Brake System Overview

A brake turns the kinetic energy of a hundred-tonne aircraft into heat in a matter of seconds. So every design choice in the system serves two ends: stop reliably (multiple fallbacks) and shed heat safely (so it does not get hot enough to burst a tyre). This is the entry article for the braking group (09–15, the thickest group in the chapter); it gives the whole picture — the wheel, the carbon discs, the two piston sets, the BSCU, and the five-mode fallback chain — and the later articles open each block.

Per FCOM DSC-32-30-10:

The main wheels are equipped with carbon multidisc brakes, which can be actuated by either of two independent brake systems. The normal system uses green hydraulic pressure, whilst the alternate system uses the blue hydraulic system (backed up by the hydraulic accumulator).

And the hub is one computer. 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.

1. The wheel — built to prevent a heat-burst or a pressure-burst

Each main wheel is two forged aluminium-alloy halves bolted together with high-strength tie-bolts, and almost everything on it serves heat management and burst prevention. Per AMM 32-41-00:

Each wheel has: ... - a heat shield which decreases the heat transfer from the brake to the wheel and tire - vent holes which give a sufficient flow of air to cool the brakes - thermal fuse plugs which protect the wheel and tire from too much heat (Each thermal fuse plug has a eutectic core that melts at a specified temperature.) ... - a pressure relief valve which protects the tire from over-inflation (The pressure relief valve will rupture at a specified pressure.)

Two of these are directly pilot-relevant, and they protect against opposite causes of the same failure — a burst tyre:

• Thermal fuse plug. When the brake overheats and the heat reaches the wheel, the plug's low-melting-point (eutectic) core melts first and deliberately releases the high-pressure gas from the tyre — letting the tyre deflate gently rather than explode. This is why standing for a period without rolling after heavy braking (e.g. after a rejected take-off) is treated carefully: the heat soaks across and can melt a plug to deflate the tyre.
• Pressure relief valve. From the inflation side — if pressure climbs too high, the valve ruptures to vent, preventing an over-inflation burst.

One guards against a heat-burst, the other against a pressure-burst — opposite causes, the same protective aim. The wheel also carries a heat shield, vent holes, rotor drive keys (driving the brake rotor discs with the wheel), tapered roller bearings, and an axle sleeve to protect the axle from heat.

2. Carbon multidisc brakes — where kinetic energy becomes heat

The brake is a carbon multidisc stack: a set of rotor discs that turn with the wheel, interleaved with a set of fixed stator discs. On braking, the pistons clamp the stack and rotor/stator friction converts kinetic energy to heat. Carbon is chosen because it tolerates high temperature, is light, and has a large heat capacity — well-suited to absorbing the large heat load of a landing. Brake life is counted in landings (1500 basic / 2500 alternative — see the Overview).

3. Two piston sets — a physically-isolated redundancy

Each brake has two independent sets of hydraulic pistons:

• Set 1 — pushed by normal braking, on green hydraulics.
• Set 2 — pushed by alternate and parking braking, on blue hydraulics or the accumulator.

The reason is physical isolation. If green is leaking or contaminated, blue can still brake through set 2 — and vice versa. This is the hardware basis of "two independent systems": not one piston set with a switched hydraulic source, but separate pistons each driven by its own system. It pairs with the dual brake manifolds at the leg (one feed for normal, another for alternate — see Main Landing Gear).

4. The BSCU — the dual-channel brake brain

The BSCU runs four main functions (normal, alternate, autobrake, antiskid) plus three secondary ones (residual-pressure check, temperature monitoring, wheel-speed output to other systems). Dual-channel means two independent internal channels, one active and one monitoring/standby, swapping when the gear is selected DOWN or a channel fails (see BSCU Architecture and BITE).

The core of antiskid (the principle here; the detail in Antiskid). Per AMM 32-40-00:

The BSCU controls the anti skid function. To do this it compares the actual wheel speeds with the aircraft speed. The BSCU then releases the brake of the wheel that starts to be in a skid condition.

