Now, where do the corrosive agents come from? This is another area where you will get differences of opinion. In addition to the small amount of corrosive agent found in air, the primary source is the result of the combustion process. When fuel burns, the exhaust gases are corrosive. Some of these gases enter the crankcase via the "blow by" and the corrosive agents are deposited into the crankcase and the oil. You can observe the corrosion caused by exhaust by looking at your aircraft belly behind the exhaust stack. Rivets in this area often corrode.
When an engine oil is exposed to heat, a process called "oxidation" is started. When oxidized oil is mixed with moisture as discussed earlier it often forms an acid which attacks metal surfaces. This is a concern relating to certain sump pump preheaters which may overheat and degrade the oil as it heats the engine. Another problem with these heaters is that as they heat the oil, driving the moisture out, the vaporized moisture will rise and again condense on cooler parts of the engine. This condensed moisture will begin the corrosion process on that engine part.
Finally, we need to discuss the metal. Clearly everyone agrees that exposed metal is the most vulnerable, but all metal in the engine is open to corrosion attack, even metal that receives the protection of oil.
Brand new engines are very susceptible to corrosion. Lycoming says, "Our experience has shown that in regions of high humidity, active corrosion can be found on cylinder walls of new engines inoperative for periods as brief as two days." As mentioned earlier, engines that use break-in oil (mineral oil) develop a protective varnish coating. This can happen during about 50 hours of operation. This is why the aircraft dealer tells customers of new aircraft "Fly often." This advice applies to rebuilt and overhauled engines as well.
Normally you would expect a well-lubricated engine to have the essential internal parts covered with oil and not subject to rust. So why can metal with a coating of oil corrode? Here is where we must toss the oil and water do not mix concept out the window. Moisture can be present in oil. Oil companies have long recognized that, and have recently begun modifying their products to address the issue.
Other considerations and unknowns
One point that engine rebuilders and overhaulers agree on is that there is no single and clear culprit for corrosion. Many years ago some blamed engine preheaters. It was discovered that "total engine" heaters were fine but the so-called "sump pump" heaters can be a menace. They heat only the oil, and subsequently the air in the sump which leads to moisture, which condenses and creates water which in turn creates the moisture we talked about. In actual tests in a glass container, one could watch moisture condense on the upper inside portions of the container and rain down into the oil. It was clear that heating only the oil was a major problem unless the entire engine was heated to prevent the water vapor from condensing on the colder upper internal parts of the crankcase.
If every corrosion problem in all engines were considered, even the steel used in the parts is a factor. Darrel Buldoc, a successful and highly respected engine rebuilder, identified a very significant number of incidents of Lycoming cam followers that showed substantial corrosion after as little as a few hundred hours of operation. He and other rebuilders have concluded that the problem results from the steel used.
Some have accused another culprit, aviation’s "new" 100LL fuel. It is known that the combustion of fuel creates by-products including acids. However, the newer fuel formulas designed to increase octane, reduce lead and other emissions may be producing by-products that may still be identified as contributing to corrosion. Some point to the increased sulfur content of the fuel. More research into this subject needs to be done.
OK, so what is the answer? Although there is some consensus, here again, there is no simple, concise, all-inclusive answer. Once more, let’s distinguish between "active" and "inactive" engines. Without getting embroiled in the earlier discussion what constitutes active and inactive, let’s assume regular means run more often than every 30 days.
Continental states, "The best method of reducing the likelihood of corrosive attack is to fly the aircraft at least once every week for a minimum of one hour." (An irony of aviation: the more you fly your plane, the longer it lasts).
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