Beyond TBO and Oil Analysis: Split from 7MA2 Cook-Out/Fly-In

[3958v]

Great video!! Check out the latest Sport Aviation article on the subject of TBO and engine failures. Seems to question the wisdom of overhauling a good running engine. I know they are different beasts but I know that with our construction equipment there really seems to be no real good predictor of when a good running engine might fail other than if it has recently been worked on. Although I dont do it I do believe oil samples could help. I wish there was more research on running engines well past TBO. I know some A&Ps who dont think its a bad idea but would not want there names in the logs if anything . I know I overhauled mine at 1400hrs and it did not run one bit better and was harder to start. Bill K

[jrenwick]

I sold a 170 in 1996 or thereabouts, with a fairly high time O-300 engine. In 2003 I ran across it again, and the owner at that time said the engine was still running great at 2200 hours SMOH. He didn't seem worried about it at all.

[hilltop170]

When I bought my 170 the second time, the engine had 1750hr SMOH. It was still running very good.

On teardown, the crank was found to be cracked 1/3 of the way around and the rear oil pump bearing journals that are machined directly into the accessory case were egg shaped. There is no way to tell how much longer it would have run before failure.

On the other hand, almost everything else was in tolerance. The cylinders were worn but still had good compressions.

I think if I was to intentionally run an engine past TBO, I would start an oil sample program several hundred hours before TBO and send in a sample at every oil change and carefully watch for change.

[GAHorn]

If you began an oil analysis program late in an engine's life.... such as the one you re-purchased, Richard.... How would that program have detected that cracked crank and worn pump before it failed?

Not likely, in my opinion.

The oil pump wear would only show high aluminum which would be expected in any high time engine and you'd likely have continued operation until destruction.

[hilltop170]

If you began an oil analysis program late in an engine's life.... such as the one you re-purchased, Richard.... How would that program have detected that cracked crank and worn pump before it failed?

Not likely, in my opinion. .

First, just so nobody misunderstands my statement, I would not recommend running any engine past TBO, that is why I got a ferry permit when I bought the plane and flew it from it's tie-down directly to the repair shop where the engine was removed and overhauled. I did not enjoy that 10 minute flight at all.

I guess I should not have mentioned TBO at all in the comment and should have worded the statement differently such as, I regularly use oil analysis on my engines to monitor their health so I can see when conditions start to change, specially when close to TBO where you can expect the rate of change to increase. Without a base line started when the engine is still healthy, it would be hard to determine when it is time to do something.

Second, you are exactly correct, the oil analysis would have shown nothing to indicate the crank was cracked. There was a comment made by another member that their engine was running very well even though it had high time. That's why I mentioned the crank crack. It had nothing to do with the oil analysis comment.

The oil pump wear would only show high aluminum which would be expected in any high time engine and you'd likely have continued operation until destruction.

What harm does it do to monitor the oil in the later stages of an engine's life? Again, I'm not suggesting anyone run an engine past TBO, but if someone exercising their own free-will decision decides to, after a base line has been established well before TBO, at least a judgement call can be made when contaminant level changes increase over time to the point where action needs to be taken. It might very well be before recommended TBO. That's why I do it.

[GAHorn]

...operation until destruction.

What harm does it do to monitor the oil in the later stages of an engine's life? Again, I'm not suggesting anyone run an engine past TBO, but if someone exercising their own free-will decision decides to, after a base line has been established well before TBO, at least a judgement call can be made when contaminant level changes increase over time to the point where action needs to be taken. It might very well be before recommended TBO. That's why I do it.[/quote]

I don't mean to imply that "harm" would occur by late-TBO oil analysis. I only meant that oil analysis started late would have much less informational value from a trend-monitoring aspect than one which was carried from early in the engine's operational history.