Rudder and Yaw

Yaw is the one axis the A330 flies with a single surface. Per FCOM DSC-27-10-20:

Yaw control is achieved by one rudder surface.

Behind that single surface, however, sits the most distinctive architecture in the whole flight-control chapter. In pitch and roll you fly the sidestick and ask for a result — give me this much. In normal flight you barely touch the pedals at all. The rudder is the surface with two identities: most of the time the computers work it for you (coordinating turns, damping Dutch roll, trimming for asymmetry), so you leave it alone; but it is also the surface you physically push for engine-failure control, crosswind, decrab, and ground steering. And it differs from every other surface in one decisive way — Normal Law's envelope protection does not protect the yaw axis. Nothing in the system stops you over-using the rudder, and over-using it can break the fin.

This article follows the yaw path from pedal to surface: rudder-by-wire signalling, the three parallel servocontrols, the speed-scheduled travel limiter, turn coordination and the yaw damper, manufactured pedal feel and rudder trim, and the BCM that catches yaw when everything else is gone.

Two warnings up front, because they re-frame everything below:

[!warning]- The rudder is rudder-by-wire, not a cable-and-pulley surface. Clear the classic mental model.

There is no mechanical cable from the pedals to the rudder. Pedal movement is read by position transducers and sent to the flight control computers, which drive three hydraulic servocontrols. The two pilots' pedals are rigidly interconnected to each other by a rod, but that rod is between the captain and first-officer pedal sets — it is not a run to the surface. If you carry an A330ceo "the rudder is the one mechanical surface" model into this chapter, every degradation case will read wrong.

[!warning]- The rudder travel limiter is not a structural-damage protection. It will not stop you breaking the fin.

The travel limiter scales down the maximum single-direction deflection as speed rises, which reduces the load from a single large input. It does nothing about rapid, reversing pedal inputs (pedal reversal), where the loads add up and can exceed the structural limit. The FCTM says so explicitly (§4). The "soft wall" the limiter provides is for over-deflection in one direction; it is not a railing that makes aggressive footwork safe.

1. One rudder, two identities

The maintenance source states the full job of the rudder in one passage — note how many different tasks ride on this single surface. Per AMM 27-20-00:

The rudder control mainly provides the yaw control of the aircraft. Associated with the ailerons and spoilers, it ensures automatically the roll/yaw coordination during turns and the damping of the Dutch roll. It is also used for the aircraft guidance on the ground. In case of total loss of the normal servoing, the rudder also permits the yaw control of the aircraft by means of an electrical back-up.

Four roles hide in that sentence, and they map directly onto the rest of this article:

• Primary yaw control — the surface that yaws the aircraft.
• Automatic coordination and damping — worked together with ailerons and spoilers by the computers, so the pilot does not normally touch the pedals in flight (§5).
• Ground guidance — the pedals steer on the ground, where you use the rudder actively.
• An electrical back-up for the total-loss case — the BCM (§7).

The single most important contrast with pitch and roll is protection. Normal Law holds the aircraft inside the angle-of-attack, load-factor, and bank limits no matter what the pilot commands — but only in pitch and roll. The rudder, "as on conventional aircraft", has no such protection (Flight Control Fundamentals §8). That is the thread running through travel limiting (§4), turn coordination (§5), and the whole "be gentle on the rudder" discipline.

2. The control chain — pedals to surface

The human-language version of the loop: you push the pedals → pedal-position transducers turn "how much" into electrical signals → those go to the PRIMs, the SECs, and the BCM → the computers compute "how much rudder" for the active law → they drive three hydraulic servocontrols onto the surface → surface-position feedback closes the loop. The AMM states the chain and its multiple input sources. Per AMM 27-20-00:

In manual mode, the rudder is controlled from the pedals or the side sticks. Associated position transducer units send electrical signals to the Flight Control Primary Computers (FCPCs), Flight Control Secondary Computers (FCSCs) and Back-up Control Module (BCM). The computers generate command orders to the servocontrols, depending on different control laws. Three electrohydraulic servocontrols, simultaneously active, actuate the rudder.

