Don't believe there is a limit on hours or cycles. I believe that it is more, on-condition. There are other "aging fleet" issues that will probably drive them out of the skies before the sheet metal does. Wiring, tubing, and all of the things that deteriorate are likely to make them less and less desirable to the operators. I would not be surprised to see inspection schedules become more and more expensive, not only in terms of the actual procedure, but for frequency and the downtime on the airframe.
I used to read the FAA's MRR Summaries for the DC-9 and some issues, maybe 70% of the items detailed there were corrosion and cracks found in C and D checks. (at Republic and Eastern, mainly)
I've also been told that when Sunworld ceased operations one of their DC-9-14 airframes was one of the top two or three for time/cycles and, for that reason, it was re-acquired by the factory to be tested to destruction. Can anyone confirm that? (Maybe N9102)
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Photo © Brian Peters
Happiness is not seeing another trite Ste. Maarten photo all week long.