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Moments after a single-engine aircraft collided with a plane towing a glider over north Boulder on Saturday, an experienced flight instructor who was in the air heard a "mayday" call from the glider pilot.

Reuben Bakker, who was flying two passengers in the glider, reportedly saw the other plane "out of the corner of his eye" and cut the line between his aircraft and the tow plane moments before the collision. He then navigated through fire and smoke to land safely.

Tracey Spence, who manages the Boulder-based flight school Journeys Aviation, said Bakker made a distress call on a shared radio frequency immediately after the mid-air crash that killed three people.

"He announced 'mayday, mayday, mayday,' and said there was a collision and gave the location and said one of the airplane's parachutes had deployed," Spence said, adding that Bakker remained calm throughout the distress call.

Spence, who said Saturday's mayday call was the first he's ever heard, repeated the information back to Bakker and promised to pass it on to someone on the ground and ask them to call 911. Bakker and his two passengers -- a woman and her 11-year-old son on a flight for the boy's birthday -- landed safely at the Boulder Municipal Airport.

The three people who were in the two planes that collided at 8,000-plus feet over U.S. 36 and Broadway about 1:30 p.m. Saturday died.

Bob Matthews, 58, of Boulder, was piloting his Cirrus SR20 plane, and his brother Mark Matthews, 56, of Englewood, was a passenger, according to investigators. Alexander Howard Gilmer, 25, of Evergreen, was piloting the Piper Pawnee that was towing the Schweizer 2-32 glider.

Bob Matthews was a Boulder-based lawyer with the firm Faegre & Benson, and his brother was an engineer living in Englewood. Gilmer worked for Mile High Gliding, based at the Boulder Airport.

Investigators from the National Transportation Safety Board cleared the crash debris from the scene Sunday and were continuing to interview more than 30 witnesses Monday, including the three survivors who were in the glider. Numerous witnesses in the area reported seeing a ball of fire and a trail of smoke in the sky after hearing a loud boom.

Some people reported seeing occupants of one, or possibly both, planes jump after the collision.

The full investigation could take up to 10 months to complete, and a final determination on the probable cause of the accident will come two months after that.

But Jennifer Rodi, the NTSB's lead investigator for the crash, on Monday released more details about the collision, including the positions of the two aircraft when they hit.

The Piper Pawnee, which was headed west at about 70 mph, was level but on its way up to 10,000 feet at the time of the collision, Rodi said. The Cirrus, which took off from the Erie Municipal Airport for a recreational flight and was headed south at an unknown speed, also was level but might have been descending slightly. The Cirrus hit the Pawnee at a 90-degree angle, Bakker told investigators.

Neither aircraft made any maneuvers before the impact, Rodi said, and it doesn't appear the two planes had any communication at all with each other or air-traffic controllers.

Spence confirmed that he heard no communication from either aircraft over the shared radio frequency while flying in the area Saturday.

At the time of the crash, Spence said, visibility in the skies was about nine miles. On a clear day, he said, pilots can see up to 70 miles away.

"It was somewhat reduced due to haze," Spence said, adding that the clouds shouldn't have created any danger. "Normally that is acceptable."

Investigators said the glider pilot reported clear conditions and said that -- before the crash -- it was one of the smoothest flights in his memory. Survivors on the glider snapped photos and video during their flight, according to investigators, but it's unknown at this time whether they got any shots of the collision.

Tuesday, investigators will do a 3-D reconstruction of the collision in Greeley -- where the wreckage has been towed -- to try to determine how and why the planes hit.

Boulder Municipal Airport Manager Tim Head said mid-air collisions are extremely rare, and everyone in the Boulder-area flying community is "still soaking it in."

"In fact, we were just saying that it's nice to have a snow day so everyone can do that," Head said.