Electrical Backup (BCM/BPS) — the Rudder's Self-Powered Last Resort
The electrical back-up is the rudder-only survival system that flies the rudder when the normal computers or aircraft electrics are gone. It is built from two Back-up Power Supplies (BPS) — generators driven by the hydraulics — and one Back-up Control Module (BCM) that reads the pedals, computes yaw, and drives one rudder servo at a time. Note: the A330 has no EHA/EBHA electro-hydrostatic actuators; this BCM/BPS is the electrical backup.
The electrical back-up provides the yaw control of the aircraft if the rudder normal servoing by the flight control computers is not operational. The electrical back-up transmits the pilot orders from the rudder pedals to the rudder and ensures Dutch roll damping. — AMM 27-99-00
1. What it is — two BPS and one BCM
Per AMM 27-99-00, the electrical back-up consists of:
- two Back-up Power Supplies (BPS) — each an electrical generator supplied from one hydraulic system, providing power to the BCM;
- one Back-up Control Module (BCM) — an electronic module powered by the BPS that:
- acquires the pedal position transducer signals,
- computes the yaw order,
- acquires the rudder position transducer signals,
- servos the Yellow rudder servo-control (or Blue if Yellow is unavailable).
[!warning]- It makes its own electrical power from hydraulics — that is the point The BPS is a hydraulically-driven generator (AMM 27-99-00): the back-up does not depend on the aircraft electrical network at all. With two BPS (one per hydraulic system, B and Y), the BCM has power as long as one hydraulic system runs — which is why a total aircraft electrical failure still leaves the rudder flyable from the pedals. This is the directional counterpart of the purely-mechanical THS trim wheel.
2. When and how it activates
Per AMM 27-99-00, in normal operation the BCM is held off by inhibition signals from FCPC1 and FCSC1. The back-up activates when:
- those inhibition signals are absent (from a computer failure or a centralized electrical power supply failure) — the BPS activate and power the BCM if the corresponding hydraulic system is available;
- there is no inhibition signal from FCPC2/3, and the BCM is energised by at least one BPS — then the BCM becomes operative automatically.
The BCM then selects and controls one rudder servo at a time — Yellow in priority, Blue if Yellow is unavailable.
[!note]- The inhibition logic is what keeps it dormant until truly needed (integrative synthesis) The BCM is gated by two inhibition layers: FCPC1/FCSC1 (whose loss arms the BPS) and FCPC2/3 (whose loss lets the BCM go operative). Only when the normal flight-control computers can no longer be servoing the rudder do those inhibitions disappear and the BCM takes over (AMM 27-99-00). This prevents the back-up from fighting the normal system — it engages automatically but only as the genuine last resort, matching the reconfiguration map.
3. How it flies the rudder
Per AMM 27-99-00, once operative the BCM:
- processes the rudder order from a dedicated pedal-position transducer in the PFTU;
- computes the yaw-damper order from a dedicated rate gyro inside the BCM;
- drives the Yellow (or Blue) rudder servo.
So even at the lowest level, the pilot still has pedal rudder control plus Dutch-roll damping — but, as noted in the overview, no turn coordination in this mode.
[!note]- The BCM is fully self-contained — own sensors, own power (integrative synthesis) The BCM has its own rate gyro and reads its own pedal transducer in the PFTU, powered by its own BPS (AMM 27-99-00) — it shares nothing with the normal flight-control computers except the pedals and the rudder servos. That independence is exactly what makes it trustworthy after a total computer/electrical loss: there is no common element left to fail.
4. Counterintuitive points
[!warning]- The A330's electrical backup is rudder-only — no EHA/EBHA The 27-99 electrical back-up powers only the rudder (BCM/BPS); the A330 does not use electro-hydrostatic actuators (AMM 27-99-00).
[!warning]- It is hydraulically powered, not electrically powered The BPS generate electricity from a hydraulic system — the back-up survives a total aircraft electrical failure (AMM 27-99-00).
Self-test
[!note]- Q1. What does the electrical back-up consist of, and what does it control? Two BPS (hydraulically-driven generators) + one BCM; it controls the rudder only (one servo at a time — Yellow priority, Blue backup).
[!note]- Q2. How is the BCM powered, and why does that matter? By the BPS, generators driven by the hydraulics — so it works through a total electrical failure as long as one hydraulic system runs.
[!note]- Q3. What activates the BCM? Loss of the FCPC1/FCSC1 inhibition (computer/electrical failure) arms the BPS; with no FCPC2/3 inhibition and ≥1 BPS energising it, the BCM goes operative automatically.
[!note]- Q4. What does the BCM use to fly the rudder? A PFTU pedal-position transducer (rudder order) and its own rate gyro (yaw-damper order); no turn coordination.
Key takeaways
| Point | Detail |
|---|---|
| Scope | rudder only electrical back-up; no EHA/EBHA on A330 |
| BPS | two, each a generator driven by one hydraulic system → powers the BCM |
| BCM | reads pedals + rudder position, computes yaw order + yaw-damper (own rate gyro), drives Yellow (or Blue) rudder servo |
| Activation | FCPC1/FCSC1 inhibition lost → BPS arm; no FCPC2/3 inhibition + ≥1 BPS → BCM operative |
| Independence | own power (BPS), own gyro, own PFTU pedal transducer; no turn coordination |
References
- AMM 27-99-00 (Electrical Back-up — Description and Operation) — provides yaw control if rudder normal servoing by the flight control computers is not operational; transmits pedal orders to the rudder + Dutch-roll damping; two Back-up Power Supplies (each an electrical generator supplied from one hydraulic system, powering the BCM); one Back-up Control Module (acquires pedal-position transducer signals, computes yaw order, acquires rudder-position transducer signals, servos the Yellow rudder servo-control or Blue if Yellow unavailable); activation in absence of FCPC1/FCSC1 inhibition (computer or centralized electrical power supply failure) with BPS powered if hydraulic system available, BCM operative automatically with no FCPC2/3 inhibition and ≥1 BPS; rudder order from a dedicated PFTU pedal-position transducer, yaw-damper order from a dedicated rate gyro in the BCM; selects one rudder servo at a time (Yellow priority / Blue backup).
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.