Oil is to an aviation piston engine what blood is to the human body: a crucial element in keeping it alive. Though blood fulfills the task by carrying nutrients to the various cells in the body, oil accomplishes the same function by creating a microscopic film between metal parts that keeps them from grinding each other into oblivion from heat and friction. Oil also carries the nasty by-products of combustion away from the critical parts of the engine, and holds them in suspension until your next oil change. Oil also disperses the intense heat created during combustion. In all, oil is your engine's best friend.
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."
How Oil Works
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.
Important Oil Information
| When learning about oil, it's valuable to know what the different terms mean, and what the facts are about different types. Since many myths abound, we talked to our experts and compiled some terms, facts and interesting information about your engine oil.
Mineral Oil—Non-synthetic oils come from earth-bound minerals and are said to be "mineral oils." The term describes a base-lubricating oil distilled from crude oil. In the aviation world, the term "mineral oil" refers to a pure (no-additive) oil often used to break in new or newly rebuilt piston engines. This type of oil helps "seat" the rings, valves and other components. Because it has no additives, it isn't designed to carry away contaminants.
Approved Oil—This refers to any brand name of oil that's produced under the specification SAE J1966 (for mineral oil), or SAE J1899 (ashless dispersant grades).
Ashless Dispersant—This used to be called "detergent" oil. It's an additive in aviation engine oil that prevents sludge and other harmful deposits by making them soluble and carrying them away. The contaminants end up in the oil filter or remain suspended in the oil until it's changed.
Multiviscosity—An engine oil that meets more than one SAE grade by passing tests at both high and low temperatures. These oils pump quicker at startup (compared to fixed-grade), flow faster to the upper valves and produce a thicker film at higher temperatures. The first number designates the viscosity at low temperature, and the second refers to its high-temperature viscosity.
Mixing Multigrade and Single-Grade Oils—While this used to be a bad thing, today's oil formulations allow mixing of multigrade and fixed-grade oils. So, a pilot may use something like a 15W-50 grade in the cold winter months, and then switch to a W100 in the summer without issue.
Drain Oil When It's Hot—Though this may be difficult in some airplanes, experts recommend you drain the oil only when it's hot. This avoids the settling of dirt and water on the parts of a cold engine, and more of the contaminants stay suspended in the hot oil, to be carried away from the oil pan.
Why Oil Turns Black—When straight mineral oil turns black, it's because it has oxidized and needs to be changed. Dispersant oil is designed to turn black as it carries away soot and other contaminants. Change oil based on engine time, not on color alone.
Synthetic Oil—Synthetic oil is formulated in a laboratory, not distilled from earthly components. In aviation, 100% synthetic oil hasn't caught on as much as in the automotive world, because it doesn't perform well with aviation fuel. The lead by-products of combustion in an aviation engine (because of the fuel) aren't compatible with synthetic oils. AeroShell has, however, developed a partially synthetic oil that performs well in aviation engines.