It says "when an instrument letdown...is necessary" that you shall use an
instrument letdown. Isn't that sort of circular?

It doesn't quite say that... it's more like "when an instrument
letdown...is necessary to an airport... that you shall use =OUR=
instrument letdown to =THAT= airport." This looks like it precludes
shooting an approach to ABC and then slipping off to land at DEF, and it
certainly precludes home-grown approaches unless otherwise authorized by
the administrator.

Jose
r.a.misc snipped. I don't follow that group
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