My last column was all about how often you should change your airplane’s oil.
It received a number of comments, which I always appreciate.
Many of the comments were about the recommendation by engine manufacturers to change the oil every four months.
One reader emailed me, noting he had several questions about oil change intervals.
“One guy said that way back in the past, the oil change interval was a recommended 50 hours or six months. If so, when did it go from six months down to every four months? I believe the answer is that the four months was a result of decrease aircraft usage.”
I did not remember that, so I emailed my friend, Paul McBride, who writes the Ask Paul column for General Aviation News, to see if he knew about such a recommendation. He had not heard of it, so if there had been such a recommendation, it was well over 50 years ago.
But why do manufacturers recommend 50 hours or every four months with a full oil filter?
This recommendation reflects the main cause of lubricant-related failures in aircraft engines, which is corrosion leading to camshaft and lifter spalling.
This, in turn, is mainly caused by aircraft inactivity, just as our reader thought.
When an engine sits, there is almost always water in the oil. Some comes from exhaust blow-by, but it is mainly from water condensation.
When an engine sits inactive, warm moist air is drawn in during the day. As it gets cooler at night, the water drops out and is left in the oil or on engine parts.
Over time, the water combines with the sulfur in the oil, which comes from unburnt fuel in the blow-by past the rings. This forms an acid that attacks the cam and lifter surfaces, forming surface rust.
When the engine is finally started, the rust acts as a lapping compound which, in turn, starts the wear process that eventually leads to failure.
These failures are almost always on low usage aircraft that sit a lot. It is very rare to see this on an engine that is used regularly all year long.
Based on this data, engine manufacturers came up with the recommendation to change the oil every four months.
Four months as opposed to three or five months was thought to be a reasonable compromise. The idea is that the recommendation will help eliminate a long inactivity period, because the longer an engine sits, the more rust accumulates.
Several readers pointed out that there would be a lot less rust accumulating on an aircraft engine sitting in Arizona than one in Florida.
That is true, but the engine manufacturers cannot write a recommendation for every situation. If they tried, they would have to put in a table that listed the time for oil change vs. normal relative humidity.
Expert Recommendations to Get to Full TBO
What would I recommend to get to full TBO?
The first step is to get rid of the water in your oil during normal service.
To do that you need to check the accuracy of your oil temperature gauge.
Take the sending unit out of your airplane and put it into a container that’s sitting on a hot plate. Heat the oil up to 180°F, measured with an accurate thermometer in the oil. Now make a mark on your oil temperature gauge.
When you fly, check this gauge and make sure that your oil temperature is at or near the mark.
The only way to get rid of water in the oil is to boil it off, which is why it needs to reach 180° while you are flying.
As oil goes through your engine, it will pick up about 50°F above sump temperature. If your sump temperature is below 160°F, the oil will not get above 212°F and the water will not boil off. Instead, it will just accumulate in the sump.
Second: Do not just start your engine on the ramp to coat all of the parts, thinking this will help extend the life of your engine. This just adds to the fuel contamination and does not evaporate any water.
Third: Fly your plane as often as possible. Plan a flight where you get the oil temperature up long enough to drive off the water.
Fourth: Depending on how much and when you fly, you need to plan your oil changes.
A lot of pilots living in northern climates fly a lot in the summer and very little in the winter. As winter comes, pilots may just put the plane away where it will sit during the cold months, and then change the oil in the spring so they can fly on fresh oil.
Wrong plan. You should change the oil before you put the plane away. This way you will start the period of inactivity with no water or acid build up.
Also, I would recommend putting a quart or two of Phillips Anti-Rust Oil in the crankcase with the oil change. This will help protect your engine and you can still go fly should the occasion arise.
Now for the pilot who only flies 12 hours a year: I do not have a perfect answer for how often to change your oil.
What I would do is change the oil at least every year and add some of the Phillips Anti Rust Oil to every change.
I would also start a savings account for that new engine you may need in a few years.
Bob Barnes says
If you change the oil, add Camguard, and then fly the airplane every weekend for 1 hour, it would still take almost a year to get to the 50 hour mark. I find it hard to believe you would see corrosion with this type of usage. It seems that there is always the assumption of long periods of inactivity when there are lower hours on a plane but that isn’t necessarily the case.
