How is hydraulic fluid cooled on the ground?

• A. Ram air across a heat exchanger
• B. Electric fan pulls air for the exchanger
• C. Natural convection and radiation
• D. Water-to-oil cooler

Answer: (B)

On the ground, the hydraulic system relies on forced air to remove heat. The hydraulic fluid flows through an oil cooler (a finned heat exchanger), and an electric fan pulls ambient air through or across this cooler. That forced convection is needed because there isn’t enough ram air from motion when the aircraft is stationary, so the fan provides the airflow necessary to carry heat away efficiently and keep the fluid within its safe operating temperature.

Ram air cooling would work if the aircraft were moving fast, but on the ground it isn’t reliable. Natural convection and radiation alone would be too slow to handle the heat load. A water-to-oil cooler is another type of cooling used in some systems, but the scenario described here specifically uses the fan-driven air flow across the exchanger to achieve ground cooling.