What is the reason for an approach (or a whole airport) to be NA as
an alternate? Is there some attribute an approach has to have to be
worthy of being filed as an alternate?

The one that got me wondering was KAST (Astoria, OR) which is NA except
for the VOR Rwy 8 when it also has ILS/LOC and GPS approaches.

--
Ben Jackson

http://www.ben.com/