Shortly after takeoff from San Francisco International Airport, a United Airlines jumbo jet came so close to colliding with a small Cessna plane that it set off the United plane's collision avoidance alarm system.
According to audio recordings released from Saturday's incident, United Flight 889, bound for China with 268 passengers and crew members on board, acknowledged a routine clearance for takeoff.
"Cleared for takeoff … United triple 889," the pilot said.
But as the jumbo jet climbed through 1,100 feet, the airport tower controller, who waa also in contact with the pilot of the small Cessna place, realized the planes are too close and getting even closer.
The controller instructs the Cessna pilot, who is flying south, to go behind the United plane.
"7-echo maintain zero separation … pass behind that aircraft," the controller said.
"7-0 will pass behind him," the Cessna pilot acknowledged.
The controller then told the United pilots to "start heading to your right" and to "maintain visible separation."
"The light aircraft turned away from the United airline, that is how the captain of the airliner saw the underside of that airplane," Michael Barr, from the USC Aviation Safety and Security Program, said.
The Cessna ultimately passed behind the United plane.
"It was pretty darn close," Barr said.
The planes were 300 feet apart vertically and 1,500 feet apart horizontally. Rules dictate that at that phase in flight planes should maintain a distance of 500 feet vertically and one-and-a-half miles horizontally.
It was so close that a collision avoidance alarm, called a TCAS alert, went off in the cockpit of the United plane warning the pilots to descend to avoid a midair crash.
The United crew was clearing not happy about the situation.
"OK, that set off that TCAS," the United pilot said.
"We need to talk," the pilot told the controller.
"Roger," the controller acknowledged.
United Airlines asked the National Transportation Safety Board to investigate the incident, and a spokesman for United told The Associated Press that a near miss such as this is "unusual" for the airline.
The Federal Aviation Administration is also investigating and will take "strong measures to make sure something similar does not occur in the future," FAA spokesman Ian Gregor told the AP.
Gregor told ABC News that the air traffic controller should have seen the small plane and jet heading for each other earlier, but once the controller realized what was happening, the pilots were immediately contacted.
The Associated Press contributed to this report.