Trimmable Horizontal Stabiliser

The two elevators handle the transient pitch task — they slam a few degrees in a fraction of a second to make a manoeuvre. The Trimmable Horizontal Stabiliser (THS) handles the steady task: it moves the whole horizontal tailplane to carry the trim load, so the elevators can sit near neutral with their full travel held in reserve. It is also the single most architecturally important surface in the chapter, because it is the only flight control on the A330 that carries a mechanical input. When every flight-control computer is dead and the aircraft is electrically dark, the pitch-trim wheel still turns the THS through a cable-and-chain run — the last-ditch pitch channel named by the one word also in the fly-by-wire overview (Flight Control Fundamentals).

This article builds the THS from the surface inward: what it does, the two-input/one-actuator architecture, the three control modes, the electrical motor chain, the hydraulic actuation, the fail-safe load paths and brakes that stop it ever running away, the mechanical trim-wheel run, the automatic-trim behaviour in normal law, and the failure handling crews actually use.

[!warning]- The trim wheel is a mechanical signalling channel, not a mechanical force channel.

It is tempting to read "the THS can be mechanically controlled" as "the wheel mechanically muscles the stabiliser, so it works with no hydraulics at all." It does not. The wheel mechanically commands the actuator's control valves; the force that drives the screwjack is always hydraulic. Per FCOM, mechanical control is available if either the blue or the yellow hydraulic system is functioning. Lose both Blue and Yellow and the trim wheel turns against a stabiliser that is now braked in place — there is no muscle behind it. This is the fundamental difference from an old cable-driven tailplane, and it is the spine of every THS failure case.

1. What the THS does — the slow trimming surface

In trimmed flight three forces act in pitch, and they do not share a single point of application. Per FCTM AS-CG:

For pitch control, there are three forces that must be considered when airborne: The weight applied on the CG; The lift applied on the Center of Pressure (CP); The downward force created by the Trimmable Horizontal Stabilizer (THS).

Because weight (at the centre of gravity) and lift (at the centre of pressure) act at different stations, they form a couple. Per FCTM AS-CG:

Because the first two forces are not applied at the same point, they create a pitching moment, that must be counteracted by the THS setting.

That is the THS's entire job: set a download (or reduced download) on the tail that exactly cancels the wing/weight couple, so the elevators have nothing to hold and keep their full authority for manoeuvring and the flare. The THS is therefore a slow surface — capped at 1° per second of travel (§5) — that walks the aircraft's balance point, not a fast surface that flies manoeuvres. The two elevators plus this one stabiliser make up the A330 pitch axis (Elevators).

The travel split between the two surfaces tells the same story. Per FCOM DSC-27-10-20:

The maximum elevator deflection is 30 ° nose up and 15 ° nose down. The maximum THS deflection is 14 ° nose up and 2 ° nose down.

The strongly nose-up-biased THS range (14° up against only 2° down) reflects that the trim job in normal operation is almost always to hold the nose up against a nose-heavy couple.

2. Architecture — two inputs, one actuator, one surface

The whole system reduces to one idea: two command inputs (electrical and mechanical) converge on one electro-hydraulic actuator, which drives one screwjack, which moves one stabiliser. The control side (which way, how much) is kept separate from the actuation side (the force), exactly as for every other surface in the chapter.

        CONTROL (command)                         ACTUATION (force)
  ┌──────────────────────┐
  │ sensors → FCPC feed: │
  │  ADIRS  accelerometers│        one motor       ┌───────────────────────────────┐
  │  FMGEC  SFCC  LGCIU   │       at a time via    │  PTA ──► control valves        │
  │  radio height        │       a clutch          │   │                           │
  │        │             │   M1 ┐                  │   ▼                           │
  │  ┌─────▼──────────┐  │   M2 ┼── 3 electric ───►│  ┌──────────┐                 │
  │  │ 3 PRIM (FCPC)  │──┼── M3 ┘    motors        │  │Blue motor│┐                │   ┌─────┐
  │  │ P1 master      │  │                         │  │Yel. motor│┴ differential   │══►│ THS │
  │  └────────────────┘  │                         │  └──────────┘  + ball screw   │   └──┬──┘
  └──────────────────────┘                         │   no-back brake · 2 POB       │      │
                                                    │   jam protection · shear pins │   elevators
  trim wheel ═══ mechanical run ════ override ═════►│   double load path (LSD)      │
  (handwheel)    cancels electrical command         └───────────────┬───────────────┘
                                                                     │ position (RVDT)
   FCDC ◄────────────────────────────────────────────────────────────┘
    └──► ECAM  (F/CTL SD page: THS position)

