No - that's not what they are doing. Those lines are live.

What they are doing is making an equipotential bond between the line and
helicopter. The helicopter as basically a conducting object in free air
has inherent capacitance to ground and the the reactance is sufficient
to cause a not insignificant current to flow at 1/2 mil volts 50-60Hz

If the line were made dead, it would have been earth strapped at both
ends before anyone was let near it (standard operating procedure).

And for a quicky dodgey calcL

Capacitance of sphere of radius 1m (tiny helicopter) in free space is
about 0.1nF

Reactance at 60Hz approx:

1 / (2f x pi x C) = 27MOhms

Current at 254kV to earth (440kV phase-phase, for 3 phase) = V/R
= about 10mA

Which is enough to hurt and be a danger.


In reality I think the presence of a nearby ground plane (the ground)
plus the fact the helicopter is longer than 2m and has lots of sticky
out bits is likely to push the current a fair bit higher.

Plus some of those lines you speak of may be higher than 440kV.

So on that basis, whilst a small bird can sit on an HV line, if an
ostrich got up there, its feet would probably tingle


Now cometh a debate re can an ostriches sit on a power line;!()(

What lines are higher then the 400 kV grid then?..


--
Tony Sayer