Roy Smith wrote
I have actually seen one airport served by half a dozen approaches,
including an ILS, where all the approaches were noted /A\na except one
NDB approach.

Any idea why?

My understanding was that an approach got the /A\ thingie if the navaids
required to fly it were not monitored.

That sort of makes sense - the NDB probably had remote monitoring
because it was part of an airway once, long ago. I think the last of
the NDB airways in the US were decomissioned a couple years ago,
though.

If you look up the alternate mins for KCLL (College Station, TX) you
will see that the ILS and LOC BC go NA when the tower closes, but the
VOR approach doesn't. I suspect this is because CLL VORTAC must be
remotely monitored because it forms the basis of multiple STAR's
serving Houston and Dallas, whereas the ILS transmitter is probably
only monitored in the tower cab.

Michael