5. The four-layer fallback — each step more primitive, more robust

Walking down the chain:

• Layer 1 (green): pedals or autobrake, with antiskid.
• Layer 2 (blue): on loss of normal braking or low green pressure, an automatic changeover sends pedal demand to set 2 on blue, the BSCU still driving four servovalves for antiskid.
• Layer 3 (blue/accumulator): with the A/SKID & N/W STRG switch OFF, or the alternate's electrical control unavailable, it drops to pure pedal-proportional pressure with no electrical power — hence the towing mode. If blue is unavailable, the accumulator takes over. Per AMM 32-40-00: accumulators (filled from the Blue hydraulic system) give sufficient pressure for at least 7 full operations of the brakes.
• Layer 4 (accumulator): the PARK BRK handle, on the accumulator or blue, to a maximum of 175 bar; the accumulator holds the brakes for at least 12 hours (see Parking and Ultimate Emergency Braking). Per AMM 32-40-00: ...let the available hydraulic supply pressure (to a maximum of 175 bar) go to the brakes.

The pattern: green pushes set 1 (normal), blue pushes set 2 (alternate and parking); antiskid steps down from yes → yes → no → no; the electrical dependence steps down from needed → needed → not needed → not needed. The further down the chain, the more primitive but the more robust.

[!warning]- Five misconceptions this article corrects (1) Normal and alternate do not share one piston set with a switched source — there are two independent piston sets (green pushes set 1, blue pushes set 2), physically isolated. (2) A melted thermal fuse plug is not a burst tyre — it is an active protection: the eutectic core melts to deflate the tyre gently rather than let it explode. (3) Alternate braking is not on yellow — it is blue (plus the accumulator). (4) Towing does not use normal braking — it uses alternate-without-antiskid (no electrical power needed); the accumulator gives at least 7 full applications. (5) The BSCU does not work only in normal braking — it runs all four modes and antiskid, still driving the servovalves in alternate-with-antiskid.

Self-test

[!note]- Q1. What do the thermal fuse plug and the pressure relief valve each protect against, and how do they differ?

Both protect against a burst tyre, from opposite causes. The thermal fuse plug protects against a heat-burst: its eutectic core melts when the brake overheats and deflates the tyre gently. The pressure relief valve protects against a pressure-burst: it ruptures to vent if inflation pressure climbs too high. One acts on heat, the other on pressure.

[!note]- Q2. Each brake has two piston sets. Who pushes each set, on which hydraulics, and why two sets?

Set 1 is pushed by normal braking on green; set 2 by alternate and parking braking on blue or the accumulator. Two separate sets give physical isolation — if one hydraulic system leaks or is contaminated, the other still brakes through its own pistons. It is not one piston set with a switched source.

[!note]- Q3. What four main functions does the BSCU control, and what does "dual channel" mean?

Normal braking, alternate braking, autobrake, and antiskid (plus secondary functions: residual-pressure check, temperature monitoring, wheel-speed output). Dual channel means two independent internal channels, one active and one monitoring/standby, swapping when the gear is selected DOWN or a channel fails.

[!note]- Q4. Give the four-layer fallback order with each layer's hydraulic source and whether it has antiskid.

Normal (green, set 1, antiskid) → alternate with antiskid (blue, set 2, antiskid) → alternate without antiskid (blue/accumulator, set 2, no antiskid) → parking/ultimate (accumulator, set 2, no antiskid). Antiskid steps down yes/yes/no/no and the electrical dependence steps down needed/needed/not/not — each layer more primitive but more robust.

[!note]- Q5. Give three brake hard numbers.

Maximum brake pressure 175 bar; the accumulator gives at least 7 full brake applications; the parking brake holds for at least 12 hours.

Key takeaways

References

A330 specifics per FCOM DSC-32-30-10 (General — carbon multidisc, green normal / blue alternate + accumulator, BSCU dual-channel four functions, fuse-plug burst protection, cooling fans, BSCU swap at landing), AMM 32-40-00 (Wheels and Brakes — description and operation: the braking modes with their hydraulic sources, pistons and antiskid, the antiskid logic, accumulator ≥ 7 applications, parking 175 bar, retraction braking), and AMM 32-41-00 (Wheels — two aluminium-alloy halves, thermal fuse plug eutectic core, pressure relief valve, heat shield, vent holes, rotor drive keys, axle sleeve, tyre sizes). The overview diagram and the "more primitive, more robust" framing are integrative syntheses of the AMM modes and the FCOM General. Carbon-disc wear measurement and brake assembly are maintenance-layer. Each sub-system is developed in articles 09–15.

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.