Brake Temperature and Cooling

The carbon discs turn kinetic energy into heat, so temperature management is the brake system's health gauge. This article covers how temperature is measured (BTMU + thermocouples), how the WHEEL-page colour changes, what to do at each temperature band (fan on / do not set the parking brake / maintenance action), and the most deceptive trap of all — the indicated temperature falling sharply when the fan is on does not mean the carbon has actually cooled.

Per AMM 32-47-00:

The brake temperature system measures the temperature at each brake and supplies this data to other systems. The system includes a sensor at each brake and a Brake Temperature Monitoring Unit (BTMU) for each pair of brakes... The brake temperatures are shown on the WHEEL page of the SD.

This is the parent system article for the two case studies (gear-up brake-temperature rise; the 500 °C cargo case).

1. Measuring — chromel-alumel thermocouples and the BTMU

Per AMM 32-47-00:

The temperature sensor is a chromel alumel thermocouple. It supplies an output voltage in proportion to the temperature difference between the ambient temperature (measured at the BTMU) and the temperature at the thermocouple.

Four BTMUs sit on the two MLG bogie beams, each handling a pair of wheels (the left bogie's forward BTMU reads wheels 1 and 2, for example), compensating and amplifying the thermocouple voltage to 1–9 V; the BSCU then converts it to ARINC 429 for the ECAM. The BTMU measurement tolerance is itself banded: ±10 °C between 200 and 300 °C, ±25 °C outside that range — tightest accuracy exactly around the critical BRAKES HOT threshold.

 Measure (1/brake)       Monitor (1/pair)            Collect          Display
 ─────────────────       ────────────────            ───────          ───────
 temperature sensor ──► BTMU 3GW1/3GW2/4GW1/4GW2 ──► BSCU ──────────► WHEEL page (SD)
 ×8                      ×4 (2 per MLG bogie beam)   analogue→digital  wheel symbol colour:
 chromel-alumel          cold-junction comp + amp     (ARINC 429)       · grey = normal
 thermocouples           computes 1–9 V ∝ temp        → ECAM            · green = hottest > 100 °C
                         open/short monitoring        → SDAC if hot     · amber = > 300 °C + BRAKES HOT
 ═══════════════════ Cooling (independent manual system) ═══════════   EWD: BRAKES HOT
 BRK FAN P/BSW 8GD ──ON──► relay ──115 V AC──► electric fan per MLG     + single chime + MASTER CAUT
 (white ON light · amber HOT)  │ only with gear down and locked │ draws air across the brakes
                               └ fan coaxial with the tachometer (drive shaft through the centre)

2. WHEEL-page colours — grey, green, amber

Per AMM 32-47-00:

The color of the symbol is: normally gray / GREEN for the hottest brake of more than 100 deg.C (212°F) / AMBER for the brake temperature of more than 300 deg.C (572°F) and the warning message BRAKES HOT shows on the EWD / an aural warning (single chime) operates / the master caution lights come on.

Green marks only the hottest brake (above 100 °C) — it is not a warning, just "this is currently the hottest". Amber is the BRAKES HOT warning, with hysteresis. Per FCOM PRO-ABN-BRAKES:

This alert triggers when the temperature of at least one brake is above 300 °C. The alert disappears when the highest brake temperature goes below 290 °C.

The 10 °C hysteresis (trigger 300 / clear 290) stops the warning flickering when the temperature hovers near 300.

3. The brake fan — manual, 115 V AC, only with the gear down

Per AMM 32-48-00:

The brake cooling system is a manually controlled system... usually used on the ground but can also be used in flight if a landing is necessary immediately. The system is not available when the landing gear is retracted.

And: The circuit cannot be made unless the landing gear is extended and locked down. The pushbutton (8GD) ON drives a relay feeding 115 V AC to a three-phase fan motor in each MLG wheel; the impeller draws air across the brakes. The fan is coaxial with the tachometer (the tacho drive shaft passes through the centre) — a compact arrangement. It cannot run with the gear retracted because the closed wheel bay makes a running fan pointless and risky, so gear-down-and-locked is a hard interlock.

4. The big trap — fan-on temperature drop is not real cooling

Both the FCOM and the FCTM warn of this specifically. Per FCOM PRO-ABN-BRAKES:

Note: The brake temperature sensor is located close to the carbon, but not inside. Consequently, when the brake fan is ON, the indicated temperatures decrease rapidly.

And per FCTM PR-NP-SOP-100:

when the brake fans are running, the indicated brake temperature will be significantly lower than the indicated brake temperature when the brake fans are off... when the brake fans are switched off, it will take several minutes for the indicated brake temperature [to rise].

The sensor is near the carbon, not inside it. The fan first cools the air around the sensor, so the indicated value drops sharply while the core of the carbon is still hot. Therefore:

• Do not be fooled by the low reading after the fan is on — it does not mean the carbon can immediately take another heavy application (such as a second reject).
• To judge whether the brake has truly cooled, look at the value that settles a few minutes after switching the fan off, or use the cooling chart.
• This is exactly the caution in the 500 °C case study: a reading like 500 °C means completely different things with the fan on versus off.

