Normal Braking

Normal braking is "electrically-controlled, hydraulically-operated" worked out in full: you press the pedal → it becomes an electrical signal → the BSCU computes → a servovalve turns current into precise hydraulic pressure → the carbon discs clamp. It runs on green hydraulics by default, with antiskid, and is the main ground deceleration. Per AMM 32-42-00:

The Normal braking system is used to decrease the speed of the aircraft when it moves on the ground. The system is electrically controlled and hydraulically operated. The necessary brake control signals are sent from the brake pedals or supplied automatically from a computer, the BSCU... The hydraulic supply is from the Green Main Hydraulic Power System.

This article takes the manual-braking main chain apart. Antiskid is in Antiskid; autobrake in Autobrake; the alternate system in Alternate Braking.

1. Pedal to signal — the transmitter unit

The pedals are paired and mechanically linked (the two left, the two right), so the left and right MLG can be braked independently or together. When pressed, per AMM 32-42-00:

The transmitter unit... has two potentiometers each with four tracks. One potentiometer is for the left MLG and one is for the right MLG... When a brake pedal is operated, the related potentiometer sends an output (to the BSCU) in proportion to the angle through which the brake pedal moves.

Why four tracks: redundancy plus a consistency check (four equal voltages let the BSCU cross-compare and catch a sensor fault). Note that in normal braking the pedal has no real hydraulic feel — the feel is artificial, produced by a low-pressure system in proportion to travel. (That low-pressure system actually belongs to the alternate brakes — see Alternate Braking — and normal braking borrows it for feel.)

2. The servovalve — current into precise pressure (the key to the whole system)

Each brake has one servovalve in the MLG manifold — a flapper-nozzle plus slide-valve electro-hydraulic servo with two coils, one per BSCU channel. Per AMM 32-42-00:

Each servovalve has two coils. A coil is related to each of the BSCU channels (1 and 2). The active channel supplies a control current to its related coil to: - get the correct quantity of pressure in the brake line - connect the brake line to return to release the pressure.

Its states are the key to the entire braking system:

This one table runs three articles. Normal braking meters pressure precisely with control current (current ↔ brake pressure). Antiskid drives the current to its maximum so the servovalve releases instead (see Antiskid). Alternate braking leaves the servovalve de-energised but supplied, so the slide valve opens fully for maximum pressure (metering is then done elsewhere — see Alternate Braking).

3. The automatic selector valve — green fails, blue takes over

This is the automatic junction between normal and alternate braking. Per AMM 32-42-00:

The hydraulic pressure operates the automatic selector valve 5202GG. This valve connects: - the Green hydraulic pressure to the system and isolates the Blue Main Hydraulic System from the Alternate braking system - the Blue hydraulic pressure to the Alternate braking system when the Green hydraulic pressure is not available.

In normal braking, green pressure holds this valve so the brakes are on green and blue is blocked. The moment green pressure drops (the brake selector valve de-energises or the green system loses pressure), blue pressure shifts the valve and the brakes are connected to blue automatically. A throttle valve (5201GG) controls the speed of the changeover so it is not an abrupt, shock-inducing switch. This is the physical realisation of "normal fails, alternate takes over automatically" from the overview — no crew action, one hydraulic valve doing it on its own.

4. The wheel brake — carbon heat pack with automatic wear compensation

The brake unit is a piston housing + torque tube + heat pack. The piston housing is held against rotation by a torque pin to the brake rod, and carries two independent flow paths and two independent piston sets (one normal, one alternate). The heat pack, per AMM 32-42-00:

The heat pack contains carbon-composite components that include rotor discs, a pressure plate, stator discs and an end plate. The pressure plate and the stator discs have slots... that engage splines on the torque tube. The rotor discs have slots in the outer edge, which engage with the torque bars installed in the MLG wheel.

So the rotor discs turn with the wheel (engaged by the wheel's torque bars) and the stators are fixed (splined to the torque tube); the pistons clamp them and the friction turns kinetic energy into heat. Wear is compensated automatically. Per AMM 32-42-00:

The mechanism makes the piston go back to its initial position when the brakes are released. It also makes allowances for wear and keeps the distance between the piston and the pressure plate constant.

As the carbon thins with use, this retraction mechanism "absorbs" the wear so the pedal-travel feel stays consistent. A wear pin protrudes from the piston housing and recedes as the discs wear — on the ground, how far the wear pin protrudes shows how much carbon remains. A heat shield sits between heat pack and wheel, and between torque tube and axle, to block heat.

5. When manual braking is available, and residual-pressure monitoring

Manual braking is not "press and it always brakes" — it has availability conditions. Per AMM 32-42-00:

Manual braking is available when the Green hydraulic pressure is more than a specified value and one of these conditions occur: - the Left MLG or the Right MLG is on the ground - the average computed wheel speed is more than a specified value - the ground speed (from the ADIRS) is less than a specified value.

