American Airlines is extending the cancellation of its Boeing 737 Max 8 flights from June until mid-August.
The move is to “provide confidence” to customers during the peak summer travel season, the airline said in a letter.
Last month, 737 Max aircraft were grounded worldwide following fatal crashes involving Ethiopian Airlines and Indonesia’s Lion Air.
Boeing is developing new software for the jet’s anti-stall system, which has come under scrutiny since the crashes.
Senior officials at American Airlines said they were “confident” that the Boeing upgrade would be approved by US aviation regulators before 19 August, despite its decision to ground its fleet of 24 planes until then.