BSCU Architecture and BITE

Articles 09–14 kept saying "the BSCU does this, the BSCU does that". This article develops that dual-channel brain shared by braking and steering on its own: how the two channels divide the work and alternate, what functions it runs, how its BITE self-tests, how it tests braking and steering itself before every landing, and how its faults degrade the system. It is the finale of the braking group and the control hub of the chapter.

Per FCOM DSC-32-30-10:

A dual channel Brake and Steering Control Unit (BSCU) controls all braking modes and functions which are the following: Normal braking / Alternate braking / Autobrake / Antiskid. The BSCU performs the following secondary functions: Checks the residual pressure in the brakes / Monitors brake temperature / Provides discrete wheel speed information to other aircraft systems.

1. Two channels, one active, alternating each landing

            ┌──────────── BSCU 3GG ────────────┐
 elec sub-system 1 ─┤ Channel 1 (SYS1) ◄─monitor─►  │
                   │   master of the whole system when active │
 elec sub-system 2 ─┤ Channel 2 (SYS2)               ├─► controls:
                   └─────────────────────────────────┘   · brake selector valve 4GG / servovalves
              power-up → Ch1 active                       · antiskid (increase servovalve current to release)
              each DOWN landing-lever selection            · autobrake programmes
              or a channel failure → changeover            · retraction braking
              after landing, the active channel is logged   · nosewheel steering servovalve 5112GC
              in EEPROM; next landing uses the previously
              standby channel (forced alternation)

 ════════════════ BITE (in each channel) ════════════════
  (1) power-up self-test (ground power-on / recovery after a loss > 5 s)
  (2) continuous fault monitoring — flight memory (60 faults, kept 64 flights) / ground memory (20, cleared next flight)
  (3) pre-landing system function test (NLG locked down → absorber compressed): tests braking + steering + 0.7° nose wheel → CAT III B
  (4) ground maintenance test (MCDU; inhibited in flight: first engine start + 3 min ... touchdown 80 kt + 30 s)
  fault found → log to EEPROM + send to CMS + send to FWS (ECAM)

The changeover timing, per FCOM DSC-32-30-10:

A changeover between the two BSCU systems takes place at each DOWN landing lever selection, or in case one system fails.

So: at power-up Channel 1 is active; selecting the gear down switches to the channel that was standby at the last landing (the EEPROM remembers); a channel failure switches immediately; and the two channels monitor each other. Why forced alternation each landing: to ensure both channels are regularly used in earnest, so one cannot sit idle and be found failed only when needed. Silent faults are the most insidious, and forced alternation flushes them out.

2. Everything the BSCU controls

One BSCU ties together almost every article from 09 to 17 — it is the real hub of A330 braking and steering.

3. BITE — three kinds of test and fault recording

Each BSCU channel, per AMM 32-46-00, contains BITE for: - the Normal braking system - the Alternate braking with anti-skid - the Alternate braking without anti-skid - the Brake temperature system - the Steering system - the interface with other specified systems. When a fault is found and confirmed, per AMM 32-46-00:

When the software has made sure that a failure has occurred and is correctly identified, the BSCU puts the data into a specified area of the EEPROM.

The data is then available to the central maintenance system and the flight warning system. Fault records split into flight and ground memory. Per AMM 32-46-00:

If a failure occurs when the aircraft is in flight, the failure data is put into the flight memory. This memory has the capacity to keep the data of 60 failures. Each data entry is kept in the memory for 64 flights... If a failure occurs when the aircraft is on the ground the failure data is put into the ground memory. This memory has the capacity to keep the data of 20 failures... All this data is erased at the start of the subsequent flight.

A given fault is logged once per flight; an intermittent fault logs the first occurrence plus a count (up to four). This lets maintenance distinguish a persistent fault from an occasional flutter.

4. The automatic pre-landing function test (worth knowing as a pilot)

The BSCU does not wait for trouble — it tests braking and steering automatically before every landing. Per AMM 32-46-00:

The system function test occurs automatically during the aircraft's usual sequence of operations... The test starts when the aircraft prepares to make a landing (Nose Landing Gear locked down) and continues until the landing gear shock absorbers are compressed.

It tests three things: normal braking (can the active channel energise/de-energise selector valve 4GG, detect a stuck-open valve, build and release green pressure); NWS steering (can it control the steering selector valve, detect stuck-open, drive the servovalve) — and during this the nose wheels turn 0.7°; and the alternate servovalves' current/pressure, release, and independent monitoring. Per AMM 32-46-00:

During the test the nose wheels move through a small steering angle (0.7 of a degree each side of the aircraft centerline).

