A United Airlines pilot made an emergency dive to avoid a US Airways flight over Hawaii last month after a traffic controller did not notice the two Boeing 757s were on a potential head-on collision course.
The planes' onboard warning systems alerted both pilots that their aircraft were too close, according to the Federal Aviation Administration, which is investigating the late-night incident that occured on April 25.
The jetliners were travelling at 33,000 feet about 321.86 kilometres northeast of Kona, on the Big Island.
Los Angeles-bound United Flight 1205 had just taken off from Kona when the pilot took evasive action to avoid the Kona-bound US Airways Flight 432 from Phoenix.
"We've got an emergency descent going!" the United pilot radioed to the Honolulu control tower.
The plane then dropped 600 feet in 60 seconds.
"It was a really violent, scary experience," United passenger Kevin Townsend, of San Francisco, told CBS News. "It felt kind of like the plane had gone dead in the air and started dropping."
Regulations require planes to be separated 5 miles (8.04 kilometres) laterally or 1,000 feet vertically.
Preliminary radar data showed they were separated at their closest point by 5.3 miles (8.6 kilometres) laterally and 800 feet vertically, the Honolulu Star-Advertiser reported. CBS News said they came with 2.2 miles (3.54 kilometres) of each other.
That's close for airliners traveling about 600 mph, Peter Forman, a retired pilot and aviation expert, told Hawaii News Now.
"Sixty miles an hour is one mile a minute. Six hundred miles an hour is 10 miles a minute closure rate," he said.
The FAA and the National Transportation Safety Board are investigating.