JimH in CA says
We fly our C175B 1-2 days a week for about 1 hours, and one 2 hour x/c flight a month and we are barely get to 40 hours in 6 months.
We were doing 50 hour oil changes, but after changing a cylinder, our oil use dropped to 8-9 hrs per qt from 3-4 hrs. We use Phillips 20w-50 and a full bottle of Camguard.
We lean aggressively during taxi and cruise.
But the oil analysis showed a huge jump in lead, about 3x to 3,000 ppm. The oil filter is clean with a few very small copper and carbon particles.
2 years later we had 3 stuck exhaust valves, from the lead/coke build up on the stem end of the valve.!
We are now doing 30 hr oil changes and watching the lead levels . If the lead is still over 1,200 ppm, we’ll go to 25 hrs on oil.
FYI see Lycoming SB 388C.
John Robertson says
The O-200/300 class engines have marginal exhaust valve cooling, in conjunction with limited valves stem support in the guide, when in the open position. To mitigate exhaust valve problems I suggest being especially particular to ensure that the latest valve/guide combinations be installed, that the valve face contact be perfectly concentric and the full recommended width. It is important to ensure the best valve guide bore finish possible and has 100% contact within the guide boss.(accurately reamed bore)
With 100LL I doubt that you actually would have excessive lead fouling, unless there was cylinder blow by. I would think that it’s all hard carbon in the guide. I don’t see the relevance of Lycoming SB 388C to this engine, which has a completely different design to the Lycoming cylinder.
I believe that no matter what you do, these engines are never going to run full time without touching the cylinders. We just get the best service from them that we can.
JimH in CA says
These cylinders were new TCM nickel units from Airpower with 350 hours on them. I have no way of knowing what the valve guide material is or the reamed clearance, or the ability to disassemble new cylinder valves.
Most A&P and IAs I know will not do this work on cylinders.
The valve faces have a full contact and have a light gray color, as viewed with a borescope, so no lead deposits on this end of the valves.
The spark plug electrodes are all also light gray with no lead deposits at all.
The reamed deposits were brown in color, so they appeared to be lead compounds, vs black carbon.
I referred to SB 388C in it’s recommending a 300-400 hour inspection of the exhaust valve/ guide clearance/ sticking, and implying that it should be applied to Continental engines as well.
So, I will be checking the exhaust valves in another 300 hours on my GO-300.
Howard Riley says
Use Cam Guard and keep the engine compartment warm!
Flying B says
TBO on engines is BOTH hours of use AND Years in service. Shocking News: Almost all engines make it to TBO. For trainers at a flight school they hit hours first. For the personal airplanes who fly much less they hit the TIME portion first. Most owners want to ignore the time part. Sorry, that is not the way it works.
Matt says
Actually, if you’re Part 91, you’re free to ignore both time and hours if you properly monitor and care for the engine.
Klaus M says
Thanks Flying B, the years between TBO are by far a bigger problem then hours. Manufactures recommend 12 years, I find a part 91 engine can make it 20 years if all the basic engine care practices are followed. After 20 years the operator is pushing their luck. Aluminum deteriorates, steel parts fracture, bushings wear, rust, sludge, carbon build-up, seals and gaskets harden…etc. Ask any overhaul shop what they see with old engines.
If an FBO rented out an aircraft with a +12 year since overhaul engine and it failed let’s see who wins the law suite. The FBO’s first and last statement in the trial: “The engine ran great, had great compression and the oil analysis was great before it quit”. Prosecutor: “What does the engine Manufacture recommend?” “And why do you think that is?”
victor t Connor says
Hey Ben! Let’s have more tips on extending TBO. I’m sure there’s more than oil!
Neil fogle says
Why is no one talking about cam guard. How about draining the oil into a container , boiling it and putting the oil back in the engine. How about sealing the engine or installing a desiccant or dip stick heater.
scott k patterson says
What percentage of pre TBO failures are from oil contamination vs actual component failures and assembly errors?