Four things to read off the diagram, each opened up below:

1. The THS is fed by an unusually wide set of sensors. Accelerometers, FMGEC (autopilot), SFCC (slat/flap configuration), ADIRS (speed/attitude), radio height and LGCIU all reach the FCPCs, because automatic trim must know the load factor, the configuration and whether the aircraft is about to land (the 100 ft freeze, §8).
2. The electrical output is one motor at a time — P1 through motor 1, P2 through motor 2, P3 through motor 3, selected by a clutch (§4).
3. The trim wheel enters from the other end of the actuator, through an override mechanism, touching no computer — the only "electrics-dead, surface still moves" path on the aircraft (§7).
4. The real muscle is two hydraulic motors driving a ball screw (§5). The three electric motors only command; control and force are separate.

2.1 Component list

2.2 Key numbers — the operational layer

These are the figures a pilot reasons with. (The actuator's structural design loads are kept separate in §6, because they are a different layer — never collapse the two into one sentence.)

3. The three control modes — and why mechanical always wins

The AMM resolves the whole surface into three control modes, and this is the key that unlocks everything else. Per AMM 27-40-00:

There are three control modes for the THS: autoflight (electrical control); manual (electrical control); standby (mechanical control).

• Autoflight (electrical). The autopilot is engaged; the FCPCs trim for you. This is the cruise norm — you do not see the wheel move (though it does turn, because the mechanical run is geared to the actuator).
• Manual (electrical). Hand-flying, but the FCPCs still autotrim per the active law. Note the trap in the wording: manual here means hand-flown, not trim wheel — trimming is still electrical and automatic.
• Standby (mechanical). Only this mode is the wheel itself. Per AMM 27-40-00:

In the standby mode the command signals are transmitted mechanically from the control wheels to the override mechanism. The override mechanism cancels the autotrim signals from the FCPCs. It transmits the mechanical command signals directly to the hydraulic motors of the THS actuator.

So in normal and alternate law you never touch the wheel — trim is automatic; only when electrical control is lost (Direct Law, or a stabiliser-control fault) does the wheel turn from ornament into lifeline. And the instant you do turn it, the override cancels the electrical command. That priority is built into the hardware, not chosen by software — covered mechanically in §7.

4. The electrical channel — three motors, one at a time, stepping down

The electrical side is not "three motors turning together." It is "one motor working, the next taking over if it fails." Per FCOM DSC-27-10-20:

In normal operations, the PRIM 1 controls the elevators and the horizontal stabilizer, and the green hydraulic jacks drive the left and right elevator surfaces. The THS is driven by N° 1 of three electric motors. If a failure occurs in PRIM 1 or the associated hydraulic systems or hydraulic jacks, the system shifts pitch control to PRIM 2. PRIM 2 then controls the elevators via the blue and yellow hydraulic jacks, and controls the THS via the N° 2 electric motor. If neither PRIM 1 nor PRIM 2 are available, the system shifts pitch control to SEC 1 for elevator control, and to PRIM 3 for THS control via the N° 3 electric motor. In case of 3 PRIM failure, SEC 1 controls the elevator. Electrical control of THS is lost. THS actuation is still available through manual pitch trim wheel control.

There is a counter-intuitive point buried in that ladder. Predict the answer before reading on: with all three PRIMs failed, the elevators go to SEC — so does the THS go to SEC too?

[!warning]- All-PRIM-lost sends the elevators to a SEC, but THS electrical control is simply gone — not handed to the SEC.

