It starts in a treehouse about 15-20 feet up and goes about 50 yards and the cable wraps around a tree trunk about 8 feet up. He is a tinkerer and eventually attached one of those big lifetime planes to it.

No joke.
Have yet to have a major injury in 8 years and the kids love it. My wife does not. All handmade so no kit. Just a big cable with a pulley and clamps where it wrapped around the tree. I think he cut up a car tire to cushion the tree from the cable. Usually they will stop just short of the tree but once my oldest hit the end pretty hard. No harm done. They either have to jump down from the plane or someone has to run it back to the start.