Tuesday, May 7, 2013
There’s More To Oil Than You Think!
Taking care of your engine with aviation lubricants
Figuring out which oil to use requires understanding a few concepts—notably, the type of oil (straight mineral, ashless-dispersant or synthetic), viscosity and additives. Which type to use also depends on what kind of engine you have and what kind of flying you do. I'm going to start with the traditional horizontally opposed piston engines used in most GA airplanes, and then move on to more exotic alternatives. But first, a warning: Before changing the oil in your airplane, check the owner's manual! It may specify particular oils. More likely, it will provide general guidance, or refer to a service bulletin (SB) or service instruction (SI) from the engine manufacturer.
Most piston aircraft and engine manufacturers approve two major types of oil for use: straight mineral oil and ashless-dispersant (AD) oil. Straight mineral oil is pretty much what comes from a refinery, and it's a good lubricant, but it won't keep your engine clean. During break-in, that can actually be a good thing! Ben Visser, a former AeroShell lubrication expert, told me that using straight mineral oil for break-in "goes back to the old hard chrome used to bring cylinders into specification, where you had to actually wear down the parts—the particles worn off work as a lapping compound."
On the other hand, constantly polishing the inside of the engine isn't ideal for long life—as you polish, clearances increase, which eventually will lead to oil loss, and debris from polishing has to accumulate somewhere. That's where AD oil additives come in: Instead of allowing particles of metal to agglomerate (or burn down to ash), AD oils disperse metals (and other contaminants) and flush them out of the engine at the next oil change. So, AD oil is almost always recommended for use after break-in. In some cases, AD oil is recommended during break-in!
Both straight mineral and AD oils are rated for viscosity—a measure of resistance to flow, or in practical terms, the "thickness" of the oil, which depends on temperature. Viscosity is expressed by the "weight" of the oil, measured on either of two scales—the familiar Society of Automotive Engineers (SAE) or the commercial aviation scale (the latter is basically double the former). Typically, you'll use a higher-viscosity oil at higher ambient temperatures. For the Continental O-470 in my Skylane, Cessna recommends SAE 50 (or 100W) above 40° F, SAE 30 (or 60W) at lower temperatures.
There also are variable-viscosity oils, which are thin at low temperatures but thicken at high temperatures. A 10W-40 oil is an oil that acts like SAE 10 at room temperature, but like SAE 40 at operating temperature. That's ideal for pilots who live in the snow belt; you don't want to try starting the engine at below-freezing temperatures with an SAE 40 oil. On the other hand, the SAE 10 oil that's good for starting won't do a good job of protecting your engine on a long cross-country.
Variable viscosity oils start with a low-viscosity base oil (for a 10W-40, starting with an SAE 10 base oil) with viscosity modifiers (VMs) added. Think of VMs like small balls of yarn. At low temperatures, they have no real effect on the overall viscosity of the oil, but as the temperature rises, the yarn (actually polymers, basically, plastics) unroll, and as they do so, the oil thickens. Most variable-viscosity oils are built on natural mineral base oils, but there also are fully synthetic lubricating oils that offer several advantages over mineral oils (among other things, longer oil change intervals). They're not used in piston-aircraft engines because the lead from aviation gas can interact with the oil to produce sludge, which needs to be removed by changing the oil.
Other types of engines, particularly turbines, don't use leaded aviation gas and can use synthetic oils. Which brings up a big difference between synthetics and conventional mineral-based oils: Conventional oils can be freely mixed. If you're flying a piston-engine airplane on a long cross-country and find yourself a quart low, you don't have to top up with the exact same oil—any aviation oil with a viscosity approved for your engine will be fine. Not so for turbine engines. Different manufacturers may use completely different synthetic chemicals, and mixing them can have unpredictable results. Even oils from the same vendor may be incompatible. Another significant difference is oil-change interval. For piston-engine airplanes using mineral-based AD oil, the interval is usually listed as 50 hours (if you have a replaceable oil filter) or half that (for older airplanes that only have an oil screen). But the oil manufacturers add an additional limit: You should change the oil at least every four months, regardless of how many hours have been flown. Why? Because oil picks up water and acids. According to ExxonMobil, "Changing your oil on a calendar schedule as well as a flight hour schedule helps ensure you get contaminants out."
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