[COEXERJ145]

I'm not sure. It would depend on how many cycles the aircraft have right now. IIRC, Northwest plans to fly their DC-9's to roughly 114,000 cycles. That is the point where the wing box would require serious maintenance which would be cost prohibitive.

[Paleo Conservative]

Northwest plans to fly their DC-9's to roughly 114,000 cycles.

If they're serious about flying that many cycles, shouldn't they re-engine them? When they reach their cycle limit, they could transfer the engines to some second hand MD-80's.