In late August, American Airlines accidentally flew the wrong plane from Los Angeles to Hawaii, a mix-up that violated Federal Aviation Administration regulations.

The mistake, first reported by aviation blogger Brian Sumers, occurred on Aug. 31 — just days after the airline began flying Airbus A321 planes on the L.A. to Hawaii route (other planes in American Airlines’ fleet have long flown this route).

A spokesman for the airline confirmed to The Washington Post that a version of the A321 plane that was not certified to make long flights over water — as on the route between Los Angeles and Hawaii — was accidentally flown that day.

The mistake was caught midway through the flight, Sumers reported, but a decision was made to continue to Hawaii. The airline canceled the return flight and the empty plane was flown back to Los Angeles.

“Immediately when we realize what happened, we notified the FAA and we are working and fully cooperating with them,” spokesman Casey Norton said. “We also have an ongoing, thorough review of our policies and procedures.”

Planes that fly routes with no suitable landing areas are required to have a certification called ETOPS (Extended-range Twin-engine Operational Performance Standards) which is primarily an administrative requirement, but it also calls for the planes to be equipped with extra oxygen and a fire suppression cannister.

The A321 planes can be either ETOPS certified or non-ETOPS certified. Both planes have the same number of life vests, rafts, and engine range, Norton said.

According to Sumers, airlines don’t certify all planes, just the ones that are used to fly the long-range routes over water:

Obtaining ETOPs certification is not difficult but it is a complicated process, and so there’s no reason to certify aircraft that won’t fly on long routes over water. As a result, American has two types of A321s in its standard first class and coach configuration — the A321H, which can fly to Hawaii, and the A321S, which cannot.

Yet flying a non-ETOPS certified plane, as American did on Aug. 31, is a big mistake and it is extremely rare, Norton acknowledged.

Sumers, who writes about aviation issues, said that he first learned about the mistake from pilots chatting online anonymously.

“It’s really rare,” Sumers said. “Something like this, when you talk to airline people, they say ‘How does this happen? This can’t happen.'”

American Airlines would not say what caused the mistake. But Norton said that immediately after it occurred, the airline upgraded the software on the plane so that “the correct aircraft is identified to fly the correct route.”