American Airlines flew the wrong plane in error from Los Angeles to Hawaii last month, breaking Federal Aviation Administration regulations.
The mix up occurred on August 31, just a few days after it began operating Airbus A321 planes on the LA to Hawaii route.
American said the plane used that day was not ETOPS certified to fly long flights over water.
The error was noticed midway through the flight but the decision was made to continue to Hawaii.
The return flight was cancelled and the plane flew back to LA empty.
"Immediately when we realized what happened, we notified the FAA and we are working and fully cooperating with them," said spokesman Casey Norton.
"We also have an ongoing, thorough review of our policies and procedures."
Planes which cover long distances predominantly over water must be ETOPS (Extended-range Twin-engine Operational Performance Standards) certified under FAA rules.
ETOPs aircraft are required to carry oxygen tanks and a special fire suppression equipment in the cargo hold.
The mistake was brought to attention by aviation blogger Brian Sumers, who said the mistake is extremely rare.
"It’s really rare. "Something like this, when you talk to airline people, they say ‘How does this happen?’" Sumers said.
