Which condition allows the portable cooling fans to be used?

Using portable cooling fans is essential when the brakes are too hot. This condition is critical because when the brakes overheat, they can lose effectiveness, leading to potential safety hazards. Overheating can cause brake fade, which compromises the vehicle's stopping power, increasing the risk of accidents. By deploying cooling fans, the airflow helps to dissipate the heat, allowing the brakes to cool down to a safer operating temperature. This practice promotes safe operation and prolongs the lifespan of the braking components, ensuring that the aircraft or vehicle can operate reliably and efficiently.

The other conditions, such as brakes not cooling or being dirty, might not necessitate the use of cooling fans directly, as they address different maintenance or operational issues. Overinflated tires are also unrelated to brake cooling needs. Thus, the specific condition of brakes being too hot directly justifies the use of the portable cooling fans.