American Airlines shockingly sent more than 100 passengers on a wrong plane from Los Angeles to Honolulu Aug. 31 and the error was only noticed when it was already halfway through its journey.

The passengers of American Airlines flight 31 left L.A. on board an Airbus A321. This was the correct plane variant, but not really the right one. It turned out the airplane that was supposed to take off had a different tail number. What is even more disturbing is that the plane used lacked the Extended-range Twin-engine Operational Performance Standards (ETOPS), said Brian Sumers, an expert who first reported the incident in his blog. This certification is required by the Federal Aviation Authority (FAA) for planes flying long-haul routes that are far from emergency landing sites, including this five hour flight. The FAA forbids non-certified planes from flying such routes, according to the Verge. American Airlines cancelled the return flight to L.A.

"Immediately when we realize what happened, we notified the FAA and we are working and fully cooperating with them," Casey Norton, spokesman for American Airlines, told the Washington Post. "We also have an ongoing, thorough review of our policies and procedures."

While the A321 plane was not certified to fly to Hawaii, the flight itself was safe. The wrong plane has similar technical specifications with the certified A321 aircraft, including the safety gears available to passengers and crew. The lack of certification appears to have been due to American Airlines decision not to apply for one since the plane will not be flying extended operation routes, according to Sumers. That process takes time in addition to the fact that it is expensive.

It is not yet clear whether American Airlines or any of its personnel is criminally liable. The company is also quiet as to what caused the mix-up. An upgrade, however, on the airline's software system was undertaken after the incident, according to Norton in the Washington Post report.