The problem is the wood spar. Although they may still be very strong,
the FAA has an AD that says if there are any nails backing out or if
any part of the spar is rubbing on a rib, the spar is unairworthy. It
is very challenging to find a Champ that does not have one of those
spar problems. Most people seem to agree that it doesn't make the plane
unsafe to fly.

It's more than just nails coming loose. The wood shrinks across
its width with age, and as the aluminum rib doesn't shrink, the nails
force the wood's grain apart and creates lengthwise cracks and the
spar's shear strength suffers. As the ribs move up and down, they
elongate those nail holes and the spar begins to crack vertically.
Additionally, the mechanics who do the AD properly have been finding
cracks in the spars at both the strut and butt attach points, both
causes for real concern. We rebuilt a pair of Champ wings some years
ago, long before the AD, and found extensive cracking across the grain
of the spars through the nail holes as described by a Service Alert. It
was only detectable when we had the wing all apart and bent the spars a
bit when the cracks appeared. Scared us pretty good; we replaced the
spars.

Dan