Do you know how a Van De Graaff generator works?
Now imagine the twin discs (main rotor and tail rotor) of a helicopter going into a cloud of positively charged particles.
If you get the correct mix of conditions, then BANG! Followed by several pairs of brown pants. Subsequently followed by a bill of hundreds of thousands for lots of very expensive components that will need to be changed.
Which is why the Met Office is now trialling a system that shows offshore pilots areas where the 'triggered lightning risk' is greatest:
http://www.oilandgasuk.co.uk/downloadabledocs/1227/6