Alternate Braking

Alternate braking is the second line of defence after normal braking fails — on blue hydraulics, pushing the second piston set, with the pedal demand carried hydraulically (not as an electrical signal). It has two layers of its own: with antiskid and without antiskid. Per AMM 32-43-00:

The Alternate braking with anti-skid system is a secondary electrohydraulic braking system... It automatically becomes available if: specified failures occur in the Normal braking system / the pressure of the Green main hydraulic power supply is less than a specified value. Braking inputs are made only at the brake pedals. These operate a low-pressure hydraulic system. This system causes the Blue main hydraulic power supply to go to the second set of pistons in the brakes.

Three words separate it from normal braking: blue (not green), second piston set (not first), hydraulic pedal transmission (not an electrical signal). The hydraulic source is blue — verified against three places in the source library, not the "yellow" of generic accounts.

1. Two physically independent sets of servovalves

The overview said the two piston sets are physically isolated. Further: normal and alternate each have their own servovalve manifold:

Why alternate antiskid is "coarser": normal braking has 8 servovalves controlling each wheel individually; alternate has 4, each controlling a wheel pair — so alternate antiskid releases in pairs, less finely than normal's per-wheel control. That is one of the costs of the downgrade.

2. The low-pressure system — master cylinders make pedal force into pressure

Normal-braking pedals are electrical signals (the transmitter unit in Normal Braking), but the feel has always come from this low-pressure system — a reservoir plus two master cylinders. Per AMM 32-43-00:

The master cylinders are mechanically connected to the brake pedals. The left pedals of each pair operate one master cylinder and the right pedals of each pair operate the other master cylinder. The master cylinders give artificial feel in proportion to the pedal travel and supply a control pressure to the dual valve.

So this system is dual-purpose: normal braking uses only its artificial feel; alternate braking brings its control pressure into play to drive the dual valve.

3. The dual valve — control pressure amplified to brake pressure, capped at 175 bar

Per AMM 32-43-00:

The dual valve 5403GG controls the quantity and rate of increase in the brake pressure and keeps the quantity of brake pressure to a maximum limit (175 bar). This valve includes two pressure reducing valves.

It has two independent mechanisms (one per MLG side): the master cylinder's control pressure drives a pressure-reducing valve that meters blue-system pressure to the brakes proportionally, reaching balance when brake pressure equals the control pressure. 175 bar is the alternate-braking pressure ceiling — preventing the alternate mode from delivering excessive brake pressure.

4. The alternate servovalve — single coil, permanently energised (the opposite quiescent state)

Per AMM 32-43-00:

The electrical control of these servovalves is different: only one coil is used / the coil is permanently energized with a minimum current to get the pressure to the brakes quickly / the coil control current is increased to a maximum to release the pressure in the brakes (anti-skid).

Against the normal servovalve (two coils; no current = released; control current = metered), the alternate servovalve is energised and applying pressure by default, and the BSCU only drives the current to maximum to release for antiskid. Both share the antiskid action — maximum current releases (the BSCU's uniform method), but their baseline states are opposite.

The changeover to blue is, again, the automatic selector valve 5202GG. Per AMM 32-43-00: The automatic selector valve 5202GG also operates to connect the Blue hydraulic pressure to the dual valve 5403GG. When green pressure drops, this valve shifts to blue and sends blue pressure to the dual valve — no crew action. The crew sees, per AMM 32-43-00, the Wheel page... shows the captions AUTO BRK and NORM BRK in amber. If the BSCU cannot control the anti skid function, ANTI SKID shows in amber.

5. The downgrade to "without antiskid"

Without-antiskid is one step below with-antiskid. Per AMM 32-44-00:

The system is automatically available when the anti-skid function is not available. This occurs when: the A/SKID & N/W STRG switch is set to OFF / electrical control of the Alternate Braking with Anti-skid System is not available.

Normal braking (green · piston set 1 · 8 servovalves per-wheel · electrical · autobrake · antiskid)
   │ low green pressure / normal-braking fault → automatic selector valve shifts to blue
   ▼
Alternate with antiskid (blue · piston set 2 · 4 servovalves in pairs · hydraulic pedals · no autobrake · antiskid)
   │ A/SKID OFF / alternate electrical control lost
   ▼
Alternate without antiskid (blue or accumulator · hydraulic pedals · servovalves fully open · no antiskid → manual)

Without antiskid, the accumulator and the ≤ 1000 PSI rule come in. Per AMM 32-44-00: If the Blue hydraulic supply is not available, accumulators (filled from the Blue hydraulic system) give sufficient pressure for at least seven full operations of the brakes. As electrical power is not necessary to operate the brakes, this system is usually used when the aircraft is towed. And per FCOM DSC-32-30-10: Control is achieved by the pedals (acting on the dual valves). Alternate servo valves are fully open... The pilot must modulate brake pressure at, or below, 1 000 PSI in order to avoid wheel locking.