5. The temperature-handling gradient (numbers to know)

Per FCOM PRO-NOR-SOP-04:

When one brake temperature is above 500 °C (or above 350 °C with the brake fans ON), avoid applying the parking brake, unless operationally necessary.

Why avoid the parking brake when hot: it clamps the carbon for a long time, and at very high temperatures that worsens carbon oxidation/damage and can trap heat — so use chocks to hold the aircraft instead. The "350 °C with fans ON" threshold is an extension of the §4 trap: with the fan on, an indicated 350 corresponds to a fan-off 500, so the threshold is lowered accordingly.

6. Carbon wear and the take-off temperature limit (two FCTM "whys")

Per FCTM PR-NP-SOP-100:

Carbon brake wear depends on the number of brake applications and on brake temperature. It does not depend on the applied pressure, or the duration of the braking... the only way the pilot can minimize brake wear is to reduce the number of brake applications.

This breaks a strong intuition: carbon brakes are not worn by harder pressing but by more applications. So one long, steady application beats several taps, and autobrake LO (holding a constant deceleration, Autobrake) saves carbon by reducing the number of applications.

Why a take-off brake-temperature limit at all. Per FCTM PR-NP-SOP-100:

The maximum brake temperature limitation for takeoff ensures that in the case of a hydraulic leak, any hydraulic fluid that touches the brake units does not ignite in the wheel well after the landing gear retraction.

The take-off brake-temperature limit is not about the brake itself but about a hot brake igniting potentially-leaking hydraulic fluid once retracted into the wheel well — which is why a hot brake is either allowed to cool, or the gear is left down to cool after take-off before retracting.

[!warning]- Six misconceptions this article corrects (1) A fan-on temperature drop does not mean the carbon has cooled — the sensor is near the carbon, not inside; the fan cools the sensor while the core stays hot. (2) Green on the WHEEL page is not a warning — it marks only the hottest wheel above 100 °C; amber is the BRAKES HOT warning. (3) Carbon wear does not depend on how hard you press — it depends on the number of applications and the temperature, not pressure or duration. (4) You may not freely set the parking brake when hot — above 500 °C (350 °C with fans on) avoid it and use chocks. (5) The brake fan is not available at any time in flight — only with the gear down and locked, and only when an immediate landing is needed. (6) The take-off brake-temperature limit is not about protecting the brake — it is about a hot brake igniting leaked hydraulic fluid in the wheel well.

Self-test

[!note]- Q1. Trace the temperature path, and how are the BTMUs arranged?

Eight chromel-alumel thermocouples (one per brake) → four BTMUs (one per wheel pair, two on each MLG bogie beam) → the BSCU → the WHEEL page. Each BTMU compensates and amplifies its thermocouples to 1–9 V; the BSCU converts to ARINC 429 for the ECAM.

[!note]- Q2. What temperatures correspond to grey/green/amber, and what are the BRAKES HOT trigger and clear values?

Grey = normal, green = the hottest brake above 100 °C (advisory), amber = above 300 °C (BRAKES HOT). BRAKES HOT triggers above 300 °C and clears when the highest brake drops below 290 °C — a 10 °C hysteresis.

[!note]- Q3. Does a fan-on temperature drop show the carbon has cooled? Why?

No. The sensor is near the carbon, not inside it, so the fan cools the air around the sensor and the indicated value drops sharply while the core of the carbon is still hot. The true state is the value that settles a few minutes after the fan is switched off, or per the cooling chart.

[!note]- Q4. Above what temperature should you avoid the parking brake, and why?

Above 500 °C (or 350 °C with the fans on). The parking brake clamps the carbon for a long time, and at very high temperatures that worsens oxidation/damage and traps heat — so use chocks instead. The 350 °C fan-on threshold reflects that a fan-on reading understates the true temperature.

[!note]- Q5. What does carbon wear depend on, and what is the take-off temperature limit for?

Carbon wear depends on the number of brake applications and the temperature — not on the applied pressure or the duration. The take-off brake-temperature limit ensures that, in a hydraulic leak, fluid touching a hot brake does not ignite in the wheel well after retraction.

Key takeaways

References

A330 specifics per AMM 32-47-00 (Brake System Temperature — eight thermocouples and four BTMUs, chromel-alumel, 1–9 V, grey/green-100/amber-300, tolerances, X on failure), AMM 32-48-00 (Brake Cooling — manual control, 115 V AC three-phase fan, coaxial with the tachometer, available only gear-down-and-locked, limited in flight), FCOM PRO-ABN-BRAKES (BRAKES HOT trigger 300 / clear 290, the sensor-near-not-inside trap, PREFER CHOCKS), FCOM PRO-NOR-SOP-04/21 (avoid the parking brake above 500 °C / 350 °C with fans, maintenance action), and FCTM PR-NP-SOP-100 (carbon wear is per-application not per-pressure, the take-off-temperature-limit rationale, the fan-off recovery delay). The temperature-path diagram and handling gradient are integrative syntheses of the AMM/FCOM/FCTM text. The cooling-chart minute figures belong to the performance/QRH brake-cooling chart and are not invented here. The thermal fuse plug protecting against tyre burst is in Wheel and Tyre Damage. Brake fans are an option (the fleet here is confirmed fitted); individual tail differences defer to the actual configuration.

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.