Plane Exploding in Mid-Air

There are far too many variables to give you a good answer.

If you want to read about plane crashes though, this is the place.

http://ntsb.gov/ntsb/query.asp

Depends on how high it was, how much explosive was used.
If I recall correctly an Air India plane was brought down over the Atlantic by explosive hidden in a portable music centre.

When Pan Am flight 103 exploded over Locherbie, Scotland debris was scattered over 845 square miles. More info http://en.wikipedia.org/wiki/Pan_Am_Flight_103

What everyone said.

Google
airplane crash 'debris pattern"

dolores haze said:

When Pan Am flight 103 exploded over Locherbie, Scotland debris was scattered over 845 square miles. More info http://en.wikipedia.org/wiki/Pan_Am_Flight_103

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I didn't see this in the above article, but the bomb was timed so that it would explode while the plane was over the ocean. The flight was delayed to the point that the bomb exploded while the plane was still over land, and this greatly helped the investigation.

By contrast, there's TWA 800 which exploded over the ocean, though closer to the ground and not far from land, shortly after takeoff (the explosion was witnessed by people on Long Island). From the above link, it's clear that such events have conspiracy theories surrounding them, but this one is infamous for its conspiracy theories, perhaps most notably that it was destroyed (accidentally or intentionally) by a US Navy missile:
http://en.wikipedia.org/wiki/TWA_800