That half-line "or the side sticks" is worth pausing on: the lateral law adds the turn-coordination share of yaw onto the rudder from your roll demand. So the sidestick counts as a rudder input — but you never feel as if you are pushing the rudder, because the computer is doing it for you (§5).

Who drives which servo is stated precisely by FCOM — and the order of degradation is the part to memorise. Per FCOM DSC-27-10-20:

In normal operation, PRIM 1 controls the green hydraulic servo control, PRIM 2 controls the blue hydraulic servo control, and PRIM 3 controls the yellow hydraulic servo control. If all the 3 PRIMs fail, SEC 1 controls the green hydraulic servo control. In case of a total electrical failure, or loss of rudder control due to flight control computers failure, the Backup Control Module (BCM) controls the yellow hydraulic servo control, or the blue hydraulic servo control, if the yellow hydraulic servo control is not available.

The AMM adds the wiring detail that explains why the back-up assignments fall the way they do. Per AMM 27-24-00:

Each servocontrol is connected to one Flight Control Primary Computer (FCPC). In addition: the upper and lower servocontrols are connected to the Back-Up Control Module (BCM), the middle servocontrol is connected to one Flight Control Secondary computer (FCSC).

Put the two together with the hydraulic split (middle = Green, the upper and lower pair = Blue and Yellow):

   pedals (CAPT ══ rigid rod ══ F/O) + PFTU + PDFU      RUD TRIM panel
                    │ position transducers                    │
                    ▼                                          ▼
   ┌─────────────────────────────────────────────────────────────┐
   │  3 PRIM (FCPC)        2 SEC (FCSC)              1 BCM         │
   │  laws · travel limit  yaw damper · trim · t.l.  back-up yaw   │
   └──┬──────┬──────┬──────────┬──────────────────────┬──────┬────┘
      │P1    │P2    │P3        │S1 (backs middle)      │BCM   │BCM
      ▼      ▼      ▼          ▼                       ▼      ▼
   ┌──────┐┌──────┐┌──────┐  (Green)            (Blue)│  (Yellow)
   │MIDDLE││UPPER ││LOWER │                           │
   │GREEN ││ BLUE ││YELLOW│  3 servos in parallel ────┘
   └───┬──┘└───┬──┘└───┬──┘  (all active in normal op)
       └───────┼───────┘
               ▼
         ┌───────────┐
         │  RUDDER   │
         └───────────┘

Reading the ladder off the diagram: normally P1/P2/P3 each work one servo → if all three PRIMs fail, SEC 1 alone works the Green (middle) servo, and a single servo is enough to control the rudder → if even the computers are gone, or the aircraft loses all electrical power, the BCM works the Yellow (or, if Yellow is unavailable, the Blue) servo. What this means for the pilot is not the computer names but the intuition behind them: the yaw path stacks PRIM → SEC → BCM electrical back-ups on top of three independent hydraulic systems, so making the rudder genuinely uncommandable takes an extremely improbable multiple failure.

3. Three parallel servos and effort synchronisation

This is the rudder's most counter-intuitive feature. The elevators and ailerons run "one servo active, the other damping (following)", because two active actuators driving the same surface would fight each other — one wanting more, one wanting less, wasting hydraulic power and stressing structure. The rudder does the opposite: all three servos are active at once. Per FCOM DSC-27-10-20:

The rudder is actuated by 3 independent hydraulic servo controls operating in parallel. In normal operation, the 3 servo controls are simultaneously in active mode. In case of an electrical or hydraulic failure, the corresponding servo control is in damping mode.

So how do three active servos not tear at one another? The answer is in the actuator's internal description: differential-pressure transducers measure each servo's effort and the computer synchronises them. Per AMM 27-24-00:

Two differential pressure transducers sense the load developed by the actuator. One pressure transducer sends the load value to the FCPC to synchronize the loads between the servocontrols in the active mode.