With no antiskid backstop, too much pressure locks a wheel and bursts a tyre — so watch the triple indicator, cadence-brake, and keep pressure below 1000 PSI. A shut-off valve (9GZ) energises on low blue pressure in flight to isolate the accumulators (This prevents leakage of the accumulators), preserving those precious applications for the ground. The park/alternate accumulator is shared with the parking brake — see Parking and Ultimate Emergency Braking.

6. The triple indicator — why alternate has a pressure gauge and normal does not

Per FCOM DSC-32-30-10: Brake pressure, as well as accumulator pressure, are indicated on a triple indicator located on the center instrument panel. And for normal braking: No brake pressure indication is provided.

The logic is clear: normal braking has antiskid and full BSCU metering, so the pilot need not watch pressure; alternate (especially without antiskid) needs the pilot to modulate pressure manually to avoid a lock, so it must have a gauge (the triple indicator: top = blue/accumulator supply, lower two = left and right brake pressure). The presence or absence of a gauge itself tells you whether the mode requires you to manage pressure by hand.

[!warning]- Seven misconceptions this article corrects (1) Alternate braking is not on yellow — it is blue (verified against three places in the source library). (2) It is not electrically signalled — the pedal demand is carried hydraulically (master cylinder → dual valve); electricity is used only for antiskid. (3) Normal and alternate do not share servovalves — two independent sets (normal 9–12GG, dual-coil, 8; alternate 1GY/2GY, single-coil, 4). (4) Alternate antiskid is not as fine as normal — its 4 servovalves control wheel pairs. (5) Switching to alternate needs no crew action — the automatic selector valve shifts to blue on its own. (6) Without antiskid you may not press freely — keep pressure ≤ 1000 PSI, cadence-brake, watch the triple indicator. (7) Alternate braking has no autobrake, and its initial pedal feel is more sensitive — initial pedal force/travel produces more braking than in normal.

Self-test

[!note]- Q1. How is the pedal demand carried to the brakes in alternate braking, and which two components are key?

Hydraulically. The master cylinder turns pedal force into a control pressure (and provides artificial feel); the dual valve amplifies that into brake pressure, capped at 175 bar, by metering blue-system pressure. So the demand path is mechanical-to-hydraulic, not an electrical signal.

[!note]- Q2. Give the three-mode downgrade order and each trigger.

Normal braking → (low green pressure or a normal-braking fault; automatic selector valve shifts to blue) alternate with antiskid → (A/SKID & N/W STRG OFF, or alternate electrical control lost) alternate without antiskid. Each step loses a capability: autobrake, then antiskid precision, then antiskid altogether.

[!note]- Q3. How does the alternate servovalve differ from the normal one, and why is alternate antiskid coarser?

The alternate servovalve uses one coil, permanently energised at minimum current to apply pressure (maximum current releases for antiskid); the normal one uses two coils, with no current = released and control current metering. Alternate antiskid is coarser because each MLG has 2 alternate servovalves controlling wheel pairs, versus 4 normal servovalves controlling each wheel.

[!note]- Q4. Without antiskid, why keep pressure ≤ 1000 PSI, and how many applications does the accumulator give?

With no antiskid backstop, too much pressure locks a wheel and bursts a tyre, so the pilot modulates pressure at or below 1000 PSI by cadence braking. If blue is unavailable, the accumulator (filled from blue) gives at least seven full brake applications.

[!note]- Q5. Why does alternate braking have a triple indicator while normal braking has no pressure indication?

Normal braking has antiskid and full BSCU metering, so the pilot does not need to watch pressure. Alternate braking (especially without antiskid) needs the pilot to modulate pressure by hand to avoid a lock, so a gauge is provided — the triple indicator (blue/accumulator supply on top, left and right brake pressure below).

Key takeaways

References

A330 specifics per AMM 32-43-00 (Alternate Braking with Anti-Skid — blue / second piston set / hydraulic pedals, the low-pressure system and master cylinders, the dual valve at 175 bar, the single-coil permanently-energised servovalve, the automatic selector valve shift to blue, the amber AUTO BRK/NORM BRK/ANTI SKID captions, paired antiskid release), AMM 32-44-00 (Alternate Braking without Anti-Skid — trigger, accumulator ≥ 7 applications, shut-off valve 9GZ, triple indicator, towing), and FCOM DSC-32-30-10 (the pilot-level four modes, the ≤ 1000 PSI rule, the triple indicator, autobrake inoperative, the more-sensitive initial feel, the no-pressure-indication contrast). The braking-chain diagram and the servovalve comparison are integrative syntheses of the AMM and FCOM text. Part-level mechanical construction is maintenance-layer; the dual shuttle valve and the park/alternate accumulator coupling with the parking brake are in Parking and Ultimate Emergency Braking. 175 bar (the AMM alternate pressure ceiling) and 1000 PSI (the FCOM manual-modulation target) are distinct values with distinct meanings — do not conflate them.

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.