American Airlines, one of three U.S. carriers that fly the 737 MAX, is canceling almost 10,000 more flights while Boeing fixes software on the jet grounded by the Federal Aviation Administration after two deadly overseas crashes.

The latest round of cancellations from the Fort Worth, Texas-based company brings the total number of scrapped flights to 115 a day through Aug. 19. The 737 MAX, the best-selling model in Boeing's history, has been sidelined since mid-March. Southwest Airlines, which also flies 737 MAX jets, dropped flights involving the planes through Aug. 5.

The extended cancellations suggest the grounding of the aircraft, whose anti-stall system Boeing is modifying after it was faulted in the fatal crashes of Lion Air and Ethiopian Airlines flights, may last longer than initially expected.

American Airlines CEO Doug Parker and President Robert Isom notified employees of the new cancellations in a letter Sunday, in which they said they are “highly confident” the 737 MAX will be recertified for commercial use before mid-August.

“By extending our cancellations through the summer, we can plan more reliably for the peak travel season and provide confidence to our customers and team members when it comes to their travel plans,” they said. “Once the MAX is recertified, we anticipate bringing our MAX aircraft back on line as spares to supplement our operation as needed during the summer.”

Parker and Isom said they have been in “continuous contact” with federal regulators, including the FAA and National Transportation Safety Board, and are “pleased with the progress so far.”

American said last week it would be cutting 90 flights daily through June 5. Though the additional cancellations will curb revenue during the lucrative spring break period as well as travel-heavy summer months, the effect would have been much worse with a more widely-flown aircraft. Boeing only began delivering the 737 MAX in 2017, and just 67 of the jets were operating in the United States.

The FAA joined its international counterparts in parking the jetliner in mid-March after more than 300 people were killed in the two crashes, which occurred five months apart. Boeing has since been under scrutiny from Congress, as well as federal regulatory agencies.

The Chicago-based planemaker acknowledged that both crashes involved an issue with the 737 MAX’s anti-stall software known as the Maneuvering Characteristics Augmentation System, and promised the aircraft would be among the safest in operation when flights resume. In the meantime, Boeing said it would cut production of the 737 MAX airliners to 42 a month from 52.