In plain terms: every servo reports "how hard I am pushing", and the FCPC trims its commands so the three share the load evenly and move in step — none over-pushing, none lagging. That effort synchronisation is what eliminates force-fighting; three forces combine into one clean surface movement.

Each servo carries the two-mode behaviour common to all EFCS servocontrols (Servocontrols and Actuation), and on the rudder it picks up extra ground duty. Per AMM 27-24-00, the damping mode:

This mode prevents flutter in the event of multiple failures. The servocontrols also dampen rudder movement in high winds on the ground, and limit the speed of the deflection of the rudder when electrical or hydraulic power is not available.

When a servo loses its hydraulic supply or electrical signal, it automatically reverts to damping — it follows the surface without applying force, so it neither drags the other two nor lets the surface flutter freely.

Inside the actuator — three hardware protections

A failed or over-pressed servo does not become dead weight. The AMM describes three internal protections that keep a degraded servo controlled.

Isolation with a self-supplied accumulator. Per AMM 27-24-00:

The inlet (4) and return (5) blocking valves close. The servocontrol is isolated from the aircraft hydraulic system. The accumulator (7) is permanently connected to the return line of the servovalve (1).

After a hydraulic failure the inlet and return blocking valves close, isolating that servo, and a built-in accumulator — permanently connected to the servovalve's return line — keeps the servo supplied so it can hold a stable damping instead of going slack.

Overpressure relief and anticavitation. When an external gust drives a chamber pressure too high, an overpressure valve shifts the mode-selector valve toward damping so the chambers interconnect and bleed off, rather than the actuator or fin structure taking the load; an anticavitation valve makes up oil during interconnection so the chamber does not cavitate. Per AMM 27-24-00 (overpressure protection) the mode-selector valve moves under the action of its spring, and the actuator includes anticavitation valves.

What this means for the pilot: this is the hardware floor beneath the rudder's "no protection" story. Software (the travel limiter, §4) limits your input; the actuator hardware absorbs external loads (gusts, ground winds) and keeps a failed servo in controlled damping. It is exactly why three parallel servos with one failed still leave two driving a full-authority rudder.

4. The rudder travel limiter — a speed-scheduled soft wall

This is the highest-frequency exam topic in the chapter, and the most misread. The need first: at low speed the rudder must deflect a long way to generate enough yaw force (ground steering before rotation, engine-failure trim); but at high speed that same deflection puts a huge aerodynamic load on the fin. So the system dynamically caps the maximum deflection as a function of speed — faster means a smaller allowed maximum. Per FCOM DSC-27-10-20:

The maximum rudder travel deflection gradually reduces as the speed increases, to avoid high structural loads. In the case of a failure that causes loss of the Rudder Travel Limit function, the rudder deflection limit stops at the last value reached. At slats' extension, full rudder authority is recovered. In all cases, the available rudder deflection provides sufficient yaw control within the entire flight envelope. This includes the case of maximum asymmetric thrust.

Reading it line by line:

• "gradually reduces as the speed increases" — a continuous schedule, not stepped. Its input is calibrated airspeed (Vc) from the ADIRUs. Per AMM 27-90-00: The Vc information from the ADIRUs is used by the FCPC (and FCSC via the FCPC) to compute the rudder and pedal travel limitation.
• "limit stops at the last value reached" — if the travel-limit unit fails, the limit freezes at the value in force at the moment of failure; it does not jump open or slam shut.
• "At slats' extension, full rudder authority is recovered" — a safety fallback. If airspeed data is lost (ADR failure) or the unit fails, extending the slats restores full authority, because slats mean a low-speed configuration where a large deflection is safe. The AMM states the fallback directly: per AMM 27-90-00, the computers calculate the rudder and pedal travel limitation if ADR data is lost (full rudder and pedal travel at flap/slat extension).
• "sufficient yaw control... including maximum asymmetric thrust" — even at high speed with the deflection capped to its minimum, what remains is enough to hold the yaw of an engine failure.

