plane proximity
 

Flew up to Canada and back in the last coupla weeks.

On the Delta Connection flight from SAT to DTW it appeared
to me there was a very close encounter with another aircraft.

I just happened to look out the window when I see another
plane whiz by going the opposite direction.

That plane was at a slightly lower altitude but close enough
I could make out the livery clearly that it was an American
Airlines jet with the engines mounted on either side of the
tail... a la CRJ.

Don't know if I was the only one who saw this given no other
passenger said a thing. I hope at least the pilots were aware.

The close proximity freaked me out a bit.

The pilots knew. Assuming you were above 29,000 feet (technically FL290, but for the sake of simplicity let's call it 29,000 feet), you were in RVSM airspace (reduced vertical separation minimums) which allows for 1,000 foot vertical separation. Both planes knew where the other was, and no safety or other rules were compromised.

The RVSM comment is correct. 1000 feet doesn't look like much but there is quite a bit of space between you. And both planes are equipped with TCAS (Traffic Collision Avoidance System) so if there was any danger of running into each other, the systems in both planes would have been issuing advisories to the pilots.

It could have very well been an MD-80, and I'm sure it was much farther away than it looked.