Judging by the number of questions I receive in the mail and at air shows, the No. 1 lubricant related question is: “”What is the best thing to do to prevent rust and corrosion during prolonged storage or during very low usage times?””
Judging by the number of questions I receive in the mail and at air shows, the No. 1 lubricant related question is: “”What is the best thing to do to prevent rust and corrosion during prolonged storage or during very low usage times?””
Pilots also ask about synthetics, wondering if they provide better rust protection than straight mineral-based oils.
Let’s tackle synthetics first. Many mechanics and pilots have stuck a screw driver into a quart of semi-synthetic oil and another one in a quart of straight weight grade 100 AD oil. When the tools are removed, they have observed that the coating of the oil from the grade 100 oil is much thicker than on the semi-synthetic oil. This leads to the conclusion that the straight weight 100 oil leaves a thicker oil film on the parts, and will therefore provide better rust protection during storage.
The problem with this logic is that the thickness of the oil film on the tools is directly related to the viscosity of the oil. The viscosity of a grade 100 oil is about three times that of a 15w-50w oil at 70? F. However, the engine is not at 70? when you shut it off. Repeat the tool experiment at 210? and you’d observe almost identical coatings on the tool from each of the oils.
Does this mean that semi-synthetic oils are as good as straight grade oils at protecting an idle engine? Yes and no. In lab rust tests, the semi-synthetic oils will perform as well as, or even better than, straight grade mineral based oil. Several of the semi-synthetic oils on the market contain some antirust additives that improve their performance. In the real world this performance advantage may or may not be realized. One of the advantages of semi-synthetic oils is they tend to run cooler. This is a real advantage in the summer, especially if your engine is running too hot. But in the winter it can be a disadvantage, as it can lower the oil temperature below a targeted 170?-190?. This, in turn, can result in more moisture in the oil, which can lead to increased rust activity in the engine.
Bottom line, semi-synthetic oils should offer equal antirust performance in your engine, but only if the oil temperature is high enough to ensure that the moisture is boiled out during normal flight operation.
What are the steps necessary to reduce ? or at least minimize ? rust/corrosion activity in your engine? Rusting and corrosion are caused by a lack of oil film protection on engine parts and/or moisture and byproducts of combustion in the oil. There are a lot of small steps that may help reduce rusting, but the three major steps are to reduce the amount of moisture in your engine, change oil frequently and fly as much as possible.
The moisture in oil comes from two sources: blow-by gases past the rings, and moisture fallout from the heating and cooling of your engine when it sits idle. Whenever your engine is run, the combustion process produces water. Piston rings do not provide a perfect seal, so some combustion gases enter the crankcase during normal operation. These gases contain some water and unburned fuel, which also contain things like sulfur, which can combine to increase rust and corrosion during a period of inactivity.
Things that help reduce rust and corrosion are factors such as getting the oil temperature up to 170? to 190? to boil out the water and minimizing the temperature variation of the engine during storage. A heated hangar is nice, but expensive. You can also reduce the temperature variation with an engine blanket and a small amount of heat. This should include plugging the cowling and exhaust. But an engine heater in an unprotected engine can actually increase rusting activity, especially if there is moisture in your oil.
The second step is to change oil before any prolonged period of storage or inactivity. If the plane is to be inactive for several months and is in a humid area, you should consider using preservative oil. In addition, the less you fly, the more often one should change the oil. Never go more than three months without changing the oil and filter.
Finally, fly as much as possible. Do not go out to the airport and start your engine up for a few minutes and then put it back in the hangar. When you start your engine, you need to fly it at least long enough to heat up the oil and get rid of all the moisture. So maybe a $100 hamburger should be stretched out to a $200 hamburger. It may save you money in the long run.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].
