ATA 32 LEARNING PATH
Read the chapter in sequence
01 · WHEEL / BRAKE HARDWARE
The brake starts as a mechanical energy-conversion device
The A350 nose wheels are unbraked. Braking is applied at the Main Landing Gear wheels. On the A350-900, the source describes eight braked main wheels; on the A350-1000, each MLG carries six wheels and brakes for a total of twelve main wheels.
Each wheel assembly is made from two wheel halves joined by tie bolts, with inflation and over-pressure protection. Main-wheel thermal protection includes fusible plugs that allow tire pressure to be released if excessive wheel temperature is reached. This is why high-energy braking events require controlled ground-safety precautions around the wheel area.
The brake units are multi-disc carbon brakes. Hydraulic pressure acts on the piston housing to clamp the rotating and stationary disc stack, converting aircraft kinetic energy into heat. Wear indicators provide a direct maintenance reference, while brake rods transfer braking torque into the landing-gear structure.
02 · BRAKING ARCHITECTURE
Two wheel groups, multiple braking modes, redundant control
The source divides the main-wheel brakes into two hydraulic groups. On the A350-900, brakes 1–4 form the forward group supplied by the Yellow hydraulic system, while brakes 5–8 form the rear group supplied by the Green system. The two groups can operate independently in different braking modes.
The Braking Control System (BCS) application is hosted in CPIOMs and works with Remote Braking Control Units (RBCUs). The CPIOMs perform higher-level braking computation and coordination, while the RBCUs provide local control/monitoring authority and can retain braking capability even if the AFDX network or CPIOM path is unavailable.
The normal circuit uses Normal Brake Selector Valves (NBSELVs) and individual Normal Servo Valves (NSVs). The selector valves connect the high-pressure aircraft hydraulic source to the brake circuit when braking is required, while servo valves meter pressure to individual brakes and support anti-skid control.
The alternate circuit uses stored hydraulic energy in accumulators, Alternate Brake Selector Valves (ABSELVs) and Alternate Servo Valves (ASVs). Shuttle valves select the applicable pressure path to the single-cavity brake. This architecture lets the brake remain usable even when the primary hydraulic/control route is lost.
03 · NORMAL BRAKING
From pedal or automatic demand to individual brake pressure
In normal mode, braking demand can come from the brake pedals, Auto-Brake or retraction braking. Brake Pedal Transmitter Units (BPTUs) convert pedal movement into electrical demand signals. CPIOMs and RBCUs then command the selector and servo valves needed to establish the requested brake pressure.
The normal braking system is split into redundant control sides. The training material describes a regular side changeover associated with flight-cycle logic, reducing the chance that a dormant failure remains hidden indefinitely. If the normal CPIOM/AFDX path is unavailable, RBCUs can assume the required control functions for a reduced but still useful braking capability.
Retraction braking is a special function used to stop wheel rotation as the gear retracts. Unlike ground braking, the source notes that retraction braking does not use anti-skid protection. The operational objective is different: stop wheel spin before the gear enters the bay rather than optimize tire/runway friction.
04 · ANTI-SKID & AUTO-BRAKE
Closed-loop braking: command is continuously corrected by wheel response
Anti-skid compares wheel-speed information from tachometers with aircraft motion data from ADIRS. If a wheel tends toward excessive slip, servo-valve pressure is reduced or adjusted so the wheel can recover rotational speed. The target is efficient braking with reduced risk of wheel lock, tire damage and unstable directional behavior.
Normal braking availability depends on conditions including aircraft-on-ground logic, hydraulic availability and anti-skid selection. Auto-Brake adds another layer: automatic deceleration is armed and then engaged only when the required ground/aircraft conditions are satisfied.
The source distinguishes Rejected Takeoff (RTO) and landing Auto-Brake functions. Landing Auto-Brake depends on conditions such as spoiler deployment and NLG ground state. The function can be disarmed or stopped when key arming conditions are lost or when the crew takes over through sufficient brake-pedal input.
This distinction matters in troubleshooting. A report that “Auto-Brake did not operate” does not immediately implicate a brake valve. The engineer must verify the complete chain of arming conditions, aircraft-state inputs, automatic-command logic and actual hydraulic response.
05 · BRAKE TO VACATE
Braking can be linked to a runway-exit objective
Brake to Vacate (BTV) is the A350 landing Auto-Brake function that uses runway/airport-navigation information to calculate a deceleration profile toward a pilot-selected runway exit. The objective is not simply a fixed deceleration rate; it is to manage deceleration so the aircraft reaches the selected exit at an appropriate speed while minimizing unnecessary runway occupancy.
The training source connects BTV with Airport Navigation Function data and PRIM processing. It also discusses runway-overrun warning/protection functions that evaluate whether the available runway and current braking performance are sufficient. For learning purposes, this is a useful example of system integration: ATA 32 braking hardware participates in a higher-level operational objective using navigation, flight-control and aircraft-state data.
06 · ALTERNATE & EMERGENCY
What changes when normal braking is lost?
