Retro Pilot wrote:
how long has it been since airports have gone from 3-to-4 letter
identifiers?

I'm not sure what you mean by this. If you mean an identifier like KABE, where
the "K" indicates a U.S. airport, I first saw that in 1989, and it had been that
way for many years. I haven't seen four-letter Ids in the States where the first
letter wasn't "K."

also, when did ATC go from 2000 feet vertical separation above FL290 to 1000
feet?

IIRC, sometime last year, but it could've been a bit earlier. The new collision
avoidance systems make closer separation standards possible.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.