Dec. 3, 2009 -- Days before Thanksgiving, a United Express and Frontier Airlines plane came disturbing close to colliding midair, the Federal Aviation Administration says. The FAA is now investigating the incident during which the two jets were apparently so close to each other that they had to take evasive action to head off a crash.
The incident happened on Nov. 23 around 7 a.m. as the two jets were approaching Denver International Airport.
One of the planes, United Express flight 6764 from Lincoln, Neb., operated by regional airline Skywest, was in a string of planes following a set path heading to the airport.
The other plane, Frontier flight 1539 from Omaha, operated by parent airline Republic, was flying along a parallel path. Controllers needed to put the Frontier jet into the same route as the United Express plane to line them up for landing.
That's when the mistake happened, an aviation source told ABC's Denver affiliate KMGH, which first reported the story.
Controllers gave the Frontier pilots an incorrect heading -- forcing them to make a U-turn, the source told KMGH. That put the jet right in the path of the United Express plane -- which was just 200 feet above it and only a little more than a mile-and-a-half away, closing quickly, just seconds apart.
"When you've got airplanes coming within 200 feet of each other or aimed at each other, it's a very dangerous situation," said ABC News aviation consultant John Nance.
On board the two regional jets, collision avoidance systems sounded in the cockpits, forcing the jets to take evasive action to avoid a collision, an FAA spokesman told KMGH.
"You need to follow that command right now," Nance said. "So your heart rate goes up and you're immediately involved."
Planes Close to Colliding
KMGH, said that in the control center, the planes merged on radar and that the aviation source told them "they were within a blink of an eye of colliding."
The FAA said in a statement that it is "investigating a serious loss of the required separation between aircraft."
"We want to fully understand how this event occurred and ensure all the necessary safety measures are in place," the FAA said.
A third plane, Frontier flight 615 was next in line behind the United Express flight and also lost critical separation with the oncoming plane, but was not in immediate danger.
In the end, all the planes landed safely and it is unclear how much, if anything, those early morning passengers knew about the incident.