The elevators reconfigure cleanly: SEC 1 takes them. The THS does not — Electrical control of THS is lost, and only the manual trim wheel remains. The reason is functional: only a PRIM (FCPC) computes autotrim and drives the three trim motors; the SECs have no THS electrical function at all. This is exactly why Direct Law always carries "USE MAN PITCH TRIM" — with all PRIMs gone, autotrim is gone, so the crew must trim by hand. The asymmetry between elevator handover (clean) and THS handover (none) is the most-missed fact about this surface.

How are the three trim motors kept to "one at a time"? An electromagnetic clutch. Per AMM 27-44-00, the PTA has:

Three brushless DC motors. Each three brushless DC motor has an electromagnetic clutch. In normal control, one clutch is energized and the others de-energized.

and the commanding computer drives that motor's valves. Per AMM 27-44-00:

The Flight Control Primary Computer (FCPC) can control the motors to open the control valves.

Chaining the mechanism end to end (synthesis, each link sourced above): the commanding FCPC energises one motor's clutch and drives that brushless motor → the motor turns the PTA output → the PTA opens the actuator's control valves → Blue/Yellow pressure drives the two hydraulic motors → the differential and ball screw move the THS → position transducers feed position back and the valves null. Changing computers means changing which clutch is engaged — the surface does not jump; only the motor issuing the command changes hands.

5. Hydraulic actuation — screwjack, two motors, differential, half speed

Whatever commands it, the force that moves the stabiliser is hydraulic. Per AMM 27-44-00:

The THS actuator is an electrohydraulic unit.

and:

The Blue and the Yellow hydraulic systems supply hydraulic fluid pressure to the motors. The hydraulic motors operate the ball screw jack which moves the THS. A differential gear connects the hydraulic motors to each other.

The differential gear is the elegance of the design: it lets both motors drive the screw together and lets one motor drive it alone. So losing one hydraulic system does not jam or freeze the THS — it slows it. Per AMM 27-44-00:

If one hydraulic supply to the THS actuator becomes unserviceable, the related POB becomes in operation. The POB stops and holds the hydraulic motor shaft. The power differential operates at a low speed. The motor, that stays in operation, operates the THS actuator at half speed.

The Pressure-Off Brake (POB) is the part that does the holding. Per AMM 27-44-00:

Two POBs are installed on the motor driven shafts. If a loss of hydraulic supply occurs on one circuit, the related brake stops the hydraulic motor. The other motor moves the screw jack to the necessary position.

For the pilot this is a deliberately soft degradation: lose Blue or Yellow and trimming continues at half speed, barely noticeable except on a large re-trim. Only when both Blue and Yellow are lost is the THS truly stuck — both motors are unpowered, and (as §6 shows) the brakes hold it at its last position. The electrical trim rate, by the way, is itself capped. Per AMM 27-44-00, the maximum operating speed is limited by the PTA in the electrical mode to 1 degree per second.

6. Fail-safe load paths and the protective brakes

A runaway or a structural failure of the THS is catastrophic — an out-of-control trimming surface can drive the aircraft to a pitch attitude the elevators cannot recover. So the THS is wrapped in more passive protection than any other surface: a brake that prevents it ever back-driving, a doubled load path so a fracture cannot release it, and a jam protection that locks rather than fights.

6.1 No-back brake — the THS only goes where it is driven

Per AMM 27-44-00:

The THS actuator has a no-back brake of ratchet and pawl type. The no-back brake keeps the ball screw jack in its last position. It also stops all the movements of the THS with aerodynamic loads.

A ratchet-and-pawl is a one-way lock: when a motor or the wheel actively drives the screw, it turns; the moment the driving effort stops, the pawl bites and the aerodynamic load — however large — cannot back-drive it. With all hydraulic power gone the no-back and the POBs lock together. Per AMM 27-44-00:

If there is a complete loss of hydraulic power to the THS actuator, the POBs and the no-back brake operate. They hold the THS actuator ball screw-jack in its last position.

