The conclusion of these facts is that if (and I emphasize "if") preheaters are a cause of corrosion, they are only one small aspect in a very complicated process.
Back to the three considerations relating to leaving the preheater plugged in continuously. Keep in mind the facts mentioned above.
How often is the aircraft flown?
If a pilot flies frequently (once a week or more and for at least 35 minutes and with the oil temp gauge reading 180 F or more), he will burn off some of the moisture in the oil and provide the internal parts with a coating of oil.
Where is the aircraft located?
If it is located outside where air movement can cause significant temperature changes in the engine which leads to condensation (i.e. liquid moisture on internal parts) you increase corrosion potential. Aircraft not located in a hangar are subject to wind or air movement and thus greater temperature changes. Aircraft located in hangars with covers on them are not as vulnerable. I should emphasize that even in hangars, the engine should be covered to prevent air movement through the engine.
The worst case scenario is if the preheater is controlled by a thermostat. This will cause the heater to turn on and off which is a condition you want to avoid.
What type of preheater does it have?
If the aircraft has an oil sump only heater (i.e. the preheater does not heat the cylinder heads), the heat on the oil can drive out moisture in the oil, which may condense on the colder parts in the cylinder area. Cylinders are, of course, air cooled and located some distance from the heat source. Oil sump only heaters cannot put excessive heat for fear of boiling the oil. Consequently that single source of heat requires a longer time for the heater to heat the entire engine. This creates the temptation to leave the preheater on continuously or at least for extended periods. Consequently many mechanics recommend Tanis preheaters because they put heat directly on the cylinder head allowing more heat applied evenly throughout the engine.
Remove the dipstick
A suggestion that many pilots who use Tanis preheaters follow is to remove the oil dipstick when the plane is in the hangar and the preheater is heating. This has been shown to reduce the humidity by up to 50 percent as the warm moist air rises out of the engine and is replaced with cooler, dryer air. Although the combustion process produces moisture in the crankcase, there is no effective way for that moisture to escape. This suggestion provides for that escape.
All this seems reasonably logical and straightforward. Yet, the big debate will undoubtedly continue. In spite of what the testing oil companies and preheater manufacturers do, we still can only speculate on exactly what is going on inside that engine. We merely can observe the end result when the engine is torn apart. It is true, a relatively small percentage of engines experience corrosion. Yet as long as corrosion remains an issue, aircraft owners want an answer to "Why?" and "What should I do?". In regards to the question "Why?" the preheater companies, the engine manufacturers, the oil and fuel suppliers all say "Don't blame me" and point in another direction. "What should I do?" Their answers vary based on their product.
It would be unfair to those who took the trouble to read this far not to attempt to at least answer the question "What should I do?" Based on the facts presented here, the recommendation is: "If a pilot flies about once a week or more (following the criteria as mentioned earlier) and has a heater that heats the entire engine, he or she can leave the heater plugged in while their aircraft is covered in a hangar. Otherwise, the preheater should be started a few hours before they intend to fly."
In the end, however, the practice that is followed is still up to the pilot. He or she must decide what to do. Hopefully this information will help you educate them when it comes time to make that decision. AMT
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