Skiers will nearly free-fall out of the starting house, gathering speed like a sky diver.
This altered their velocity, and enabled them to match speeds with other sky divers.
For an extra fee, jump centers will often send along another sky diver to videotape the event.
In any case, sky divers often brush aside the risk.
The safety board said there have been 38 crashes involving planes carrying sky divers in the past 10 years.
To an experienced sky diver, this became a matter of instinct.
"I've seen similar injuries in sky divers whose parachutes fail to open," he said.
The airport continued to operate late yesterday afternoon, sending planes into the air for other sky divers.
Instead, it was what was under each sky diver.
When the craft hit the ground, the rescuers said, the sky divers appeared to have slammed forward, one on top of the other.