The cognitive anchor: the THS is a "set-and-hold" surface. You can drive it somewhere, but it never creeps back on its own — so even in a total electrical-and-hydraulic failure the aircraft keeps the trim it last had and does not abruptly lose its pitch balance.

6.2 Double load path, shear pins and the LSD

The structural parts of the THS actuator are fail-safe: a primary path carries the load, an idle secondary path waits to catch it. Per AMM 27-44-00:

The fail-safe ball screw-jack has a double load path. The secondary load path becomes in operation when there is a failure related to the primary load path.

and the fuselage attachments do the same. Per AMM 27-44-00:

The THS-actuator fuselage mounts have a double load path. The first path transmits the load and the second path has no load.

The problem with a hidden secondary path is knowing when it has engaged. Two devices report it. First, a checkable shear pin breaks under the load the secondary path now carries. Per AMM 27-44-00:

At a load of more than 2350 daN (5283.0092 lbf), the shear pin breaks. This causes the spring to release the piston inside the checkable shear pin. The two switches then send a signal through the RVDT circuit and the Flight Control Computer (FCC) senses the failure.

Second, on some build standards a Load Sensing Device turns the same event into a flight-deck message. Per AMM 27-44-00:

If there is a failure in the primary load path component, the LSD detects that the secondary load path of the THS actuator engages and it gives information to FCPC2 through the Rotary Variable Differential Transducer (RVDT) pack.

and:

The message 'F/CTL PRIM 2 PITCH FAULT' is shown on the ECAM Display Unit (DU).

[!warning]- Secondary load path "in operation" does not mean the THS is broken and unusable.

The intuitive reading — "the secondary path took the load, so the THS has failed" — is wrong. After the secondary path engages the THS is still structurally intact and still actuates; the shear pin and LSD are an early warning, telling the crew "you are now on the backup structure, get on the ground and stop testing it," not an immediate loss of function. That is the whole point of fail-safe design: the first path fracturing does not drop the surface, but the system tells you loudly not to wait. Treat a THS-related PITCH FAULT as "load-path redundancy is now in use" and handle it conservatively (Control-Surface Fault Spectrum).

6.3 Jam protection — lock, don't fight

The actuator runs two hydraulic servo loops. If one valve jams, forcing the other against it would tear the unit, so the design compares them and locks. Per AMM 27-44-00:

The jam protection device compares the two control valve movements. This device has a detent which breaks if the two control valves do not have the same movement.

When the valves disagree the device commands both hydraulic valve blocks to shut off and, per AMM 27-44-00, Then, the THS is locked. Locking a stabiliser at its current setting is far safer than letting two opposed loops fight and possibly run it away — it preserves the last good trim, and the crew still has the elevators and (if available) the wheel. This is the same philosophy as the no-back: the safe state for a THS in trouble is to stop, not to move.

7. The mechanical channel — the trim-wheel run

The standby mode is a continuous mechanical path from the cockpit handwheels to the actuator's input shaft. Per AMM 27-41-00 the mechanical control system comprises:

two graduated scales, a toothed belt, a shaft which has a sprocket and a fixed stop, chains, cables, a cable tension regulator, an actuator input drive shaft.

Turning the wheel drives that run into the actuator. Per AMM 27-41-00:

When you turn the handwheel installed in the cockpit center pedestal, the chain and cable loop move the input shaft. The input shaft moves the mechanical servoloop mechanism through an override mechanism. The override mechanism, which is installed adjacent to the pitch trim actuator (PTA), makes sure that the mechanical control cancels the electrical control.

The override is the physical embodiment of "mechanical wins." Per AMM 27-41-00, its operation is:

The cam moves a roller and releases the brake which limits the output shaft of the PTA and the mechanical input together.

At the same time a piston is pushed to operate the three override mechanism microswitches.

The mechanical control now comes on before the electrical control.

A V-shaped cam on the turning input shaft lifts a roller that releases the brake tying the PTA output to the mechanical input, and a piston trips three microswitches so the electrical command stands down. FCOM compresses the whole priority rule into one line. Per FCOM DSC-27-10-20:

Mechanical control of the THS is available from the pitch trim wheel, at any time, if either the blue or the yellow hydraulic system is functioning. Mechanical control from the pitch trim wheel has priority over electrical control.

