This goes without saying, but planes aren't like socks. You can't mix and match them. Each is uniquely identified and has a precise schedule with the airlines. That's why it's pretty shocking to hear that American Airlines used the wrong plane on August 31st to fly over a hundred passengers from LA to Honolulu on flight 31.
Passengers in Los Angeles boarded and took off in an Airbus A321. That's the right model of plane, but it wasn't exactly the right plane. Not only did this A321 have a different tail number from the plane that should have flown the route, but it wasn't certified to fly a long over-water route.
It was an Airbus, but not the right Airbus
Specifically, the A321 variant used by American Airlines lacked ETOPS (Extended-range Twin-engine Operational Performance Standards) from the FAA, according to Brian Sumers, an aviation expert who first reported on the incident. The certification is required for planes with two engines to fly long-range routes far from emergency landing sites — like that from the West Coast to Hawaii. With certain levels of certification, the planes are approved to fly for up to three hours with only a single jet engine in operation. The FAA strictly forbids the use of non-certified planes on such routes.
Although the mixup is an odd one, it may not have been a particularly dangerous one. According to comments from an American Airlines spokesperson, both A321 variants are equipped with similar safety gear, including life rafts. While the equipment on board is similar, Sumers reports that airlines only seek certification for planes that fly extended operation routes since the verification can be time-consuming and costly.
In a statement, airline spokesperson Casey Norton said, "When we noticed it, we immediately undertook an internal investigation, and we alerted the FAA." Norton added, "We are checking our internal procedures, everything that led up to the departure. ... We have gone back and made some changes to software systems."
It appears American noticed the mixup too late, however: the plane completed its flight to Hawaii instead of rerouting to LA. The plane was then immediately flown back to LA, empty, and the scheduled return flight was canceled.
It's not clear how the wrong plane ended up going down the runway at LAX, but it might have to do something with new equipment changes on the route. American only started using its A321s on the LA to Honolulu route a couple of weeks before the incident — it previously used Boeing aircraft for the flight.