Tips for Getting Your Aircraft Engine to Full TBO — General Aviation News

My last column was about how often you should change your plane’s oil.

It received a number of comments, which I always appreciate.

Many comments related to the engine manufacturers’ recommendation to change the oil every four months.

A reader emailed me, noting that he had several questions about oil change intervals.

“A guy said that in the past the recommended oil change interval was 50 hours or six months. If so, when did it change from six months to every four months? I believe the answer is that the four months were the result of a decrease in aircraft utilization.

I couldn’t remember, so I emailed my friend, Paul McBride, who writes the Ask Paul column for General Aviation News, to see if he was aware of such a recommendation. He hadn’t heard of it, so if there had been such a recommendation, it was over 50 years ago.

But why do manufacturers recommend 50 hours or every four months with a full oil filter?

This recommendation reflects the primary cause of lubricant-related failures in aircraft engines, which is corrosion leading to spalling of the camshaft and lifters.

This, in turn, is mainly caused by aircraft inactivity, just as our reader thought.

When an engine rests, there is almost always water in the oil. Some of it comes from the blowing of exhaust gases, but it’s mostly from water condensation.

When an engine is idle, warm, moist air is drawn in during the day. As it gets cooler at night, the water runs off and stays in the oil or on engine parts.

Over time, the water combines with the sulfur in the oil, which comes from the unburnt fuel in the blow past the rings. This forms an acid which attacks the cam and tappet surfaces, forming surface rust.

When the engine is finally started, the rust acts as a break-in compound which, in turn, initiates the wear process that eventually leads to failure.

These failures almost always occur on low-use aircraft that sit a lot. It is very rare to see this on an engine that is used regularly all year round.

Based on this data, engine manufacturers made the recommendation to change the oil every four months.

Four months instead of three or five months was considered a reasonable compromise. The idea is that the recommendation will help eliminate a long period of inactivity, because the longer an engine sits, the more rust builds up.

Several readers have pointed out that there would be far less rust accumulating on an aircraft engine in Arizona than in Florida.

That’s true, but engine manufacturers can’t write a recommendation for every situation. If they tried, they would have to put in a chart showing oil change time versus normal relative humidity.

Expert recommendations for achieving full TBO

What would I recommend to achieve full TBO?

The first step is to get rid of the water in your oil during normal service.

To do this, you need to check the accuracy of your oil temperature gauge.

Take the transmitter unit out of your plane and place it in a container on a hot plate. Heat the oil to 180°F, measured with an accurate thermometer in the oil. Now make a mark on your oil temperature gauge.

When flying, check this gauge and make sure your oil temperature is at or near the mark.

The only way to get rid of water in oil is to boil it, which is why it must reach 180° while you fly.

As the oil passes through your engine, it will rise about 50°F above the crankcase temperature. If your sump temperature is below 160°F, the oil will not exceed 212°F and the water will not boil. Instead, it will simply accumulate in the sump.

Second: Don’t just start your engine on the ramp to coat all the parts, thinking it will help extend the life of your engine. This only adds to fuel contamination and does not evaporate the water.

Third: Fly your plane as often as possible. Plan a flight where you raise the temperature of the oil long enough to get out of the water.

Fourth: Depending on the amount and timing of your flight, you should schedule your oil changes.

Many pilots living in northern climates fly a lot in the summer and very little in the winter. As winter approaches, pilots can simply store the aircraft where it will rest during the cold months and then change the oil in the spring so they can fly with fresh oil.

Bad plan. You should change the oil before storing the aircraft. This way you will begin the idle period without water or acid buildup.

Also, I would recommend putting a quart or two of Phillips rust preventative oil in the crankcase with the oil change. This will help protect your engine and you will still be able to go flying if the opportunity arises.

Now, for the pilot who only flies 12 hours a year: I don’t 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 Phillips rust preventative oil with each change.

I would also open a savings account for that new engine you might need in a few years.

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