Read the qualifier carefully: "at any time" is conditioned on Blue or Yellow being live, because the wheel only commands — the hydraulic motors still supply the force (§5). With both lost the wheel turns a stabiliser the brakes are already holding. The mechanical channel is a signalling path, not a force path: that is precisely how it differs from a classic cable-driven tailplane, and why "mechanical backup" on the A330 is not the same idea as on an aircraft with a manually muscled stabiliser.

8. Automatic trim and the take-off preset

In normal law the THS trims itself throughout the flight; the crew never touch it. FCOM describes the preset-then-autotrim behaviour from the ground up. Per FCOM DSC-27-20-10-20 (ground mode):

Ground mode is active on ground. It is a direct relationship between the sidestick and elevator deflection, without auto trim. The THS is automatically set at 5° up (inside the green band). Before departure, as soon as the three hydraulic systems are available, the THS is automatically set to the takeoff value, calculated by the PRIM, according to the ZFWCG entered in the MCDU and the fuel distribution. This automatic setting occurs only one time, and the flight crew can override it at any time.

Approaching the ground the THS is frozen so it cannot keep running during the flare. Per FCOM DSC-27-20-10-20:

When the aircraft passes 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode.

But 100 ft is only one of several freeze conditions. Per FCOM DSC-27-20-10-20:

Automatic pitch trim is frozen in the following cases: Manual trim order; Radio height below 100 ft for flare; Load factor lower than 0.5 g; In high speed protection.

and there is a hard nose-up limit tied to the high-angle-of-attack protection. Per FCOM DSC-27-20-10-20:

When the angle of attack protection is active, the THS is limited between setting at entry in protection and 2 ° nose down (i.e. further nose up trim cannot be applied).

The same source extends the limit to high load factor and large bank, though the published clause trails off. Per FCOM DSC-27-20-10-20:

Similarly, when the load factor is higher than 1.3 g, or when the bank angle is outside ± 33 °...

— the word Similarly carries the same restriction (nose-up trim inhibited) across to those cases. The angle-of-attack inhibition is the most important of these and is restated in the protection section. Per FCOM DSC-27-20-10-20:

As the aircraft enters the protection at the amber and black strip (αPROT), the system inhibits further nose-up trim beyond the point already reached. The nose-down trim remains available, if the flight crew pushes the stick forward.

[!warning]- Locking out further nose-up trim near the stall is what stops autotrim trimming you into an unrecoverable attitude.

If automatic trim kept adding nose-up as the angle of attack rose, it could park the stabiliser at a large nose-up setting and leave the elevators without enough nose-down authority to lower the nose again. Freezing nose-up trim at αPROT (and leaving nose-down available if the crew push) closes that door in hardware. The aerodynamic counterpart is the THS's own stall — the surface is not an unlimited nose-up device. Per FCTM AS-CG: In approach with flaps extended, there is a nose down moment. This is counteracted by THS nose up setting. and The further forward the CG is, the more the THS nose up setting is required. This can result in a THS stall, particularly in cases of push over when the pilot pushes hard on the sidestick. A forward CG with a large flap setting and a hard push-over can stall the tailplane — which is one reason the take-off trim is bounded by a green band.

The green band exists for exactly that reason. Per FCTM AS-CG:

Setting the pitch trim on ground, before takeoff, provides consistent rotation characteristics and a trimmed aircraft at V2 +10 kt.

and:

...the takeoff trim setting must also be limited, in order to cover required abuse cases at takeoff. These limitations define the green band.

The wheel's scale carries a CG reference, so the crew cross-check the take-off trim against the loadsheet CG; a setting outside the band raises the take-off configuration warning (§9).

