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In the A320 FCOM it is stated that if the brake fans are running prior to takeoff then the brake temperature must be below 150 degrees Celsius. But if the brake fans are off then our limitation is 300 degrees Celsius. Any reason behind such different limits?

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The fans cool not only the brakes, but also the temperature sensors, at different rates. The temperature difference is a measure to correct this anomaly. From the document Proper Operation of Carbon Brakes by Airbus Flight Operations Support:

... the temperature indicated in the cockpit when the brake fans are running is significantly lower than the temperature indicated when the brake fans are OFF. This is because the brake fans are ventilating the brake temperature sensor that is not located inside the material itself. Therefore, as soon as the brake fans are running, the indicated brake temperature decreases almost instantaneously.

... For all brake types, when the fans are running, the difference ranges typically from approximately 50°C at 100°C actual brake temperature up to about 150°C at 300°C actual brake temperature.

... When fans are selected, the warning will disappear instantaneously, but the takeoff must be delayed until the indicated temperature consistently decreases below 150°C (for a warning set at 300°C)

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  • $\begingroup$ @aeroaliasI still didn't fully understand what is meant by this. $\endgroup$ – Jai Jul 26 '17 at 17:55
  • $\begingroup$ @Jai sounds like the fan blowing across the sensors creates a falsely low indication, i.e. The brakes are still hot but the sensors don't read it correctly because of the cool air from the fans. Sounds like an odd arrangement to me. There must be some reason for it $\endgroup$ – TomMcW Jul 26 '17 at 18:32
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    $\begingroup$ The "odd arrangement" probably stems from the fact that they don't embed the temperature sensors within the rotor material itself since it would (most likely) require a wireless sensor to actually read the temp. Therefore the sensors are somewhere on the surface where they are affected by the moving air much more quickly than the core of the rotor is. Since this is known, they give a "fan-on" and "fan-off" target temp to account for the extra cooling of the sensor. $\endgroup$ – FreeMan Jul 26 '17 at 20:07

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