How is APU oil cooled and why is oil cooling important?

• A. Oil is cooled by ambient air through a radiator; cooling reduces noise.
• B. Oil passes through an oil cooler/heat exchanger to remove heat; cooling maintains lubricant quality, reduces wear, and prevents overheating.
• C. Oil is cooled by fuel; cooling prevents foam formation.
• D. Oil is cooled by the APU exhaust; cooling improves thrust.

Answer: (B)

Oil cooling is about removing heat from the lubricating oil as it circulates through the APU. In typical APUs, the oil is pumped through an oil cooler or heat exchanger, where heat is transferred from the oil to a cooling medium—usually ambient air flowing through a radiator or a dedicated heat exchanger. This keeps the oil within its proper temperature range so its viscosity and film strength stay appropriate for effective lubrication.

Why this matters: when the oil is kept cool, it maintains the right viscosity and lubricating properties to protect bearings, gears, and other moving parts. It also resists oxidation and sludge formation, reduces wear, and prevents oil thinning that can lead to overheating of the engine and components. If oil gets too hot, lubrication degrades, clearances can grow, wear increases, and the APU can overheat or fail.