Image Jonathan Wald @jonathanwald

This tower crane in NCY went over backwards in the Sandy storm.   

A rotating tower crane is intended to "weathervane" - eg rotate so that the tower is always pointing down wind like the tail on a wind mill.

So what caused the boom on this crane to flip up and over backwards?    

It could be that the operator left the swing brake on, and the boom was unable to rotate and maintain a down wind position.   I could also be that the aerodynamics at the top of the unfinished building were such that it created a lee ward wind in the opposite direction from the prevailing wind - lifting up the boom and toppling it over backwards. 

Actually, this is a very lucky accident - because the boom - or worse - the boom and the cab and counterweights -didn't separate from the tower and fall 90 stories.    When no one gets hurt, maybe the contractor will consider this is just a Halloween joke!

 In Singapore in 2010 a similar luffing tower crane went over backwards - just like the NYC crane above.

Image from towercraneacidents.blogspot.com

You can read the forensic discussion about the Singapore collapse at Tower Crane Accidents reported by Gaytor Rasmussen