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#10
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The limiting factor isn't generally crank/rod/ or other major
component fatigue (especially in those bullet-proof O-320s)but is probably corrosion and cam/lifter spalling. Oil analysis might catch either one. I doubt the factory has any info beyond that they would or even could release. From a strictly cumulative fatigue analysis standpoint there could have been an innocent relatively short term event in the engine life such as simply max throttle on a bitterly cold winter day that has done most of the fatigue damage. Or it could simply be long term operation at a wrong RPM where the crankshaft & prop are resonant. This type of "abuse" exists, but the wise ones (i. e. the certifiers) have decided and found that there is enough metal in this engine that it seems to be able to handle it. Hence for example there is no prohibited RPM range. Certainly the operating experience of a large fleet helps reinforce this. Whether that is true for precisely all supposedly identical engines and all conditions is impossible to say, but the history on this engine series is probably the best in the business. But engineering is an art, not a science. As one (mechanical) I can understand why the factory is reluctant to say to keep operating it. I have an O-320 E2D in a 172M last assembled over 29 years ago, & now at 1700 hrs TTSN. It has been opened only slightly for the oil pump gear AD, but otherwise it has shown no deterioration in compression, smoothness, or oil consumption over its lifetime. I'd like to think I have run it pretty carefully, but I realize I am running a slight chance. On the other hand, seeing the occasional AD show up on newly manufactured parts scares me too. The comfort I have is that I personally know the entire history of its operation. My decision is to keep operating it. Our IA once mentioned a Citabria in fish spotting duty out over the Atlantic that had 3800 hrs on it without overhaul. Unfortunately the limits of technology has to make it your call. But others are doing it too. |
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