American Airlines has admitted using an aircraft not certified to fly long distances over water on a service from Los Angeles to Hawaii last month.
The carrier says there was never any danger the A321S would not complete the flight but that it had less safety and medical equipment than an A321H, which should have been used.
An investigation into the August 31 flight is being carried out by the airline and the US regulator the Federal Aviation Administration.
American Airlines told the BBC the two aircraft have the same engines, fuel tanks and range, as well as basic safety features.
The airline said it was reviewing its safety procedures and has updated its software to ensure the correct aircraft are operating on routes in the future.