A neat piece of engineering at the pedal: the deflection is capped, but the pedal travel is not. Per FCOM DSC-27-10-20:

Regardless of the aircraft speed, therefore the maximum rudder deflection, full rudder pedal travel remains available. However, except at low speed, maximum rudder deflection is achieved before reaching maximum rudder pedal travel.

In plain terms: at high speed you can still push the pedals to the stops, but the surface reaches its capped maximum before the pedals bottom out — the remaining pedal travel is "dead". This keeps the pedal feel continuous and stops it suddenly stiffening at some speed, so the crew never has to remember "how much can I push right now".

[!warning]- The travel limiter does not prevent structural damage. It cannot stop a pedal reversal breaking the fin.

The most common misreading is that the limiter is there to stop you over-stressing the fin. The FCTM contradicts it head-on. Per FCTM AS-RUD:

Regardless of the airborne flight condition, aggressive, full or nearly full, opposite rudder pedal inputs must not be applied. Such inputs can lead to loads higher than the limit, and can result in structural damage or failure. The rudder travel limiter function is not designed to prevent structural damage or failure in the event of such rudder system inputs.

The limiter caps how far you go in one direction; it does nothing about rapidly reversing the pedals. In a reversal, the fin loads from the swing one way and back add up and can far exceed those of a single full deflection — the classic way to break a fin. So this is a soft wall that reduces a single large input's load, not a structural-limit protection. Same philosophy as "the rudder has no envelope protection" (Flight Control Fundamentals §8); the full handling guidance is in QRH Jam and Loss of Control.

5. Turn coordination and the yaw damper — why you leave the pedals alone

In normal flight the computers manage two yaw jobs for you: turn coordination (adding rudder to cancel sideslip when you bank) and Dutch-roll damping (suppressing the roll/yaw oscillation natural to a swept-wing aircraft). The lateral normal law states it plainly. Per FCOM DSC-27-20-10-30:

When the aircraft is in the flight mode, normal law combines the controls of the ailerons, spoilers (except spoilers 1), and rudder (for turn coordination) in the sidestick. The flight crew does not need to use rudder for turn coordination purpose. While in manual flight the flight crew controls the roll and heading, the flight control system automatically limits the roll rate and the bank angle, ensures turn coordination and damps the dutch roll.

A turbulence-damping layer rides on top of the yaw damper in cruise. Per FCOM DSC-27-20-10-60:

The PRIMs compute a turbulence damping command, which is added to the normal law command for the elevator and the yaw damper.

Engine failure — the blue beta target tells you how much to push. You don't touch the pedals normally, but an engine failure makes yaw your job. Normal Law re-colours the PFD sideslip index and gives you a pedal-amount reference for best climb. Per FCOM DSC-27-20-10-30:

Should an engine failure occur, the sideslip indication is slightly modified to ensure that optimum pilot rudder application is made to achieve optimum climb performance (ailerons to neutral and spoilers retracted). In the case of an engine failure at takeoff, or at go-around, the sideslip index on the PFD changes from yellow to blue... Zero, the beta target value for optimum performance with appropriate rudder application ... Accelerate if beta target cannot be zeroed with full rudder. The computation is made by the PRIM.

So an engine failure is not flown on feel: push to zero the blue beta target — that is the PRIM-computed optimum pedal amount (ailerons neutral, spoilers retracted, best climb). If full rudder cannot zero it, the speed is too low for the rudder to be effective, and the standard move is not to push harder (there is no margin left) but to accelerate until rudder effectiveness returns. This "zero it or accelerate" branch is the operational mirror of §4: large deflection available at low speed, deflection capped at high speed.

Does any of this survive degradation? Down in Alternate Law, yaw becomes yaw alternate law — damping stays, authority is limited, and turn coordination drops in one configuration. Per FCOM DSC-27-20-20-20:

YAW ALTERNATE LAW — The dutch roll damping function is available, and damper authority is limited to ± 4 ° rudder (CONF 0) and ± 15 ° (other configuration). Turn coordination is also provided, except in CONF 0.

