Rudder and Yaw — Fly-by-Wire, Three Active Servos, PFTU and the BCM Backup

The rudder is fly-by-wire: pedals signal the computers, which drive three electrohydraulic servos all active at once. It provides yaw control, automatic roll/yaw coordination, Dutch-roll damping and ground steering — with artificial feel and trim from the PFTU, a speed-dependent travel limiter, and the BCM as the electrical last resort. Unlike pitch and roll, the rudder has no Normal-law protection.

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. — AMM 27-20-00

1. Fly-by-wire rudder — three servos, all active

Per AMM 27-20-00, in manual mode the rudder is controlled from the pedals (and sidesticks for yaw orders); position transducers send signals to the FCPC, FCSC and BCM, and the computers drive the servos per the control laws. Three electrohydraulic servo-controls actuate the rudder, simultaneously active — driven PRIM 1 → green, PRIM 2 → blue, PRIM 3 → yellow (architecture).

[!warning]- The rudder runs three servos at once — not one active / one damping Unlike the ailerons and elevators (one active, one damping), the rudder uses three servos all active simultaneously (AMM 27-20-00), one per hydraulic system. This gives the single rudder surface triple redundancy across green/blue/yellow — losing any one hydraulic or PRIM still leaves two driving it. There is no mechanical cable to the rudder; the pedals are an electrical input.

2. The pedals — PFTU and PDFU

Per AMM 27-21-00 / 27-22-00, the two pedal sets (Capt + F/O, interconnected by a rigid rod) drive two mechanical units:

• the PFTU (Pedal Feel and Trim Unit) — generates the artificial feel (separate manual and AP force laws; AP forces are higher to avoid spurious commands while still allowing override), achieves the trim function, and sends pedal position to the FCPC and BCM;
• the PDFU (Pedal Damper and Friction Unit) — applies a constant friction torque (prevents unwanted pedal movement) and a speed-dependent resisting torque (feel); a mechanical fuse releases the input lever if the system jams.

[!note]- Rudder trim moves the pedals (integrative synthesis) Rudder trim is achieved by a trim motor inside the PFTU that moves the artificial-feel zero point — and therefore the pedals themselves: turn the RUD TRIM switch → FCSC → trim motor, and the pedals physically reposition (AMM 27-22-00). In AP mode the FCPC sends the trim order to the FCSC. So even though the rudder is fly-by-wire, the pedals still move with trim like a conventional aircraft, and the trim indicator reads ±30°.

3. The travel limiter and yaw functions

Per AMM 27-90-00, rudder travel limitation is computed by the PRIM and reduces the available rudder/pedal travel as speed increases (protecting the fin); full rudder and pedal travel is available at flap/slat extension, and if ADR data is lost the limitation defaults to full travel at flap/slat extension. The rudder also provides automatic roll/yaw coordination and Dutch-roll damping (with ailerons/spoilers) and ground guidance.

[!warning]- The rudder has no Normal-law protection — and travel is speed-limited Two points: as on a conventional aircraft, Normal law does not protect the yaw axis (00) — abrupt or full rudder is not envelope-limited the way pitch/roll are. The only restraint is the travel limiter, which caps how much rudder you get at speed (AMM 27-90-00), not how you use it. Full travel returns with flaps/slats out (low speed), where more rudder is needed.

4. The BCM backup

Per AMM 27-20-00 / DSC-27-10-10, on a total loss of normal servoing the rudder is flown by the BCM — direct rudder from the pedals via an independent, self-powered unit (BPS from B/Y hydraulics). The PFTU already sends pedal position to the BCM, so the handover keeps the pedal input alive.

[!note]- The PFTU feeding the BCM is why the pedals still work in a total failure (integrative synthesis) The PFTU sends pedal position to the FCPC and the BCM (AMM 27-22-00) — so when the normal servoing is lost, the BCM already has the pedal signal and can drive the B/Y rudder actuators directly. This is the directional equivalent of the THS trim wheel, achieved electrically.

5. Counterintuitive points

[!warning]- Three rudder servos run active at once One surface, three simultaneously-active servos (G/B/Y), no mechanical cable (AMM 27-20-00).

[!warning]- Trim moves the pedals, and yaw is unprotected Rudder trim repositions the pedals via the PFTU; Normal law does not protect yaw, only the travel limiter caps deflection with speed (AMM 27-22-00 / 27-90-00).

Self-test

[!note]- Q1. How is the rudder actuated, and by which computers/hydraulics? Three electrohydraulic servos, all active: PRIM 1→green, PRIM 2→blue, PRIM 3→yellow; pedals are an electrical input (no cable).

[!note]- Q2. What do the PFTU and PDFU do? PFTU: artificial feel (manual + AP laws), trim (moves the pedals), pedal position to FCPC/BCM. PDFU: friction + speed-dependent feel, mechanical anti-jam fuse.

[!note]- Q3. How does rudder trim work, and its range? RUD TRIM switch → FCSC → trim motor in the PFTU moves the pedals; AP via FCPC→FCSC; indicator ±30°.

[!note]- Q4. What limits rudder authority, and what protects the yaw axis? The speed-dependent travel limiter (PRIM-computed; full travel at flap/slat); Normal law gives the yaw axis no protection.

Key takeaways

References

• AMM 27-20-00 (Rudder — Description and Operation, CONF 23) — rudder provides yaw control, automatic roll/yaw coordination + Dutch-roll damping (with ailerons/spoilers), ground guidance; total loss of normal servoing → yaw control via electrical back-up (BCM, 27-99); manual control from pedals/sidesticks, transducers to FCPC/FCSC/BCM; three electrohydraulic servo-controls simultaneously active; PFTU + PDFU mechanically linked to pedals.
• AMM 27-21-00 / 27-22-00 — two interconnected pedal sets; PDFU (constant friction + speed-dependent resisting torque, mechanical fuse anti-jam); PFTU (artificial feel manual + AP laws with higher AP forces for override, trim function moving the pedals, pedal position to FCPC/BCM); rudder trim via RUD TRIM switch → FCSC → PFTU trim motor, AP via FCPC→FCSC; rudder trim indicator ±30° (PFTU via FCSC 1/2 on ARINC 429).
• AMM 27-90-00 / FCOM DSC-27-10-20 — rudder travel limitation computed by PRIM, reduces travel with speed, full rudder/pedal travel at flap/slat extension (and if ADR data lost); rudder electrical control PRIM 1/2/3 → green/blue/yellow servos, all PRIM lost → SEC 1 green, total electrical/rudder-control loss → BCM yellow/blue.

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.