What is the acceptable position of the brake wear indication (PIN) for an A320 to be safely dispatchable?
There are two wear pins. If the end of either pin touches the machined part of the piston housing the heat pack is worn out and the brake must be replaced. Measurement must be done when the brakes are cold. If one wear pin is missing, check to make sure the other pin is there and looks ok. If both pins are missing the brakes need to be replaced.
To answer your question the limit is 1mm. It cant be less than this value. At lengths greater than this value brake wear can be measured without any specific brake temperature value. However at values less than 1mm Airbus has a specific brake temperature of 60 degrees Celsius to measure the brake wear.
At high brake temperatures the heat from the carbon brakes causes the pressure plate to expand to which the wear pins are connected to. This can easily cause a false indication of the brakes actual lining wear and if the parking brake is not applied can cause a large false indication. Therefore the recommendation of the cold temperatures. Applying the parking brake is of importance as the actual wear of the brakes lining can be gauged.
As mentioned soo long as it's over 1mm with the parking brake ON it's possible to measure brake wear at any brake temperature.