The mnemonic: alternate yaw is not bare — damping remains, but in CONF 0 (clean/cruise) it is limited to ±4° and no longer coordinates turns. Even then, you do not fill the gap with pedals. Per FCTM AS-RUD:

For dutch roll, the flight control laws combined with the natural aircraft damping are sufficient to correctly damp the dutch roll oscillations. Therefore, the flight crew should not use the rudder pedals in order to complement the flight control laws.

The FCTM lists three abnormal cases where you do work the pedals. The first is loss of both dampers — the only case that explicitly authorises pedal use to coordinate. Per FCTM AS-RUD:

Loss of both yaw damper systems: The flight crew uses the rudder pedals as deemed necessary, for turn coordination to prevent excessive sideslip.

The other two are an abnormal-landing-gear landing (pedals for ground directional control), and a rudder-trim runaway (pedals to centre the rudder; full procedure in QRH Jam and Loss of Control). The FCTM closes the set with the point that decides everything. Per FCTM AS-RUD:

Landing with abnormal landing gear position: The flight crew uses the rudder pedals for directional control on the ground. ... In all of the normal or abnormal situations that are described above, correct rudder pedals use does not affect the structural integrity of the aircraft.

So all the "big rudder" occasions — ground crosswind, decrab, landing roll, full rudder against an engine failure, pedals to coordinate after a double damper loss, pedals to centre a runaway, an abnormal-gear landing — are safe when used correctly. The single action that breaks the fin is the repeatedly-prohibited rapid reversal. Keep the two apart: use big rudder confidently when the situation calls for it; just never slam it left-then-right.

[!warning]- Ground rudder may legitimately be large and rapid — that is not the same as the prohibited airborne reversal.

On the ground and in the flare, the rudder is your directional control, and the FCTM expects firm use. Per FCTM AS-RUD: In these circumstances, large and even rapid rudder inputs may be necessary, in order to obtain the appropriate aircraft response. The prohibition that breaks fins is the airborne, aggressive, opposite/reversing input. Do not let "be gentle on the rudder" make you timid with crosswind steering on the runway.

6. Artificial feel and rudder trim — no cable, so the feel is manufactured

If there is no cable between pedals and surface, where does the resistance you feel in the pedals come from? It is manufactured by the PFTU (Pedal Feel and Trim Unit) with springs and motors. Per FCOM DSC-27-10-20:

Inside the PFTU, artificial feel and rudder trim are achieved by two electric motors that position the artificial feel unit. In normal operation SEC 1/MOTOR 1 operate with SEC 2/MOTOR 2 synchronized as a backup.

The AMM explains why two force laws — a lighter one for manual flight and a deliberately heavier one for autopilot, so a stray nudge cannot move the pedals under AP while the crew can still override. Per AMM 27-22-00:

An artificial feel mechanism integrated in the Pedal Feel and Trim Unit (PFTU) produces forces on the rudder pedals. Two force laws are generated, one for manual control of the aircraft and the other for autopilot (AP) control. The forces generated in AP mode are higher to avoid spurious commands at the rudder pedals, while allowing AP override by the flight crew, if necessary.

Trim is a hot-standby pair like the servos, and it fails safe by mechanical locking. Per AMM 27-22-00:

In normal operation, one trim servoloop is in the active mode, the other is in standby... If the two servoings are in standby (failure of both control computers, for example), the output shaft remains immobilized and can react the artificial feel force because of the worm gear no-back.

Even with both trim computers dead, the worm-gear no-back freezes the trim at its last position and still reacts the feel force — the trim cannot run away just because its computers are lost.

In manual flight you trim with the pedestal knob; under autopilot the knob goes dead and the PRIM trims for you. Per FCOM DSC-27-10-20:

In manual flight, the pilot can apply rudder trim via the RUD TRIM rotary switch located on the pedestal. A button is provided on the RUD TRIM panel to reset the rudder trim to zero. Note: With the autopilot engaged, rudder trim orders are computed by PRIM and transmitted to the SEC for actuation. The rudder trim switch and the rudder trim reset pushbutton are not active.

