Caring For Your Aircraft

There's much hearsay about engine oil and oil changes in the piston aviation world. It seems everybody has an opinion and advice, though much of it isn't based on fact or comes from old ideas. The oil we use in our airplanes today isn't your grandfather's oil (though many of us miss oil cans), and understanding modern techniques and formulations will help us squeeze more life out of our engines. In the end, isn't that what it's all about?

We were fortunate to talk with Paul Royko, AeroShell's Technical Manager of piston-engine oil, who speaks frequently at aviation events and conferences. Though he won't admit to it, Royko is something of an aviation-oil guru, and his advice is something all pilots can benefit from. AeroShell has been providing oil to aviators since they developed the first ashless dispersant aviation oil in the early 1950s. In the 1960s, Aeroshell made headlines again when they developed their now-famous Turbine Oil 555, which was created especially for the Concorde, and could withstand the high-bearing temperatures and stresses of supersonic flight.

"The best advice I can give to pilots is simple," says Royko, "Pick the proper oil for the conditions you fly in. The other critical thing is to change the oil and filter at the proper intervals, and finally, fly often. Those three simple things will extend the life of your engine and your oil, and will help you catch problems early."

To put that advice into practice, it helps to understand the seemingly simple process of how the oil works in your engine. In a cold engine, the oil sits in the oil sump, pulled by gravity to the lowest point in the system. When you start the engine, oil is pulled out of the sump by an engine-driven oil pump. It's sent through the oil filter (or oil screen if you don't have a filter), and proceeds into the crankcase, where it lubricates the critical bearings and other components. Oil travels into the cylinders, where it creates a seal between the piston rings and the smooth walls of the cylinders, allowing compression to occur.

The oil carries away heat through convection as it travels through the engine. Typically, the oil will travel through an oil cooler, where air flow will carry the transferred heat away. Microscopic bits of metal that are the result of friction also will be carried away, and will end up in the oil filter.

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."

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.

Thus, multiviscosity oil extends the life of your engine by working well at different extremes. "A pilot should look at where he or she flies, and how often," says Royko. "That will point them to the proper multiviscosity oil if that's what's best for their operation."

Royko also recommends that pilots "spool down" their oil temperature after a long flight with a slow taxi, or by letting the engine idle for a bit before shutdown. "Racing to the hangar and shutting it down is not good for the engine. On warmup, bring the oil temp up gradually. Don't firewall the throttle a few minutes after starting either."

He says a careful oil analysis is valuable, or even cutting the oil filter yourself, removing the filter media and running a magnet through it to look for metal shavings, especially in the nooks and crannies, to check for possible issues.

There's so much more to oil than can be covered in an article. Lucky for pilots, AeroShell and others have several online resources that cover all kinds of valuable information. Whether choosing between single and multigrade oil, or discovering the best ways to handle oil changes and engine performance, there's a lot to learn. Shell has a complete knowledge center on the web at www.shell.com/home/content/aviation. Phillips 66 also has an aviation resource center online at www.phillips66aviation.com. Both offer great information to help you keep the lifeblood flowing in your engine.