic can prevent a single switch fault from producing unlimited movement.
The exact architecture varies with aircraft generation and certification basis.
Runaway stabiliser
A runaway occurs when the stabiliser continues moving without the correct command. Because the surface has high pitch authority, crews are trained to recognise persistent uncommanded trim and stop the drive using cutout switches, circuit isolation or aircraft-specific procedures.
Modern designs include monitoring intended to prevent or arrest unintended movement automatically, but crew recognition remains an important layer on conventional systems.
Trim cutout
Many aircraft provide switches that remove electrical power or hydraulic control from portions of the stabiliser trim system. If automatic or electric trim behaves abnormally, the crew can isolate it.
Manual trim may remain available depending on design, allowing the aircraft to be balanced without the failed power channel.
Redundant trim motors
Transport aircraft can use multiple motors, electrical paths or control channels so one failure does not remove all trim capability. Normal, alternate and autopilot trim commands may be separated or monitored.
Certification assesses both loss of trim and unintended trim because either can affect controllability.
Jackscrew lubrication
A mechanical jackscrew carries large loads and experiences repeated sliding or rolling contact. Correct lubrication is critical to controlling friction and wear.
FAA airworthiness material has required inspections and lubrication of stabiliser jackscrew assemblies on aircraft where excessive wear could compromise trim capability. [3]
Why thread wear matters
If a screw-and-nut system loses excessive material, backlash and structural capability can deteriorate. Maintenance measures wear through specified inspections and end-play checks rather than waiting for visible failure.
The exact inspection method and limits are aircraft-specific and controlled by approved maintenance data.
Elevator feel versus stabiliser trim
On conventional aircraft, elevator feel systems provide control forces that help pilots sense speed and loading. Stabiliser trim changes the neutral force condition so the pilot does not have to hold that elevator force continuously.
Trimming therefore does not remove pitch-control feel; it shifts the balanced point around which the elevator is used.
Why trimming with the elevator held can hide movement
If a pilot holds significant elevator while the stabiliser moves, the two surfaces can oppose each other temporarily. When the elevator is relaxed, the full effect of the stabiliser position becomes apparent.
This is one reason training emphasises correct trim use rather than treating the stabiliser as an alternative primary pitch control.
Stabiliser trim and stall recovery
During large angle-of-attack changes, the stabiliser position affects available elevator authority. Extreme nose-up trim can make nose-down recovery more difficult because the elevator must overcome a larger tail moment.
Flight-control and stall-protection systems therefore manage trim within the aircraft’s approved envelope.
Stabiliser trim and go-around
A go-around combines rapid thrust increase, flap reconfiguration and acceleration. Each changes pitch tendency. The aircraft may require substantial retrimming as it transitions from landing approach to climb.
Automatic trim or pilot trim inputs progressively unload the elevator as the new configuration stabilises.
Why trim wheels can spin during autopilot operation
On mechanically linked aircraft, automatic stabiliser trim physically drives the same mechanism connected to the cockpit trim wheels. The wheels therefore rotate even though no pilot is touching them.
The movement gives the crew a direct visual and audible cue that the stabiliser is changing position.
Position indication
Stabiliser position is displayed in units, degrees or a graphical scale depending on aircraft type. The take-off range is often marked separately.
The indication is important both for normal configuration and for diagnosing abnormal trim.
Why the tail may move even when the elevator looks neutral
From outside an aircraft, the elevator can appear nearly aligned with the stabiliser while the entire stabiliser itself sits at a noticeably different angle from the fuselage. That is a normal trimmed condition.
The aerodynamic force comes from the complete tail angle, not just visible elevator deflection.
Trim drag
If the tail produces a downward force, the wing must generate extra lift equal to aircraft weight plus that tail load. Extra lift produces induced drag.
Reducing unnecessary tail load through favourable CG position and stabiliser trim can therefore improve cruise efficiency, within the approved loading envelope.
Why aft CG can reduce drag but cannot be unlimited
Moving CG aft reduces the stabilising lever margin and can make the aircraft less longitudinally stable. Designers and operators therefore balance efficiency against handling and certification requirements.
The allowable CG envelope is a hard safety constraint, not an efficiency target to be exceeded.
Stabiliser versus stabilator
A stabilator is an all-moving tail surface used as the primary pitch control on some aircraft. A trimmable horizontal stabiliser on a conventional airliner is different: it moves slowly for trim while a separate elevator handles normal rapid pitch control.
The physical appearance can be similar, but the control purpose and actuation are different.
Maintenance access
The jackscrew and drive mechanism are often located in the tailcone or vertical-fin/horizontal-tail intersection. Access panels allow inspection of screws, nuts, gearboxes, lubrication and limit components.
Because the mechanism is structurally critical, maintenance intervals and procedures are tightly controlled.
Why a trimmable tail is worth the complexity
A fixed tail sized to cover every possible centre-of-gravity and speed condition through elevator deflection alone would create greater drag and control-force challenges. The movable stabiliser gives the aircraft an efficient way to rebalance itself continuously.
Long-range jets especially benefit because weight and aerodynamic conditions change enormously between take-off and landing.
The tail quietly rebalances the aircraft all flight
During a normal flight the stabiliser may move many times: take-off trim is set from loading data, automatic or manual trim follows acceleration, cruise Mach and fuel burn change the balance, descent and flap extension alter pitching moments, and the system retrims again for approach and go-around capability.
The elevator handles the immediate pitch manoeuvre; the stabiliser removes the steady force left behind. That separation lets a large airliner remain balanced and efficient without requiring pilots or autopilots to hold continuous elevator pressure for hours.
Verified Sources / References
- Federal Aviation Administration Transport Airplane Lessons Learned — Trimmable Horizontal Stabiliser. FAA technical explanation of stabiliser aerodynamic purpose, electric trim, jackscrew actuation and trim limits.
- FAA Transport Airplane Lessons Learned — A330 Centre-of-Gravity Management. Technical example of automatic CG optimisation and stabiliser trim-tank use.
- FAA Dynamic Regulatory System — Stabiliser Jackscrew Inspection and Lubrication Requirements. Continued-airworthiness example involving jackscrew wear and trim integrity.
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