That was the question from Bryan Gore, who flies a Citabria with an IO-320 engine equipped with an oil inverted system. It does not have an oil filter.
That was the question from Bryan Gore, who flies a Citabria with an IO-320 engine equipped with an oil inverted system. It does not have an oil filter.
During a normal one-hour practice, his engine can burn or throw out a quart or two of oil. Bryan’s question was: “”If I replace 10 to 12 quarts during 15 to 20 hours of operation over a short period of time, is there any reason to do an oil change at 25 hours?””
My initial response was to tell Bryan that the 25-hour recommendation is independent of the oil consumption rate. This is the correct answer for most aircraft that consume above “”normal”” levels of oil.
The reasoning behind this is that most oil that is burned in an engine is burned on the cylinder walls. When an engine is running, a film of oil is left on the cylinder when the piston travels down the cylinder. When the cylinder fires, some of the oil is burned.
If you have “”excessive oil consumption,”” there will be more than a normal amount of oil left on the cylinder wall. Much of this oil will be burned and go out the exhaust with the burned fuel. However, a small amount of film will be left on the wall. In combustion, the fire is “”quenched”” by the cooler wall and a very thin layer of fuel and/or oil goes unburned. This film will usually be oil and “”contaminates”” that do not burn. As the engine goes through the remaining exhaust and intake cycles, the oil and contaminates are washed or scraped off and re-enter the oil system. This means that during the normal operation of your engine, the oil consumption rate past the piston rings should not affect the oil change interval significantly. To get rid of the dirt and other contaminates, you need to change the oil.
In subsequent emails from Bryan, he explained that most of his oil consumption is not past the piston rings. His oil consumption is from aerobatic flying and the oil is being thrown out the breather tube. In Bryan’s case, the oil is well mixed with most of the dirt and contaminates in suspension. Then when he goes inverted, oil with the contaminates is blown out the breather. So when new oil is added, it is sort of like a running oil change.
In the past, some radial engine operators used this principle. Since large radial engines tend to throw oil everywhere, some operators extended the oil change intervals significantly with only a small drop in cleanliness levels in their engines. The manual for a Wright R-3350 states that oil consumption above six gallons an hour should be considered excessive.
However, it is important to remember that these operators were flying their aircraft almost every day and did not have idle time for the engines to rust. The amount of rust contaminate in your oil goes up very significantly when your aircraft sets idle, especially in a humid climate.
The bottom line for Bryan is that he could extend his oil change interval, as long as he is flying his aircraft regularly and his oil consumption remains high. He still should stay with the four-month rule and go back to the 25-hour timetable if his flying schedule slows down.
If you are in doubt as to whether or not you should extend your drain interval, I would recommend that you err on the side of too many oil changes. Remember, oil is still a lot cheaper than airplane parts.
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].