9. Operating it and its failures

Six scenes fly the machinery:

1. Pre-departure trim. Enter ZFWCG in the MCDU; once the three hydraulic systems are up, a PRIM sets the THS to the take-off value inside the green band, with the CG scale readable on the wheel. Cross-check it. A THS outside the take-off range raises CONFIG PITCH TRIM NOT IN T.O RANGE (Controls and Indications).
2. Cruise autotrim. In normal law you do nothing; the THS trims slowly (≤ 1°/s) as speed, configuration and fuel shift, and the wheel turns in sympathy. CG changes from fuel transfer re-trim the THS (ATA-28).
3. Flare. Passing 100 ft RA the THS freezes at its current setting and flare mode takes over, so the trimming surface does not keep moving through the round-out.
4. One hydraulic system lost (say Yellow). The Yellow POB brakes its motor; the Blue motor drives the screw alone through the differential at half speed. Trimming continues, barely noticeable. Only Blue and Yellow lost makes the THS inoperative, held by the POBs and the no-back.
5. Down to Direct Law (e.g. all PRIMs lost, or a severe hydraulic/electrical combination). The PFD shows amber "USE MAN PITCH TRIM" — autotrim is gone and the crew trim by hand on the wheel, which works as long as Blue or Yellow is live (Direct Law).
6. Mechanical back-up (the extreme case — all computers lost or total electrical loss). The PFD shows red "MAN PITCH TRIM ONLY"; pitch is the wheel alone (yaw is the BCM). FCOM stresses how unlikely this is — It must be noted that it is very unlikely that the backup will be used, due to the fly-by-wire architecture, and even an electrical emergency configuration or an all-engine flameout leaves Alternate Law available (Mechanical Back-up and BCM).

9.1 F/CTL STAB CTL FAULT — the one to know

The defining THS abnormal is the loss of electrical stabiliser control. Per FCTM PR-AEP-F_CTL:

The ECAM F/CTL STAB CTL FAULT is triggered when the flight control computers detect a loss of electrical control of the stabilizer with the following effects: The pitch control law reverts to alternate law. The autotrim function remains available with the elevators commanded by the SEC. In some failure cases, the manual use of the pitch trim wheel remains available to move the stabilizer. The ECAM procedure requests that the flight crew checks first if the manual pitch trim is available through the trim wheel.

Read what this is not: it is not an immediate hand-trim emergency. The law drops to Alternate, and crucially autotrim stays — the SECs hold trim through the elevators. The first crew action is to check the wheel, and if it works, to hand the trim job back to the stabiliser. Per FCTM PR-AEP-F_CTL the procedure asks the crew to TRIM FOR NEUTRAL ELEV, and adds: This action provides maximum authority to the elevators.

That is the heart of it. If the elevators are left deflected to carry trim, their remaining travel is eaten up; winding the wheel to bring the elevators back to neutral (referenced on the F/CTL SD page) returns full elevator travel for manoeuvring and the flare. The QRH/ECAM procedure pins the numbers. Per FCOM PRO-ABN-F_CTL the PFD displays USE MAN PITCH TRIM and the procedure runs MAN PITCH TRIM ... CHECK; IF MAN TRIM AVAIL: TRIM FOR NEUTRAL ELEV; IF TRIM LOCKED > 8 UP: MAX SPEED ... 180 KT; the law is F/CTL ALTN LAW (PROT LOST) with MAX SPEED ... 330/.82; and IF MAN TRIM NOT AVAIL: PITCH AUTHORITY REDUCED — Start the flare slightly earlier and Do not select CONF FULL or CONF 3.

The logic behind each branch:

• Trim locked above 8° up → limit 180 kt: a stabiliser jammed at a large nose-up setting leaves too little nose-down aerodynamic margin to balance at high speed, so speed is capped.
• Alternate Law (protections lost) → 330/.82: the normal envelope protections are gone, so a manual speed limit replaces them.
• Wheel unavailable → reduced pitch authority: flare earlier, expect more stick, and avoid the large flap settings (CONF FULL/3) that would worsen the handling.

9.2 How to actually move the wheel

FCTM gives the technique, not just the trigger. Because the THS is a large, powerful surface, the wheel is moved gently. Per FCTM AOP-10-30-20:

The pitch trim wheel is used to control pitch. Any action on the pitch trim wheel should be applied smoothly, because the THS effect is significant due to its large size.