Trim self-check — what it should read in cruise. Knowing how to trim is not enough; you need to know where a normal trim sits, or a non-zero reading will look like asymmetry. Per FCOM DSC-27-20-10-80:

When the aircraft is: In normal cruise range, In straight flight, With the autopilot engaged, With symmetrical engine thrust, and With fuel in the wing tanks distributed symmetrically, the rudder trim should stay between 1.9 ° right and 1.6 ° left. Note: This indication corresponds to a true rudder deflection within ± 1 °, taking into account the permanent offset of rudder trim indication when the aircraft is in cruise conditions (0.9 ° right, 0.6 ° left).

In cruise, a symmetric aircraft showing anything inside 1.9° R to 1.6° L is normal — do not reset it to zero in alarm. The crucial part is the note: the indicated value is not the true deflection. Cruise carries a permanent offset (0.9° R / 0.6° L), so an indicated 1.9° R is really only about 1° of true deflection. Only a reading clearly outside that window (with a symmetric configuration) flags genuine asymmetry — typically a hidden fuel imbalance or thrust asymmetry.

The PDFU — a second pedal unit. On the first-officer side sits the PDFU (Pedal Damper and Friction Unit), separate from the PFTU. Per AMM 27-21-00:

The PDFU: generates a constant friction torque to prevent unwanted rudder pedal movement due to unbalanced weight which can occur in case of disconnection between the pedals and the pedal feel and trim unit, generates a resisting torque, function of the pedal input speed, to improve pilot's feel.

The PDFU also carries a mechanical fuse: if the pedal mechanism jams, the fuse releases the input lever so the whole pedal set is not locked solid — the hardware logic behind the RUDDER PEDAL JAM procedure in QRH Jam and Loss of Control.

Same pedals are also the brakes. The first-officer pedal assembly also carries, per AMM 27-21-00, the master cylinder and the brake system interconnecting mechanism. The rudder pedals therefore do two jobs: pushing the pedal commands rudder (yaw), while pointing the toes forward commands brake pressure — two roles on one pedal, split by the yaw/brake mechanism. This is why a pedal jam can sometimes affect brake feel, and why pedal adjustment must keep both rudder reach and brake reach; the brake side is detailed in ATA-32.

7. BCM — the last electrical path for yaw

When all flight control computers fail, or the aircraft loses all electrical power, pitch survives on the THS manual trim wheel (mechanical) and yaw survives on the BCM. Its full description lives in Flight Control Fundamentals §6 and Electrical Back-up BCM/BPS; the yaw-side summary, per FCOM DSC-27-20-20-50:

The Backup Control Module (BCM) computer provides yaw damping and direct rudder command with pedals. This computer includes its own electrical generator, supplied by the B or Y hydraulic system.

Three points: the BCM works the Yellow (or Blue) servo, physically separate from the PRIM path; it carries its own generator (the BPS), driven by the Blue or Yellow hydraulic system, so it lives through a total electrical loss; and when active it behaves like yaw alternate law — there is no turn coordination, so a banked turn needs you to put in pedal yourself. The AMM shows where its pedal signal comes from — one of the PFTU's lever transducers is dedicated to it. Per AMM 27-22-00:

The lever transducers transmit the lever movement signal. Two of them are linked to the FCPC 2 and FCPC 3, the other to the backup control module.

The BCM/BPS internals, and the "A330 has no EHA/EBHA" clarification, are developed in Electrical Back-up BCM/BPS.

