In most cases, trees that are very near electrical and transmission lines are cut or trimmed by ground crews. In some areas, however, it can be very difficult for the ground crew to do the job. This is especially true for rough terrains and wetlands. That's why some companies make use of helicopters equipped with air saws in trimming trees in such difficult terrains. A typical air saw has ten rotary blades and suspended from the helicopter with a 90-foot vertical boom. Here's the helicopter and air saw in action:
Here's a similar video of the helicopter doing it's job.
The air saw is very effective and can perform the job in just a few hours which if done by a ground crew can last for several days. For more information on which trees need to be trimmed, you can visit the American Transmission Company's info about it here.
