In my last column, I described two of the most important factors in getting to full TBO with your aircraft engine: Getting the oil temperature up to around 180°F and using your plane frequently.
I recently read an article on oil change intervals in another publication. The author went on and on about what a mistake it was when the engine companies changed the oil change interval from 25 to 50 hours. To the author’s credit, he did mention in passing that there is also a four month oil change limit.
The problem is that the four month time limit is the most important criteria and the 25 hour and 50 hour limit are secondary. Most pilots hear the 50 hour change limit and stop reading, which is OK if you fly more than 150 hours a year.
But it is a real problem if you only fly 50 hours a year.
I have suggested to the engine manufacturers that they should upgrade their oil change recommendation to every four months or 50 hours (for filter equipped aircraft), whichever comes first.
In fact the University of Illinois flight school ran its Lycoming O-360 engined trainers for four months or 100 hours, whichever came first. They wrote an SAE paper on the experience that showed that there was no significant difference in engine life or performance between the 50 and 100 hour oil change intervals. They flew about 30 hours a month on average and the planes flew almost every day with no significant down time.
I am not suggesting people go to 100 hour oil changes. I am just making the point that an oil change every four months is the most critical criteria.
I have received numerous questions about what to do about really low usage aircraft. There is no real good answer here.
If an aircraft is flown less that 10 hours a year, changing the oil every three hours will not help much. Many of these planes are in museums or are stored for one reason or another.
For this situation, I would recommend putting in preservative oil that meets the Mil-C-6529C spec. Oils meeting this spec are available from Phillips and Shell.
The procedure I would use is to drain the preservative oil and replace the filter. Then fill the crankcase with an Ashless Dispersant oil meeting Mil-L-22851/SAE J1899 spec oil before flying the aircraft. Then when the aircraft is going back into storage, change the oil back to a fresh charge of the preservative oil.
If your aircraft is flown from 10 to 50 hours a year, but sits for many months with no activity, then you can use the preservative oil during the idle time, then change the oil every four months during the flying time.
If the plane is flown regularly and more than 50 hours a year, then use the four month oil change criteria.
If, however, the plane will sit many months during the year, then I recommend that you add one or two quarts of the preservative oil to the oil change just prior to the time of inactivity.
These recommendations need to be used with some common sense and I know that that flower does not grow in everyone’s garden.
For example, if the plane is stored in a very dry climate, then the changes are different than for a plane in a humid coastal area.
Also, if a plane is only flown 10 hours a year, the chance of that plane being flyable in 200 years is slim.
And if it does not reach full TBO and only makes 100 years, feel free to come back and criticize my advice.