8. Across a flight, and when it steps down

Walk the architecture through six scenes:

1. Crosswind take-off roll — ground mode: the pedals work the rudder directly (no turn-coordination layer), and the low-speed travel limiter gives you a large deflection. Per FCTM AS-RUD, on the ground, in crosswind and decrab and the landing roll, large and even rapid rudder inputs may be necessary — firm, fast ground rudder is legitimate.
2. Engine failure after V1 — push toward the good engine; the reference is the blue beta target — push it to zero (PRIM-computed optimum, best climb). If full rudder cannot zero it, accelerate (§5). After trimming and engaging the autopilot, the knob goes dead and the PRIM trims for you.
3. A banked turn in cruise — sidestick only, pedals untouched: Normal Law coordinates the turn and damps Dutch roll for you. This is §5 in the air.
4. Turbulence / Dutch roll in cruise — do not chase it with pedals. The turbulence-damping command plus the yaw damper handle it; the FCTM orders you not to complement the laws with the pedals.
5. Down to Alternate Law (e.g. multiple PRIM loss) — yaw goes to yaw alternate law: damping remains but is limited to ±4° in CONF 0 and no longer coordinates turns. In a clean-configuration turn with obvious sideslip you may add a gentle amount of pedal; Dutch roll still goes to the damper.
6. Extreme — all flight control computers / total electrical loss — the BCM takes the Yellow (or Blue) servo on its own BPS power; you steer with direct pedal-to-rudder, with no turn coordination. The "very unlikely" last resort.

The matching ECAM / failure entry points are listed here as a map; the full procedures and dispatch rules belong to Control Surface Fault Spectrum and QRH Jam and Loss of Control, and dispatch is governed by each operator's MEL.

[!warning]- Low-visibility autoland leans on the dual trim channel — which is why the PFTU has two motors.

A CAT 3 DUAL autoland requires two rudder-trim channels available; CAT 2 and CAT 3 SINGLE require one (per the QRH required-equipment table for CAT 2 and CAT 3). The SEC 1/MOTOR 1 + SEC 2/MOTOR 2 hot-standby pair in the PFTU (§6) is not redundancy for its own sake — it is what keeps the most demanding automatic landings dispatchable.

Self-test

[!note]- Q1. How many servos drive the rudder, how do they work normally, and why don't three active servos fight each other?

Three independent hydraulic servocontrols in parallel — middle on Green, the upper and lower pair on Blue and Yellow — and in normal operation all three are active at once (unlike the elevators/ailerons, which run one active + one damping). They don't fight because each servo's load is measured by differential-pressure transducers and reported to the FCPC, which synchronises the loads between the servos (effort synchronisation), so the three share the force evenly and move in step. If a servo loses hydraulic or electrical supply it automatically reverts to damping — follows the surface, applies no force — and the remaining two still deliver a full-authority rudder.

[!note]- Q2. What is the rudder travel limiter, where does it freeze if it fails, how is full authority recovered, and is it a structural protection?

It makes the maximum deflection reduce gradually as speed increases, to avoid high structural loads at high speed; its input is Vc from the ADIRUs. If the function fails, the limit stops at the last value reached; extending the slats recovers full rudder authority (a low-speed configuration where a large deflection is safe, also the fallback if ADR data is lost). It is not a structural protection — the FCTM states it "is not designed to prevent structural damage or failure" from aggressive opposite pedal inputs, because it caps single-direction deflection but cannot stop the additive loads of a pedal reversal.

[!note]- Q3. Why don't you use the pedals in normal flight, and do turn coordination and Dutch-roll damping survive Alternate Law?

Normal Law combines the rudder (for turn coordination) into the sidestick and automatically coordinates turns and damps Dutch roll — "the flight crew does not need to use rudder for turn coordination purpose". In yaw alternate law, Dutch-roll damping remains but authority is limited to ±4° (CONF 0) / ±15° (other configurations), and turn coordination is provided except in CONF 0. The one case that explicitly authorises pedals to coordinate yaw is loss of both yaw damper systems.

[!note]- Q4. With no cable to the surface, where do pedal feel and trim come from, and can you trim manually under autopilot?

