Almost all aircraft piston engines are air cooled, meaning that ram air flowing through the engine cowling wicks away the tremendous heat produced by the combustion of the fuel-air mixture inside the cylinders. It would be a mistake, however, to think that ram air alone handles all of the cooling chores. The oil that circulates throughout an engine lubricating all of the moving parts absorbs a significant amount of combustion heat.
For that reason, most aircraft engines have an oil cooler. It's a simple heat exchanger, much like an automotive radiator - a fluid transfer line encased in tightly packed thin metal wafers. Some of the ram air flowing through the engine cowling is directed to the oil cooler, where it flows across the wafers. The large surface area and small mass of the metal wafers quickly sap heat from the oil coursing through the line. It's a simple, effective method of keeping an aircraft engine cool. A thermostat prevents thick, cold oil from flowing through the cooler until it has warmed sufficiently.
Oil coolers may be mounted on the front or rear of an engine depending on the engine and airframe. For that reason, it can be difficult to examine an oil cooler during a preflight inspection.
If it's front-mounted, as on some Teledyne Continental engine models, you probably can see the cooler just inside one of the inlet holes on the nose of the airplane. If it is mounted on the rear of the engine or the engine firewall, as is typical with Textron Lycoming engines, you'll have to remove some or all of the cowling to inspect the cooler.
The good news is there is little to go wrong with an oil cooler. You can check for evidence of oil leaks caused by loose oil line connections, or cracks in the oil cooler or mounting bracket. Cracks usually are the result of vibration.
In flight, an abnormally high oil temperature indication is cause for an immediate return to the airport. It could be due to clogging or failure of the oil cooler, but that is something for the mechanic to troubleshoot.