And the goal in the backup is survival, not precision. Per FCTM AOP-10-30-20 the objective is not to fly the aircraft accurately, but to maintain the aircraft attitude safe and stabilized, in order to allow the restoration of lost systems. In Direct Law, where you have already re-trimmed by hand, sudden configuration changes will pitch the aircraft faster than you can re-trim. Per FCTM AOP-10-30-20:

The PF must avoid performing large thrust changes, or sudden speedbrake movements, particularly if the center of gravity is aft.

and:

If the speedbrakes are out, and the aircraft has been re-trimmed, the PF must gently retract the speedbrakes to give the aircraft time to re-trim, and thereby avoid a large nose down trim change.

An aft CG is more sensitive here (lower longitudinal stability, larger response to any trim change), and Direct Law has no autotrim to catch it — so slow, deliberate handling is the technique. This pairs with the forward-CG THS-stall trap of §8: forward CG risks stalling the tailplane, aft CG risks being caught out by a trim change you cannot keep up with — the two ends of the CG range, each with its own hazard.

Self-test

[!note]- Q1. What drives the THS, and why is it described as having both an electrical and a mechanical input — with mechanical always winning?

The THS is moved by a ball screw jack driven by two hydraulic motors (Blue and Yellow) through a differential gear. The motors' control valves accept two command inputs: electrical (an FCPC through one of three brushless DC trim motors, one at a time via an electromagnetic clutch) and mechanical (the cockpit handwheel through a cable-and-chain run). Mechanical priority is a hardware fact: turning the wheel rotates a V-cam that releases the brake tying the PTA output to the mechanical input and trips three microswitches, so the mechanical control now comes on before the electrical control and the override mechanism cancels the autotrim signals from the FCPCs. FCOM states the rule flatly: Mechanical control from the pitch trim wheel has priority over electrical control.

[!note]- Q2. In the screwjack/two-motor/three-motor drive, who supplies force, who commands, and what happens to trim speed if one hydraulic system is lost?

Force comes from the two hydraulic motors (Blue/Yellow) turning the ball screw; command/positioning comes from whichever of the three brushless DC motors has its clutch engaged (driven by an FCPC). Normally both hydraulic motors drive the screw together through the differential. Lose one system and its POB brakes that motor; the surviving motor drives the screw alone and the THS runs at half speed — trimming continues, just slower. The electrical trim rate is itself capped at 1° per second by the PTA. Lose both Blue and Yellow and both motors are dead; the POBs and no-back brake hold the screwjack and the THS is inoperative.

[!note]- Q3. What is the no-back brake for, and why won't aerodynamic load back-drive the THS even with all hydraulics lost?

The no-back is a ratchet-and-pawl one-way lock. When a motor or the wheel actively drives the screw it turns freely; the moment the driving effort stops, the pawl bites and stops all the movements of the THS with aerodynamic loads, holding the screwjack at its last position. On a complete loss of hydraulic power the POBs and the no-back operate together. So even in a total hydraulic-and-electrical failure the aircraft keeps the trim it last had — the THS is a "set-and-hold" surface that never creeps back on its own.

[!note]- Q4. With all three PRIMs failed, the elevators go to a SEC. Does the THS go to the SEC too?

No — and this is the classic trap. The elevators reconfigure to SEC 1, but Electrical control of THS is lost; THS actuation is still available through manual pitch trim wheel control. Only a PRIM (FCPC) computes autotrim and drives the three trim motors; the SECs have no THS electrical function. That is precisely why Direct Law always carries USE MAN PITCH TRIM — no PRIM means no autotrim, so the crew must trim by hand on the wheel (which still works while Blue or Yellow is live).

[!note]- Q5. ECAM shows F/CTL STAB CTL FAULT. What law are you in, and what is the first action?