Feel is manufactured by the PFTU with springs positioned by two electric motors; it generates two force laws — a manual one and a heavier AP one (to resist spurious input while still allowing override). Normal operation is SEC 1/MOTOR 1 active with SEC 2/MOTOR 2 as a synchronised backup. In manual flight you trim with the pedestal RUD TRIM knob (and a reset-to-zero button); with the autopilot engaged the knob and reset button are inactive — the PRIM computes trim and sends it to the SEC. The PDFU on the first-officer side adds constant friction + speed-dependent damping and a mechanical fuse against jams. Self-check: in symmetric cruise the trim should sit between 1.9° R and 1.6° L, and because of a permanent 0.9° R / 0.6° L offset the true deflection is within ±1° — a non-zero reading is normal.

[!note]- Q5. What role does the BCM play in yaw, when does it take over, and what do you lose when it is active?

The BCM is the last electrical back-up for yaw — on a total electrical failure, or loss of rudder control due to flight control computer failure, it works the Yellow servo (or Blue if Yellow is unavailable). It carries its own generator (the BPS, supplied by the Blue or Yellow hydraulic system) and uses its own sensors plus a dedicated PFTU lever transducer for pedal signal, giving direct pedal-to-rudder control. The cost: when active it behaves like yaw alternate law — no turn coordination — so a banked turn needs you to put in pedal to cancel sideslip.

[!note]- Q6. The same pedals do two jobs — what are they, and why does it matter?

Pushing the pedal commands rudder (yaw); pointing the toes forward commands brake pressure — the first-officer pedal assembly carries the master cylinder and the brake-system interconnect, split from the yaw path by the pedal mechanism. It matters because a pedal jam can affect brake feel, and pedal adjustment must preserve both rudder reach and brake reach. The brake side is covered in ATA-32.

Key takeaways

References

Per FCOM DSC-27-10-20 (Yaw control — one rudder surface; PRIM/SEC/BCM servo assignment and degradation; three parallel servos active/damping; travel limiter behaviour and full pedal travel; PFTU artificial feel and rudder trim; manual vs autopilot trim). Per FCOM DSC-27-20-10-30 (lateral normal law — turn coordination/Dutch-roll damping combined into the sidestick; engine-failure blue beta target, zero-or-accelerate). Per FCOM DSC-27-20-10-60 (turbulence damping command added to the yaw damper). Per FCOM DSC-27-20-10-80 (cruise rudder-trim self-check window and permanent offset). Per FCOM DSC-27-20-20-20 (yaw alternate law — damping authority ±4°/±15°, turn coordination except CONF 0). Per FCOM DSC-27-20-20-50 (BCM yaw damping and direct rudder command, BPS from B or Y hydraulic). Per FCTM AS-RUD (rudder use philosophy — no aggressive opposite inputs; travel limiter not a structural protection; do not complement the laws on Dutch roll with pedals; the three abnormal pedal-use cases; correct use does not affect structural integrity; large/rapid ground inputs). Per AMM 27-20-00 (rudder general and system description — multi-role yaw control, manual signal chain, three simultaneously-active servos). Per AMM 27-21-00 (rudder pedals — two sets interconnected by a rigid rod; PDFU friction/damping and mechanical fuse; brake master cylinder and interconnect). Per AMM 27-22-00 (PFTU — two force laws, trim active/standby and worm-gear no-back, lever transducer feeding the BCM). Per AMM 27-24-00 (servocontrols — each on one FCPC plus BCM/FCSC back-up wiring, active/damping modes, differential-pressure load synchronisation, accumulator isolation, overpressure and anticavitation protection). Per AMM 27-90-00 (EFCS interfaces — Vc travel-limit computation; full travel at flap/slat extension if ADR data lost). Per QRH (required-equipment table for CAT 2 and CAT 3 — RUDDER TRIM count; RUDDER JAM / RUDDER PEDAL JAM and RUDDER TRIM RUNAWAY handling). Hydraulic supply per ATA-29; brake-side of the pedals per ATA-32. The "effort synchronisation eliminates force-fighting" naming and the Green-backed-by-SEC1 / Blue-and-Yellow-backed-by-BCM rationale are integrative synthesis from the FCOM degradation order, the AMM servo wiring, and the hydraulic split, to be confirmed against those articles rather than read as single verbatim statements.

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.