An interesting concern, in light of recent threats against NYC tourist helicopters, no? When I first heard this, I thought.."hmmm..the bad guys must be getting desperate"..
A helicopter into a building..no biggie.
Now, a helicopter with someone in control dumping something over a major metro area..
Now a helicopter with a nuke aboard exploding at somewhere above ground level would cause devistation across the entire city.
1850'
Yup, I thought the same thing...