These conditions ensure normal braking is allowed only when it makes sense (touched down / wheels turning / low speed), preventing brake pressure being applied in the air. Meanwhile a pressure transducer continuously measures brake-line pressure; if residual pressure exceeds a specified value (the brake has not fully released), the wheel-release bar on the WHEEL page turns amber — flagging a possible dragging wheel. This is the BSCU's secondary "residual-pressure check".

6. The brake selector valve and in-flight braking

Per AMM 32-42-00:

The BSCU energizes the selector valve to connect the Green hydraulic pressure to the system when: - the brake pedals move more than a specified limit - the conditions necessary for an automatic braking program are available - a L/G UP selection is made - it does a brake test (this occurs before each landing or when a BITE test is set).

Note "L/G UP → connect green": this is the source of the automatic wheel-braking on retraction (see the Overview) — and retraction braking has no antiskid: The anti-skid system does not operate during in-flight braking (the wheels are off the ground, so there is no skid to prevent).

7. The dual-channel BSCU — alternation each landing

Per AMM 32-42-00:

The BSCU has two isolated channels... Only one channel (the active channel) controls the operation... After each landing the BSCU makes a record in an EEPROM of which channel was active... Before the next landing, the BSCU reads the EEPROM and makes active the channel that was standby during the last landing.

The cleverness is forced alternation: not a fixed primary/standby, but a different channel made active each landing — so both channels are regularly used in earnest, and one cannot sit idle until it fails unnoticed (see BSCU Architecture and BITE).

[!warning]- Six misconceptions this article corrects (1) Normal-braking pedal feel is not real hydraulic feedback — it is artificial feel from a low-pressure system, proportional to travel. (2) The servovalve does not just open/close the brake — it servo-meters pressure (current ↔ pressure, continuously), and maximum current releases (the antiskid action). (3) A green loss does not require the crew to switch to alternate — the automatic selector valve switches to blue on its own. (4) Carbon wear does not lengthen the pedal travel — the retraction mechanism compensates automatically, keeping the piston-to-plate distance constant. (5) Pressing the pedal does not always brake — there are availability conditions (enough green pressure + on ground / wheels turning / low speed), preventing braking in the air. (6) Retraction braking does not use antiskid — there is no antiskid in flight (wheels off the ground, no skid).

Self-test

[!note]- Q1. How is pedal force turned into an electrical signal, and why four potentiometer tracks?

The transmitter unit has two potentiometers (one per MLG), each with four tracks, sending an output to the BSCU in proportion to the pedal angle. Four tracks give redundancy and a consistency check — four equal voltages let the BSCU cross-compare and catch a sensor fault.

[!note]- Q2. Give the servovalve's states, and explain why maximum current releases the brake.

No supply + de-energised → released; supply + control current → pressure equals the current's value (servo metering); supply + maximum current → released (the antiskid action, slide valve to return); supply + de-energised → maximum pressure (alternate braking). Maximum current drives the slide valve to connect the brake line to return, so the brake releases — which is exactly what antiskid needs.

[!note]- Q3. Which valve gives "green fails, blue takes over", and what must the crew do?

The automatic selector valve (5202GG). In normal braking green holds it (brakes on green, blue blocked); when green pressure drops, blue shifts it and connects the brakes to blue automatically — with a throttle valve controlling the changeover speed. The crew does nothing; it is automatic.

[!note]- Q4. What problem does the retraction mechanism solve, and what does the wear pin show?

It returns the piston on release and compensates for wear, keeping the piston-to-pressure-plate distance constant so the pedal feel stays consistent as the carbon thins. The wear pin protrudes from the piston housing and recedes with wear, so its protrusion shows how much carbon remains.

[!note]- Q5. When is manual braking available, and why these conditions?

When green pressure is above a specified value and one of: the left or right MLG is on the ground, the average computed wheel speed is above a specified value, or the ADIRS ground speed is below a specified value. The conditions allow braking only when it makes sense (touched down / wheels turning / low speed), preventing brake pressure being applied in the air.

Key takeaways

References

A330 specifics per AMM 32-42-00 (Normal Braking — description and operation, main chain: pedals and transmitter unit, brake/automatic selector valves and throttle valve, manifold servovalves/safety valves/pressure transducers, wheel brake structure and retraction mechanism, manual-braking availability, residual-pressure monitoring, in-flight braking, dual-channel EEPROM alternation) and FCOM DSC-32-30-10 (green normal hydraulics, BSCU dual-channel). The main-chain diagram and servovalve state table are integrative syntheses of the AMM component and operation text. Antiskid (32-42 section C), autobrake (section B), and the BSCU/BITE detail are in articles 10, 11, and 15. ARINC 429 bit tables and part-level valve assembly are maintenance-layer.

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.