D/FW AIRPORT — Skies were clear and visibility was ideal in the skies over Dallas/Fort Worth International Airport last Thursday.

But News 8 has learned that the Federal Aviation Administration is investigating an incident involving two planes that nearly collided in midair south of the airport.

An alert air traffic controller prevented what would have surely been a major accident.

D/FW has seven landing strips. Thursday's incident took place with flights that took off from Runway 18-L on the airport's west side and 17-R on the east side.

An American Airlines Super 80 and an United Express regional jet were cleared for take off. The American aircraft left first.

Once airborne, the American plane flew straight ahead on its assigned heading southbound.

The United Express jet mistakenly turned southeast toward the American plane.

Both aircraft were traveling at more than 200 mph.

An alert air traffic controller who saw the dangerous situation unfolding told the United pilot to immediately turn west.

"It would have been very catastrophic," said aviation safety expert Denny Kelly. "Probably 150 people in the MD-80 and probably 60-80 in the regional jet.

Both planes were about 1,500 feet above ground. At their closest point, they were less than 7/10 of a mile and 400 feet from a mid-air accident. That's less than the distance between Reunion Arena and City Hall in downtown Dallas.

News 8 has learned the regional jet pilots appear to have mistakenly loaded the wrong departure procedure into the cockpit's computer system.

"There was a trajectory that was there that could have put have put two airplanes together," said ABC News aviation analyst John Nance. "The fact was, they let a piece of automation put them into a bad position."

That's what the FAA is looking into.

Both aircraft were accelerating and climbing at the time of the problem, a time of flight when a pilot's workload is very heavy.

All passenger jets have sophisticated collision avoidance systems on board, and cockpits are increasingly computerized.

But—especially at low altitudes—aviation experts say one of the two pilots should be looking out for other airplanes.

"If one guy has his head down doing something with the radios or the navigation gear, then someone has to look outside the cockpit window," Kelly said.

Both planes in last Thursday's incident landed safely at their destinations. Passengers were apparently unaware of what nearly took place.