Some good stuff here, but getting back to us folks using consumer grade equipment on the VHF air band:
At an elevation of about 800' ASL, using a Radio Shack Pro-2042 with a discone antenna on the roof of a single story residence, I would routinely hear commercial a/c 250-300 miles away at, usually, 35k~39k altitude.
My farthest catch was a NASA TR1 "above 60K" at 400+ miles.
As for airports, I lied in an area surrounded by hills, but 20~25 miles was my max.