Everything (conductive) in the universe has a capacitance between it and the rest of the universe. I believe the earth itself is about 1 Farad, to the rest of the universe (the earth is quite big).
The helicopter, floating in the air, has a capacitance between itself and the earth. This is not hard to calculate roughly; there is a formula for the capacitance between two spheres of given diameters and spacing. This capacitance will have a certain impedance (i.e. conductivity) at 50Hz (1/(2*pi*f*c)) and when the helicopter is wired to the power line, this capacitance will be passing a certain current. As the heli pulls away from the power line, this current is maintained through the increasing air gap, until the air gap is large enough to extinguish the arc.
The power line voltage is of course referenced to earth (the neutral wire is basically earthed) and so is the bottom of the “helicopter capacitor”
It is the same mechanism which produces the arc at the initial connection.
Peter wrote:
As the heli pulls away from the power line, this current is maintained through the increasing air gap, until the air gap is large enough to extinguish the arc.The power line voltage is of course referenced to earth (the neutral wire is basically earthed) and so is the bottom of the “helicopter capacitor”
It is the same mechanism which produces the arc at the initial connection.
Sure, but I’m surprised the current is large enough to make a noticeable effect.
I did some googling.
Firstly, I was wrong; the capacitance of the earth is only 710uF 
Using this I would say the helicopter capacitance is of the order of 200pF. That is based on a 50 inch radius so it is probably more; the rotor disk forms a part of it too. I could not be bothered to work through the complicated equations for two spheres of different diameters (one being the earth) or a sphere to an infinite plane.
At 50Hz and 440kV this gives me about 30mA, which is plenty to make a nice arc. It is also enough to stop the heart if you get the current flowing through the wrong part of the body, e.g. between the two arms. You would certainly get a really nasty shock from 30mA.
Also note that unless you touch the power line at exactly the zero crossing point, that 200pF capacitance will be initially charged with a theoretically unlimited current.
I used to work in high voltage – precision DC power supplies for scientific research, up to 500kV DC with (order of) 1ppm stability. Great fun
Christmas tree harvesting by helicopter:
Cool stuff.
Never finished my rotor rating, but it’s on the bucket list. But I did 20+hrs and even soloed. I can say it’s simply some of the most fun flying I’ve done I would encourage any fixed wing pilots to try to do it if you can afford to. Hell, if helicopters had great range and were fast traveling machines, I wouldn’t even bother with fixed wing!
