They were seconds away from a potential collision, but passengers aboard two United flights leaving George Bush Intercontinental Airport in Houston had no idea how close the planes came to crashing into each other.

The two planes leaving the U.S., en route separately to Vancouver and Mexico City, had just taken off on May 9 when air traffic control mistakenly told one of them to turn right instead of left.

The near-fatal error put one aircraft right in the path of the other departing plane, just 120 metres below it. And at the speed they were travelling, they were only several seconds away from each other laterally, just 1.6 kilometres from the airport.

"The controller then noticed that aircraft was heading into airspace normally reserved for Runway 15-Left, where United Flight 437 had just departed," the U.S. Federal Aviation Administration said in a statement. "The controller issued instructions to both pilots to safely separate the aircraft."

Air traffic control recordings caught a third-party pilot who witnessed the near collision, calling the incident "pretty gnarly."

“You all basically crossed directly over the top of each other," KHOU 11 in Houston quoted the pilot as saying.

The FAA has confirmed it is investigating the incident.

Near collisions often unreported: aviation safety expert

Shawn Pruchnicki, a former pilot and aviation safety expert, has witnessed similar near collisions in the past, including one as he was piloting a plane into New York City’s LaGuardia Airport.

“My concern is that as these events continue … our luck is going to run out and we’re actually going to have an accident,” Pruchnicki said.

Not every near collision makes it into the media, Pruchnicki said, leaving the public unaware how serious an issue it is.

Pruchnicki said it’s not enough for the airport or FAA just to punish the air traffic controller. Authorities need to figure out exactly what decisions the controller made to understand potentially systemic problems, he said.

A program called the traffic collision avoidance system (TCAS) will warn pilots when they’re too close to other planes, Pruchnicki said, but those are only used at altitude and are turned off closer to landing.

CBC News is looking to contact passengers on the flights. Please email Richard.Zussman@cbc.ca with your story.