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I believe he started flying in his late 30s/early 40s so it would be around the early 1970s.

Edit: I just asked him (he’s at the dentist) and he said around 1980, guess I was wrong about when he started flying!

I would say no earlier than October 1977:

That registration and the reference to cell phones not being available at that time puts the timeline somewhere between 1977 and 1987 or maybe 1990.

Then definitely before the DC ADIZ. It's kind of tough to make this mistake anymore.