So a slight nose-wheel movement before landing is the normal self-test, not a fault. And the result of this self-test bears directly on CAT III B capability. Per AMM 32-46-00:

The result of the NWS functional test is used as an input to make sure a CAT III B landing is possible. This is done through four BSCU validity discrete signals to the FMGEC.

A low-visibility landing needs healthy nosewheel steering (to hold the centreline after touchdown), so the BSCU feeds this "certificate of health" to the autoflight computer (FMGEC) — if the self-test fails, CAT III B capability may be lost. It is a cross-system link many do not know. The channel doing the function test also alternates each flight (the same alternation idea as §1).

5. The degradation path of a BSCU fault

Per AMM 32-46-00:

These failures can cause the BSCU: to transfer system control to the other channel of the BSCU / to cancel the automatic braking function of the Normal braking system / to cancel the Normal braking or NWS system or the two systems.

The degradation is layered: first try the other channel (lightest); failing that, cancel autobrake (keeping manual braking); only then cancel normal braking (dropping to alternate braking — 12) or NWS (dropping to differential-braking taxi — 21). Keep whatever can be kept — the consistent Airbus degradation philosophy.

6. Why some tests cannot run in flight

Per AMM 32-46-00:

The tests are prevented during flight (between first engine start plus 3 minutes and a wheel speed of 80 kts plus 30 seconds after touchdown).

This window defines "in flight": from 3 minutes after first engine start (preparing to taxi out) to 30 seconds after decelerating through 80 kt on landing (vacating the runway). Maintenance tests are barred in this window so their actions do not interfere with real braking/steering. The power-up self-test and the pre-landing function test are automatic and not in this category; what is barred is the manual ground maintenance test.

[!warning]- Six misconceptions this article corrects (1) The BSCU does not only handle braking — it handles braking and nosewheel steering (the name says Steering). (2) It is not a fixed primary/standby pair — it alternates at each DOWN landing-lever selection (forcing both channels to be used). (3) A slight nose-wheel movement before landing is not a fault — it is the BSCU's automatic function test (0.7° each side). (4) The nose-wheel self-test is not unrelated to landing capability — its result feeds the FMGEC and bears on CAT III B. (5) A BSCU fault does not immediately lose braking — it degrades in layers (change channel → cancel autobrake → cancel normal braking/NWS). (6) BITE tests can not run at any time — the maintenance test is inhibited in flight (engine start + 3 min ... touchdown 80 kt + 30 s).

Self-test

[!note]- Q1. When do the two BSCU channels change over, and why alternate?

At each DOWN landing-lever selection, or when one channel fails. Alternation forces both channels to be used in earnest, flushing out a silent fault that would otherwise be found only when the idle channel was finally needed.

[!note]- Q2. Besides braking, what does the BSCU control?

Nosewheel steering (servovalve 5112GC), plus the secondary functions: residual-pressure check, brake-temperature monitoring, wheel-speed output to other systems, and BITE.

[!note]- Q3. What does the pre-landing function test check, what is the 0.7°, and how does it relate to CAT III B?

It tests normal braking, the alternate servovalves, and NWS steering, between NLG-locked-down and absorber-compressed. The nose wheels turn 0.7° each side as the steering self-test. The NWS test result is sent (via four BSCU validity discretes) to the FMGEC to confirm a CAT III B landing is possible — a failed self-test may lose CAT III B capability.

[!note]- Q4. What is the layered degradation order of a BSCU fault?

Transfer control to the other channel → cancel the autobrake function → cancel normal braking or NWS (or both). Keep whatever can be kept: a lighter fault changes channel, a heavier one drops to alternate braking or differential-braking taxi.

[!note]- Q5. Why can the maintenance test not run in flight?

It is inhibited between first engine start + 3 minutes and 80 kt + 30 seconds after touchdown — i.e. from taxi-out to vacating the runway — so the test actions do not interfere with real braking and steering. The automatic power-up and pre-landing tests are not affected.

Key takeaways

References

A330 specifics per AMM 32-46-00 (Brakes and Steering BITE — coverage, power-up trigger, flight/ground memory 60 × 64 flights / 20, the pre-landing system function test from NLG-locked-down to absorber-compressed, the 0.7° nose-wheel movement, the NWS-test-to-FMGEC CAT III B link, the layered degradation path, the in-flight test-inhibit window), AMM 32-42-00 (BSCU channels — active/standby, power-up Ch1, EEPROM alternation each landing, mutual monitoring), and FCOM DSC-32-30-10 (the BSCU controlling all braking modes and functions, DOWN-selection changeover, the secondary functions). The architecture diagram and function table are integrative syntheses of the AMM and FCOM text. The BITE maintenance menus are maintenance-layer and are not detailed; the full CAT III B capability logic belongs to ATA-22 (this article only establishes the NWS-test-to-FMGEC interface).

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.