An aircraft system includes a fan, a differential pressure sensor, a temperature sensor, and a system controller. The fan provides airflow for at least one heat exchanger. The differential pressure sensor senses a pressure difference between an inlet of the fan and a diffuser exit of the fan. The temperature sensor senses a temperature at the inlet of the fan. The system controller is configured to receive the sensed pressure difference, the sensed temperature, and a speed of the fan, and determines an operating point of the fan based upon the sensed pressure difference, the speed of the fan, and the sensed temperature. The operating point is indicative of contamination of the at least one heat exchanger.