"George Patterson" wrote in message
news:rOyZe.1925$%L4.26@trndny02...
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.

RVSM began this past January.