EFCS Computers — PRIM, SEC, BCM and the Reconfiguration Chains

The overview named the computer set; this article is how it actually flies the aircraft — what each computer type does, who is master, and the per-axis reconfiguration chains that keep a surface controlled as computers and hydraulics drop out. The redundancy headline holds: one computer of any type can fly and land the aircraft.

In normal operation, one PRIM computer is declared to be the master (P1)... 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. — FCOM DSC-27-10-10

1. What each computer type does

Per FCOM DSC-27-10-10 / DSC-27-20-30, the functional split:

[!note]- PRIM is the brain; SEC is the direct-law fallback; BCM is the yaw lifeboat (integrative synthesis) The PRIMs compute everything sophisticated — the protected Normal law, the alternate and direct degradations, MLA, autopilot laws. The SECs are deliberately simpler: they hold direct law plus the yaw-damper/rudder-trim/travel-limit jobs, so that if all PRIMs are lost the aircraft is still flyable. The BCM is narrower still — only the rudder, only when electrics or all FCCs are gone (24). Capability decreases as you go down the list, exactly matching the law ladder.

2. Master logic

Per FCOM DSC-27-10-10:

• one PRIM is master (P1); it processes the orders and sends them to P1/P2/P3/S1/S2, each executing on its own servo-control;
• if a computer cannot execute an order, another takes over its task — except for spoiler control;
• master cascade P1 → P2 → P3;
• green hydraulic lost → P2 replaces P1 as master;
• all PRIM lost → each SEC is its own master in direct law; a single SEC suffices.

[!warning]- The spoiler exception, and the green-hydraulic master swap Two non-obvious facts: (1) the "another computer takes over" redundancy does not apply to spoiler control (FCOM DSC-27-10-10) — a lost spoiler computer is not picked up by a peer. (2) Losing green hydraulics doesn't just affect actuation — it changes the master computer to P2, because P1's pitch path leans on green (§3).

3. Pitch — the clearest reconfiguration chain

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 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.

[!warning]- When the computers run out, the trim wheel is the last pitch control Follow the chain to its end: with all three PRIMs gone, SEC 1 still flies the elevators, but THS electrical control is lost — and the THS reverts to the mechanical pitch trim wheel (FCOM DSC-27-10-20). This is the single mechanical reversion of the whole system (00, 15), and the chain shows exactly when it appears.

4. Rudder — three PRIMs, then SEC, then BCM

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... if the yellow... is not available.

Per FCOM DSC-27-10-20, the rudder reconfiguration: PRIM 1/2/3 → green/blue/yellow servos; all PRIM lost → SEC 1 (green); total electrical failure or rudder-control loss → BCM (yellow, or blue). Rudder trim is in the PFTU: two electric motors (artificial feel + trim), normally SEC 1 / motor 1, with SEC 2 / motor 2 synchronized as backup.

[!note]- The rudder never runs out of a controller (integrative synthesis) Every other axis can reach a mechanical or no-control end; the rudder instead has a four-deep electrical chain ending at the BCM (FCOM DSC-27-10-20), because there is no mechanical rudder (00). With the autopilot engaged, rudder-trim orders are computed by PRIM and sent to SEC for actuation — the manual trim switch is then inactive.

5. Roll — ailerons and spoilers

Per FCOM DSC-27-10-20, each wing has two ailerons and five spoilers. Each surface's servojack is controlled by a PRIM or SEC and powered from green, yellow or blue. The detailed surface behaviour (aileron droop, the high-speed outboard-aileron logic, spoiler retract-on-fault, symmetric-surface inhibit) is covered in the aileron and spoiler articles; the point here is that roll, like pitch and yaw, distributes its surfaces across multiple computers and multiple hydraulic systems so that no single failure removes roll control.

6. Counterintuitive points

[!warning]- Spoiler control is the one task with no peer takeover Every other order can be picked up by another computer; spoiler control cannot (FCOM DSC-27-10-10).

[!warning]- The rudder backup chain is entirely electrical PRIM → SEC → BCM, never mechanical (FCOM DSC-27-10-20) — the BCM's BPS is self-powered from B/Y hydraulics.

Self-test

[!note]- Q1. What does each computer type compute? PRIM (3): normal/alternate/direct laws, MLA, speedbrake/ground-spoiler logic, abnormal-attitude, AP & speed computation. SEC (2): direct laws + yaw damper + rudder trim/travel limit. BCM (1): backup yaw + direct rudder.

[!note]- Q2. The master cascade, and the green-hydraulic twist? P1 → P2 → P3; green hydraulic lost → P2 becomes master; all PRIM lost → each SEC is its own master in direct law.

[!note]- Q3. The pitch chain to its end? PRIM 1 → PRIM 2 → (SEC 1 elevator + PRIM 3 THS) → SEC 1 elevator, THS via manual trim wheel.

[!note]- Q4. The rudder chain? PRIM 1/2/3 (green/blue/yellow) → SEC 1 (green) → BCM (yellow/blue); no mechanical rudder.

Key takeaways

References

• FCOM DSC-27-10-10 (Flight Controls — General) — computer functional split (3 PRIM/FCPC, 2 SEC/FCSC, 1 BCM); master logic (P1→P2→P3, green hydraulic lost → P2 master, all PRIM lost → each SEC own master in direct law, single SEC suffices); peer takeover except spoiler control; one computer of any type assures safe flight and landing.
• FCOM DSC-27-10-20 (Flight Controls — Architecture) — pitch electrical control chain (PRIM1 green jacks + THS motor 1 → PRIM2 blue/yellow + motor 2 → SEC1 elevator + PRIM3 THS motor 3 → all PRIM lost: SEC1 elevator, THS electrical lost, manual pitch trim wheel); rudder electrical control (PRIM1/2/3 → green/blue/yellow servos → SEC1 green → BCM yellow/blue); rudder trim in PFTU (SEC1/motor1, SEC2/motor2 backup; AP: PRIM computes, SEC actuates); roll = two ailerons + five spoilers per wing.
• FCOM DSC-27-20-30 — PRIM pushbutton function list (normal pitch/lateral, MLA, speedbrakes/ground-spoiler logic, pitch/roll/yaw alternate and direct, rudder travel, aileron droop, abnormal-attitude law, AP-law and characteristic-speed computation).

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.