Overview — Fly-by-Wire Architecture, the Computers, and the Four Control Laws

The A330 flight controls are electrically signalled and hydraulically actuated — fly-by-wire. The pilot moves sidesticks and pedals; computers interpret the input, enforce the safe envelope, and drive the surfaces. This overview fixes the architecture the whole chapter builds on: which surfaces are electrical, the 3 PRIM / 2 SEC / 1 BCM computer set, and the four control laws that degrade from Normal down to mechanical.

The flight control surfaces are all: electrically-controlled, and hydraulically-actuated. The stabilizer can also be mechanically-controlled. Pilots use the sidesticks to fly the aircraft in pitch and roll (and in yaw, indirectly, through turn coordination). Computers interpret pilot input and move the flight control surfaces... when in normal law, regardless of the pilot's input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis. However, as on conventional aircraft, the rudder has no such protection. — FCOM DSC-27-10-10

1. Every surface is electrical — including the rudder

Per FCOM DSC-27-10-10, control by axis is:

All surfaces are hydraulically-actuated.

[!warning]- The rudder is fly-by-wire, and the only mechanical path is the THS trim Read the table carefully: the rudder is electrically controlled (rudder-by-wire) — there is no mechanical cable run from pedals to rudder. The single mechanical reversion in the whole system is the THS manual trim (the pitch trim wheel). This is the architecture of this master-library aircraft (FCOM DSC-27-10-10); the classic mental model of a mechanical rudder backup does not apply here. The yaw backup is instead an electrical unit — the BCM, introduced below.

The protection philosophy: in Normal law the computers prevent excessive manoeuvres and envelope exceedance in pitch and roll — but, as on a conventional aircraft, the rudder has no such protection (FCOM DSC-27-10-10).

2. The computer set — 3 PRIM, 2 SEC, 1 BCM

Per FCOM DSC-27-10-10:

• 3 PRIM computers (Flight Control Primary Computer — FCPC), each used for normal, alternate and direct control laws;
• 2 SEC computers (Flight Control Secondary Computer — FCSC) for direct control laws (including the yaw damper function), rudder trim and rudder travel limit;
• 1 BCM (Backup Control Module) for backup yaw damping and direct rudder command with pedals.

[!note]- FCPC = PRIM and FCSC = SEC — one set, two names (integrative synthesis) The hardware names FCPC / FCSC (used in the AMM and on the maintenance side) are the same boxes as PRIM / SEC (used by the crew and on the cockpit pushbuttons). They are not two parallel computer families. There are also 2 FCDC (Flight Control Data Concentrators) that gather data for the EIS/ECAM display — they concentrate data, they do not fly the aircraft.

One computer of any type can control the aircraft and assure safe flight and landing (FCOM DSC-27-10-10) — the redundancy headline.

Master logic

In normal operation, one PRIM computer is declared to be the master (P1)... If the master computer (P1) cannot be the master, then P2 (or P3, if P2 is not available) becomes the master. Note: When the green hydraulic system is lost, P2 replaces P1 as the master computer. In case all PRIM computers are lost, each SEC is its own master and controls its associated servoloop in direct law. A single SEC can provide complete aircraft control in direct law.

• P1 is master: it processes the orders and sends them to P1/P2/P3/S1/S2, which execute on their servo-controls.
• If a computer cannot execute an order, another takes over its task — except for spoiler control.
• Master failure cascade: P1 → P2 → P3; and notably green hydraulic lost → P2 replaces P1.
• All PRIM lost → each SEC becomes its own master in direct law; a single SEC suffices.

3. The BCM — electrical yaw backup

The BCM computer provides yaw damping, and direct rudder command with pedals, via an independent unit, in case of: total electrical failure, or loss of rudder control due to a Flight Control Computer (PRIM and SEC) failure. It includes: its own electrical generator, referred to as the Backup Power Supply (BPS), which is supplied by the B or Y hydraulic system; its own sensors (gyrometers and pedals deflection); control of the B and Y hydraulic actuators.

Per FCOM DSC-27-10-10, the BCM is the answer to "what flies the rudder when the computers or electrics are gone." It is independent: its own generator (BPS) is driven by the B or Y hydraulic system, with its own gyrometers and pedal sensors, controlling the B and Y rudder actuators. When active (yaw alternate law) there is no turn coordination.

[!warning]- The BCM replaces the old mechanical rudder reversion (integrative synthesis) Because the rudder is fly-by-wire, total electrical failure or a PRIM+SEC loss would otherwise leave no rudder. The BCM closes that gap electrically — self-powered from hydraulics, self-sensed — so even with all flight-control computers and aircraft electrics lost, the pedals still move the rudder (24). This is why "mechanical lateral backup" is the wrong frame for this aircraft.

4. The four control laws (the degradation ladder)

The system flies in one of four progressively-degraded laws, covered in detail later in the chapter:

• Normal law — full protections in pitch and roll (05–11);
• Alternate law — reduced/lost protections (12);
• Direct law — stick directly to surface (13);
• Mechanical backup — THS trim (pitch) + BCM (yaw) (15).

[!note]- The ladder is the spine of the chapter (integrative synthesis) Normal → Alternate → Direct → Mechanical is the organizing thread. Each step sheds protections and reverts toward raw control; the reconfiguration chain ties the triggers together. Keep this ladder in mind through every law and protection article.

5. Counterintuitive points

[!warning]- The rudder is electrical and unprotected Rudder = fly-by-wire (no mechanical cable), and Normal law does not protect the yaw axis (FCOM DSC-27-10-10) — pitch/roll are protected, yaw is conventional.

[!warning]- Green hydraulic loss changes the master computer Losing green hydraulics makes P2 replace P1 as master (FCOM DSC-27-10-10) — a hydraulic event with a computer-architecture consequence.

Self-test

[!note]- Q1. Which surfaces are electrically controlled, and what is the only mechanical path? All surfaces (elevator, THS, aileron, spoiler, rudder, speedbrakes) are electrical; the only mechanical path is THS manual trim.

[!note]- Q2. The computer set and the redundancy headline? 3 PRIM (FCPC) + 2 SEC (FCSC) + 1 BCM (and 2 FCDC for display data); one computer of any type can fly and land the aircraft.

[!note]- Q3. What does the BCM do, and how is it powered? Backup yaw damping + direct rudder via pedals, for total electrical failure or PRIM+SEC loss; powered by its own BPS generator driven by B or Y hydraulics.

[!note]- Q4. The four control laws, in order of degradation? Normal → Alternate → Direct → Mechanical backup (THS trim + BCM).

Key takeaways

References

• FCOM DSC-27-10-10 (Flight Controls — General) — all surfaces electrically-controlled + hydraulically-actuated, stabilizer also mechanically-controlled; axis control table (elevator/THS/aileron/spoiler/rudder/speedbrakes electrical); Normal-law protections in pitch and roll, rudder unprotected; 3 PRIM (FCPC) normal/alternate/direct laws, 2 SEC (FCSC) direct laws + yaw damper + rudder trim/travel limit, 1 BCM backup yaw + direct rudder (BPS from B/Y hydraulics, own gyrometers/pedal sensors); master logic (P1→P2→P3, green hyd lost → P2 master, all PRIM lost → SEC direct law); one computer of any type assures safe flight and landing.

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.