Plane & Pilot
Tuesday, August 9, 2011

Caring For Your Aircraft

Part IV: From selecting the correct grade to changing it at correct intervals, knowing your oil is important

All the while, the oil is leaving a residual coating on all the metal parts it touches (of about three to four microns when it's cold; twenty-five microns equals 1⁄1,000 of an inch). This coating prevents oxidation (rust), and allows the parts to stay lubricated even after the engine is shut off. Lubrication and heat transfer are the main jobs of the oil.

That's where Royko's advice comes in. Aircraft engines make water during combustion, and it's that water that causes all the problems. That water combines with the metals in the engine and the by-products of combustion to form a strong acid. If allowed to remain inside the engine, this acid corrodes the engine's parts and promotes rust. Eventually, rust could cause failure of a critical component—usually at the worst possible time.

Royko advises flying as often as possible. In today's economic doldrums, flying often may not be feasible, so he offers this advice: "The key to reducing corrosion is flying long enough at the proper oil temperature." Royko explains that pilots need to fly at an oil temperature of 180 to 190 degrees F for at least an hour so water will boil away.

"Water boils off at 212 degrees Fahrenheit, and the actual temperatures experienced by the oil inside the engine are about 50 degrees higher than at the oil-temperature gauge," Royko says. "Flying at lower oil temperatures or for less time might actually promote more moisture, not less."

With regard to oil changes, experts agree that 50 hours or four months (whichever comes first) is the proper interval for engines with oil filters. Engines with only screens should have their oil changed every 25 hours or four months. "I tell pilots to never judge oil by its color," adds Royko. "Go by the interval. If you only fly 40 hours a year, you should still do three oil changes in 12 months; one every four months, regardless of hours."

Proper Viscosity

The big news in aircraft oil is multiviscosity. Proper viscosity is one of the most critical criteria in choosing engine oil. Viscosity can be thought of as a liquid's resistance to flow.

Thus, the higher the viscosity rating, the "thicker" the oil, or the more it resists flowing. Multiviscosity oil gives pilots the better of two worlds when operating in different flying environments.

Multiviscosity oils flow very well at startup—typically when damage occurs when using fixed-viscosity oils in cold temperatures. But, they also resist flow appropriately when they're hot. To check for proper flow, Royko says pilots should get an oil-pressure indication within 10 seconds of startup. Fixed-viscosity oil might only be at optimum flow when the engine is running at normal power.


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