I don't have the book here, but I *believe* the 30nm is referenced to
the Airport Reference Point, which is the physical location from which
they began the survey of the land to lay out the airport. Mostly this
is somewhere near the center of the airport, but not always. sometimes
it is not even *on* the airport.

The last issue, #8, is implemented on some boxes and not others. I do
not remember if the 89B had that feature or not. But on some machines
it was wired to automatically switch the NAV/GPS switch back to NAV if
you had a valid Localizer freq in Nav 1. The theory was that if you
were trying to fly an ILS and fogot your NAV/GPS switch was left in the
GPS position, it would automatically default back to the NAV position
so that the data displayed on the needle you were tracking was from the
ILS and not the GPS (that you were presumably not using.)

Of course, this created the opposite problem; if you are trying to do a
GPS approach, and forget to get the ILS freq out of Nav 1, it would not
allow you to *load* the approach, let alone fly it. Infuriating.

I have not seen this feature implemented on any of the more recent
boxes.

Bottom line, you have to know what your box does and doesn't do
automatically, and when it does it, and why. An put a check of that
NAV/GPS switch in your set-up procedure for *all* your approaches, not
just the GPS ones.

Regards,

Gene