Former NTSB investigator Greg Feith, now a 9NEWS aviation expert, had been told authorities suspected four people were killed onboard on a Cirrus plane that hit the other plane, a Piper Pawnee. That plane, whose only occupant died in the crash, had a cable strung out behind that was pulling a glider.
An NTSB spokesperson tells 9NEWS the family of the Cirrus pilot indicated there were only two people on board that aircraft.
Mile High Gliding confirms the person killed in the tow plane was one of their pilots.
Rick Brough with the Boulder County Sheriff's Office says the glider unhooked from the cable just before the crash and landed safely.
Feith's sources tell him the occupants of the glider were the only survivors. They are believed to be the pilot, an 11-year-old boy and his mother. Feith's sources indicated the family was taking the glider trip as part of a party.
No names of any of the people involved were immediately released.
The NTSB says the glider went through a fireball after the crash, but it landed uneventfully at the Boulder Municipal Airport.
"I would say it's very fortunate at this point in time. I would hesitate to make too much analysis," NTSB Investigator Jennifer Rodi said.
The crash happened around 1:30 p.m. on Saturday not far from the Boulder Municipal Airport. The collision appeared to be over the US 36 and Broadway area.
Both planes went down in Boulder County open space, but the crash sites are separated from each other by a ridge. The debris field is over several miles and authorities are searching for pieces of the planes, including the sections with the tail numbers on them.
Amateur video sent to 9NEWS shows the Cirrus smoking as it had its parachute out as it fell to the ground. Witnesses also report seeing fires on the ground in the area.
"I was standing at the kitchen window and I glanced out and I saw a plume of smoke. And often where there is a ground fire, you'll see a plume of smoke coming up from the ground. And I did a double take because this plume of smoke was completely above the mountains there and there was a ball of fire coming down within that plume of smoke and then, very close to that I saw - I had to look very carefully - and it looked like an orange parachute coming down slowly," Maggie Bockman, a witness, said.
The amateur video was taken by Hugh Walton of Boulder. His home is about a half mile from the crash site.
"I realized this is a plane with a person in it and I thought, 'Oh, that is not good,'" Walton told 9NEWS. "I was just leaving my neighborhood and I saw some riders, some cyclists, pull over to the side of the road and they were pointing and looking up. So I glanced over my shoulder and I saw some smoke and some flames going on way up in the sky."
"It just was so shocking because it took me just moments to figure out that that had to have been a plane," Bockman said.
Walton's wife says she heard a boom right above their house around the time of the crash.
According to Feith, the Cirrus aircraft has a "ballistic parachute" that can be fired as a rocket. He says pilots use it as an emergency safety device to try and stop the plane from crashing. It is unclear if the pilot activated the parachute or if it fired off on its own.
Feith says his sources tell him no one at the airport heard the pilot of the Cirrus call in to Boulder Municipal Airport before the crash. That would mean the pilot of the Piper Pawnee may not have known about the Cirrus.
The NTSB says the pilots were not flying in clouds.
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