American Airlines has extended the grounding of its entire fleet of Boeing 737 Max jets until September 3, the company said on Sunday. The planes were previously banned from serving passengers through mid-August.

The airline “remains confident” that the jets will be cleared for flight once Boeing rolls out software updates and “new training elements” for the pilots and crew, which are currently in development.

Boeing’s hot-selling 737 Max’s were grounded worldwide after two of the packed planes of the series crashed in similar fashion in Indonesia and Ethiopia, killing everyone on board.