Absolute lowest altitude you can fly (legally)

Walt writes:

Ignore the political aspects of the book, although they're pretty
interesting in their own right. Just take the way they tried to figure
out a longitude fix 300 years ago and look at how it's done now.

In those days, mariners had to be very brave, or slightly stupid.

The current B-52 is what, the H model? It's changed.

Yeah, but it was supposed to be gone, replaced by fancier stuff. Now
the AF is talking about keeping it for another 40 years. It's a good
aircraft.

Who would have thought, more than four decades ago, that he could tell
his grandson "Someday you'll fly this plane, too"?

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