If normal braking or its high-pressure source is unavailable, the alternate braking mode uses accumulator pressure. With anti-skid available, alternate braking still supports pedal braking and automatic braking while the ASVs regulate pressure.
If anti-skid is lost or the alternate pressure condition degrades beyond the required threshold, the system can move to alternate braking without anti-skid. In that state, brake pressure is limited and only pedal braking remains. Emergency braking follows a similarly reduced philosophy: preserve basic braking while removing higher-level functions that can no longer be supported safely.
The key maintenance lesson is to identify not only “brakes available / brakes unavailable,” but the exact mode. Normal, alternate with anti-skid, alternate without anti-skid and emergency are different system configurations with different available commands, pressure limits and control paths.
RBCU independence is central here. The source explicitly describes braking availability even without CPIOM/AFDX support, demonstrating why local control units and emergency electrical supplies are part of the braking safety architecture.
07 · PARKING & ULTIMATE
Stored hydraulic energy can hold the aircraft without normal system pressure
Parking braking uses the alternate braking accumulators as the hydraulic source. The parking-brake handle commands Park Brake Selector Valves (PBSELVs), allowing accumulator pressure to be applied to all brakes. The source describes accumulator capacity intended to maintain parking-brake pressure for an extended period without the normal hydraulic system continuously operating.
Ultimate braking is the parking-brake hydraulic path used while the aircraft is moving. Because this path does not provide anti-skid protection, it represents a last-resort braking capability rather than a normal operational mode.
On a de-energized aircraft, emergency electrical supply to the RBCUs still permits accumulator-powered pedal braking. This is especially important for ground safety: “aircraft electrical power OFF” does not mean “no brake pressure available.”
08 · BRAKE MAINTENANCE CONCEPTS
Bleeding, wheel change and brake deactivation require system awareness
The brake circuit includes volumetric safety fuses that can close if excessive flow is detected. That affects bleeding technique: the source uses a controlled low-pressure method so air can be removed without unintentionally triggering the fuse.
For wheel-change and brake-bleeding tasks, the training material references dedicated OMT interactive functions that can apply a small controlled brake pressure without normal pedal operation. This is a good example of the aircraft maintenance system becoming an active tool in a hydraulic maintenance procedure.
Brake deactivation requires disciplined sequencing. The source stresses parking-brake OFF status before disconnecting the quick-disconnect coupling, followed by bleeding and leak checking. These are procedure-level details that must always be taken from current approved data for the specific aircraft.
09 · LANDING GEAR MANAGEMENT
Brake temperature, tire pressure and brake cooling become networked health data
Brake Temperature Monitoring System
Brake Temperature Sensors (BTSs) measure brake-unit temperature. Signals pass through Brake Temperature Compensation Modules and Wheel Remote Data Concentrators before reaching the RBCUs and CPIOM-hosted Landing Gear Management System (LGMS) application. The data are then presented on the aircraft display system and used for alerting/protection logic.
The operational reasons are clear: prevent takeoff with brakes outside an acceptable temperature condition, avoid retracting excessively hot brakes into the gear bay, and monitor abnormal residual-braking conditions.
Tire Pressure Indicating System
The Tire Pressure Indicating System (TPIS) measures wheel pressure and sends the information through wheel-interface hardware and the aircraft data chain to LGMS. The system can alert for low tire pressure or an excessive pressure difference between tires on the same axle.
This is an important evolution in ATA 32: tire pressure, brake temperature and hydraulic mode are not isolated maintenance checks. They are part of a digital condition-monitoring architecture that supports both crew awareness and maintenance decisions.
Brake Cooling Fan
The source also describes an optional Brake Cooling Fan (BCF) function controlled by LGMS. Its purpose is to reduce brake cool-down time and support faster turnaround. The function depends on ground-state logic and electrical control through SSPCs.
10 · VARIANT DIFFERENCES
How the A350-1000 expands the braking architecture
The A350-1000 uses twelve MLG wheels and brakes. The source describes the Yellow group as brakes 1–4, 6 and 8, with the Green group supplying brakes 5, 7 and 9–12. Additional brake-servo-valve manifold assemblies and volumetric fuses are used to support the larger wheel set.
Despite the additional hardware, the high-level braking-mode philosophy remains the same: normal, alternate/emergency and parking/ultimate modes are controlled and monitored through the same general CPIOM/RBCU concept, with each hydraulic wheel group capable of operating independently.
11 · TROUBLESHOOTING MINDSET
Diagnose the mode first, then the component path
- Identify the active braking mode. Normal, alternate, emergency, parking or ultimate.
- Identify the affected wheel group. Yellow group, Green group or both.
- Trace the demand. Pedal, Auto-Brake, BTV, retraction or parking-brake input.
- Trace the pressure path. Main HP source, accumulator, selector valve, servo valve, shuttle valve, brake.
- Check feedback. Brake pressure, wheel speed, accumulator pressure, temperature and related indications.
- Confirm control authority. CPIOM/AFDX path, RBCU fallback path and electrical supply.