In article 3523d.323018$Oi.300857@fed1read04,
"Leadfoot" writes:
Boeings take on aircraft service life is that it can be indefinite so long
as the sircraft is maintained properly. There has never been a requirement
to retire a Boeing aircraft after "X" number of whatevers. I suspect the
747 will fare far better than the DC-3 over a 70 year period.

While you're correct about Boeing's take on service life, the fact
remains that, at some point in its life (the end, of course) a 747
will start showing cracks in wing spars, and the fuselage pressure
vessel, and all manner of other areas, and it will become uneconomical
to repair it. That's already happening. the DC-3 series of airplanes
hasn't shown any of these behaviors. That's not too surprising,
really - The DC-3's wing structure is fairly stiff, and it uses Jack
Northrop's multi-cellular construction techniques. There are multiple
load paths there, so individual elements aren't stressed too highly.
It's not pressurized, so you're not inflating and deflating the cabin
on each flight. The 747, and, for that matter, any other jet, is much
more flexible, and has to put up with the stresses and strains of
pressurization, At some point, it's going to give.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster