The folks at Three Wing Flying Services got started on N631S's annual inspection yesterday, a day late. Mike the IA did the engine runs, uncowled the airplane, dropped the old oil and checked the cylinder compressions as instructed by TCM Service Bulletin SB03-3.
One cylinder, #2, was not very tight. It tested at 48/80 psi on a day when the minimum acceptable value based on the standard orifice was 46/80. I've gone to the logs and dug out the compression values at each annual inspection since 2007 (charted below).
You can see that #2 has always been the "weak sister" among the cylinders. Since the result has to be considered in relation to the minimum acceptance value and that in turn varies with atmospheric conditions, I made another chart (below) that shows the difference between the test result for each cylinder and the applicable minimum value.
It's also clear that #4 experienced a considerable drop this year. But that's just one data point so we'll simply note the result and see what next year's annual brings.
Over the four years since the 2007 annual, N631S's engine has accumulated a bit over 500 hours. It's currently at about 1,340 hours since overhaul. That's really pretty good for a Continental engine, as they are noted for needing a "mid-life top overhaul".
Mike will inspect the cylinders using a borescope, and assuming he doesn't see anything unusual no action is needed. But I'll be watching for any evidence of further deterioration of the #2 cylinder. The engine still does not consume any significant amount of oil between changes so if oil consumption goes up the #2 cylinder will be the likely suspect.
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5 comments:
Good luck on the rest of the inspection!
Thanks, Toria. I'm not expecting anything major to emerge. Of course, you never know 'til it's over.
Frank, You mentioned using a SOIAP program a while back ... when you wrote an article on changing oil I believe. Are you still using the program and has it revealed anyhting interesting?
I'd like to hear your comments, as you maintain a good balance between the engineering/it must be done right sort of guy and the practical, do what it needs to be safe, but not so safe we spend more time fixing than flying.
In my relatively untutored view, the SOAP results would have a lot to do with my decision on what to do withh your #2.
The compression results and history make me fell it's likely good to go for another year, but if there has been any recent change in the 'metal making' world I might well decide differently.
Hi, Dave!
Yep, I am theological about oil analysis. A sample from every oil change goes to the same lab and I monitor results to watch for any adverse trend. So far, no issues.
Also, every filter gets cut and the media examined for evidence of metal. So far, no metal!
So what do we have here? An engine with consistently low micron-level residues of all metallics in the used oil. An engine that does not deposit macro particles of metal in the filter media. An engine which does not burn oil between oil changes. And (stay tuned!) an engine which I anticipate will be OK on all cylinders for the borescope visual.
But, with a cylinder or two on the low side for the compression test. To me that all adds up to good news. I'm a strong believer in "condition-based maintenance" and I feel that I have good evidence of condition that allows continued unrestricted operation of the engine.
If I see a spike in oil consumption or any anomaly in the SOAP data on an oil change (or metal in a filter!) I will pull the cowling and have the compressions rechecked and another borescope inspection done. And react as required by the results. I certainly see NO probability of a catastrophic failure. So I'm comfortable with the program. I plan to fly the machine.
Finally, I don't believe there's a conflict between "engineering/do it right" and "practical/be safe but don't spend more time fixing than flying". If you believe in "maintain on condition", the two approaches collapse into one.
Best regards,
Frank
Thanks, Frank, for that great explanation. I understand a lot more about the thought process now. For years I pulled SOAP samples from engines and never saw any evidence of any thinking being driven by the evidence produced.
Have another great year in the sky.
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