PHOENIX (KPHO/Gray News) – A Phoenix man on a walk with his family recorded video of a close call between a United Flight and a Delta Flight in the skies near Sky Harbor on Saturday.

The Federal Aviation Administration confirmed the incident is under investigation.

Delta Flight 1070 from Detroit, carrying 245 passengers, had just been cleared to land at Sky Harbor’s Runway 8 when a collision warning went off in the cockpit around 11 a.m.

At the same time, another automated warning went off on United Flight 1724 from San Francisco, carrying 123 passengers.

The planes got within 425 feet apart in vertical distance, according to data from FlightRadar24.

Seconds later, they were within 1,200 feet apart horizontally – less than a quarter of a mile.

A former NTSB chairman said these types of planes should be at least five miles apart horizontally.

Justin Giddens, a self-described “aviation nerd,” took live photos of the close call while walking with his wife and son.

“I know the way the planes are supposed to be in the pattern on approach to Sky Harbor,” Giddens said. “It was immediate panic mode.”

In air traffic control audio obtained by CBS News, a controller had cleared the United flight to land on Runway 7 right and then told the Delta flight to turn into position to land on Runway 8.

“We do have Runway 8 in sight. Clear,” the pilot said.

That’s when the collision warning went off in the cockpit. “Descend, descend” is heard in the background.

The pilots then changed the altitude, and everyone landed safely.

“It just makes you think – oh man, they really avoided a major disaster,” Giddens said.

Embry-Riddle professor Cary Grant, a nationally recognized aviation safety expert, analyzed the video of the incident.

“Putting the ground tracks together of the aircraft, it sounds like the controller got preoccupied with other aircraft they were checking on at the frequency at the same time,” he said.

He said the crew on both planes responded appropriately.

“I don’t think there was any chance of a collision between the airplanes,” Grant said. “That’s because visibility was perfect that day.”

Delta says its flight crews “extensively train to handle uncommon scenarios such as this.”

United said its pilots “acted immediately” after hearing the flight deck warning, and officials are working with the FAA on its investigation.

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