The pitch law reverts to Alternate Law (protections lost), but autotrim remains with the SECs commanding the elevators — so it is a degradation plus a wheel-backup option, not an immediate hand-trim emergency. First action: check whether manual pitch trim is available on the wheel; if it is, TRIM FOR NEUTRAL ELEV — wind the wheel until the elevators sit neutral on the F/CTL SD page, which returns maximum elevator authority for manoeuvring and the flare. Associated limits: ALTN LAW max speed 330/.82; if trim is locked above 8° up, max 180 kt; if the wheel is unavailable, pitch authority is reduced — flare slightly earlier and avoid CONF FULL/3.

[!note]- Q6. The trim wheel is "mechanical," so can you use it with the aircraft electrically dead and both hydraulic systems lost?

No. The wheel is a mechanical signalling channel, not a mechanical force channel. It mechanically commands the actuator's valves, but the screwjack is driven by hydraulic motors — FCOM makes mechanical control available if either the blue or the yellow hydraulic system is functioning. Electrically dead is fine (that is what the mechanical path is for), but with both Blue and Yellow gone the wheel turns against a stabiliser the POBs and no-back are already holding. This is the key difference from a cable-muscled tailplane.

Key takeaways

The pilot's window onto all of this is just the trim wheel and its CG scale, the PFD trim cues, and the F/CTL SD page — everything else lives in the actuator behind the rear pressure bulkhead.

References

Per FCOM DSC-27-10-10 (pitch-axis control split — elevator electrical, stabiliser electrical for normal/alternate, mechanical for manual trim; all surfaces hydraulically-actuated); FCOM DSC-27-10-20 (elevator and THS maximum deflections; the electrical reconfiguration chain PRIM 1→PRIM 2→SEC 1/PRIM 3; loss of THS electrical control with manual wheel remaining; mechanical priority and the Blue/Yellow condition); FCOM DSC-27-20-10-20 (ground mode 5° up preset and ZFWCG take-off setting; flare-mode 100 ft freeze; automatic-trim freeze cases; αPROT nose-up-trim inhibition; the 1.3 g / ±33° "Similarly" clause); FCOM DSC-27-20-20-50 ("MAN PITCH TRIM ONLY" in red; back-up very unlikely, alternate law remains available); FCOM PRO-ABN-F_CTL (F/CTL STAB CTL FAULT procedure — USE MAN PITCH TRIM, TRIM FOR NEUTRAL ELEV, trim-locked-above-8°-up → 180 kt, ALTN LAW 330/.82, pitch authority reduced / flare earlier / no CONF FULL or 3). Per FCTM AS-CG (three-force pitch balance and THS counteracting moment; green band and rotation/V2+10 kt; forward-CG THS stall). Per FCTM PR-AEP-F_CTL (STAB CTL FAULT effects and rationale; autotrim via SEC; TRIM FOR NEUTRAL ELEV providing maximum elevator authority). Per FCTM AOP-10-30-20 (back-up and Direct-Law technique — move the wheel smoothly, objective is to stabilise not fly precisely, gently retract speedbrakes especially with aft CG). Per AMM 27-40-00 (three control modes; standby mode and the override cancelling autotrim). Per AMM 27-41-00 (mechanical run components; handwheel-to-input-shaft path; V-cam override operation and microswitches). Per AMM 27-44-00 (actuator as an electro-hydraulic unit; Blue/Yellow motors, differential, ball screw; PTA three brushless DC motors with electromagnetic clutches; one-system-lost half speed and POB action; complete-loss POB + no-back hold; no-back ratchet-and-pawl; fail-safe double load path of screwjack and fuselage mounts; checkable shear pin at 2350 daN / 5283.0092 lbf; LSD reporting to FCPC2 and F/CTL PRIM 2 PITCH FAULT; jam protection comparing valves and locking the THS; electrical trim rate limited to 1°/s; limit load 32500 daN / 73062.9 lbf). Mechanism chains marked as synthesis (the FCPC→motor→PTA→valve→hydraulic-motor→screw→feedback sequence; the forward-CG vs aft-CG "two ends of the range" framing; the Similarly clause completion for 1.3 g / ±33°) are integrative reasoning over the sourced statements above, not single verbatim manual sentences.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.