American Airlines has extended the grounding of its Boeing 737 Max planes to September 3.
It is a further sign Federal Aviation Authority officials are still some way from recertifying the aircraft.
American had taken the plane out of its schedules until August 19 and this latest extension will lead to about 115 flight cancellations per day during the busy summer travel period, the airline said.
The 737 Max has been grounded worldwide since mid-March after two fatal crashes claimed more than 300 lives.
"By extending the cancellations, our customers and team members can more reliably plan their upcoming travel on American," a statement said.
Other 737 Max operators United and Southwest Airlines have taken the planes out of their schedules until August and will likely follow American and extend that further.
The FAA still has not indicated when the planes will be cleared to fly again although Boeing CEO Dennis Muilenburg expects them to be back by the end of the year.
