Another close call in sky: United Airlines jets nearly collide in Houston

With passenger planes sometimes traveling at more than 500 miles per hour, there is little room for error, but there appears to be another case where an air traffic controller made a mistake and put two passenger jets on a collision course.

Just after 10 p.m. on May 9, two United Airlines aircraft prepared for take off at Houston's George Bush Intercontinental Airport, reports CBS News' Jeff Pegues. United flight 601 taxied down runway 9 heading for Vancouver.

Air traffic control instructed the pilot to take off; at the same time, another United flight 437 departed for Mexico City on another runway. The Vancouver-bound flight was also told by air traffic control to turn right, putting the aircraft directly in the path of flight 437.

"Uh, 601, thank you, turn right, right turn heading 3-4-0, 3-40 runway nine clear for takeoff," an air traffic controller said.

The planes were less than a mile apart, a distance which is only a second or two from impact in the air.

"United 601 stop your turn, stop your- let's just stop your climb and stop your turn, United 601," the air traffic controller said.

Roughly 150 passengers were on board each aircraft. All were unaware of the incident and landed safely.

"You all basically crossed directly over the top of each other," another air traffic controller said.

This is the third near-collision involving United Airlines planes in recent weeks.

Six weeks ago, there was another hauntingly similar close call between two passenger flights.

"We've got an emergency descent going. A plane just popped up right at our altitude," a pilot said.

A week before that, two Boeing 757s nearly collided over the Pacific Ocean.

"The NTSB is going to be looking at all of the factors," said Mark Rosenker, former head of the NTSB and CBS news transportation and safety expert. "If processes were short-cutted or if something else happened here to create this environment where this catastrophe could have occurred."

In the case of the United flights at George Bush Intercontinental on May 9, the FAA said it is still investigating the incident, but it credits the controller with catching the mistake and preventing a disaster.