American airlines (AA) has extended the grounding of its Boeing 737MAX fleet until September 3, the airline said in a statement on Wednesday.

The airline will cancel about 115 flights a day through Sept. 3 as a result of the delay, according to the statement. The company announced in April that it had grounded 24 of its existing planes until Aug. 19.

Boeing's 737MAX was grounded worldwide after two crashes in less than five months killed 346 people.

Other U.S. airlines, such as united airlines and southwest airlines, have recently added planes to replace Boeing's 737MAX to cope with demand during the summer holidays, according to CNBC.

According to Reuters, Boeing has yet to complete certification tests or formally submit a report to the FEDERAL Aviation Administration (FAA) on software upgrades and training materials.

Boeing also issued a statement on Thursday, saying it will continue to work with regulators around the world to provide information needed for software upgrades and training. The company is also working with airlines to offer its customers software upgrades once they are approved.

Boeing said it wanted to fix the plane's software to prevent faulty data from triggering the anti-stall system (MCAS) to automatically operate. According to the preliminary investigation, the MCAS systems of both planes were activated.

The FAA declined to comment Tuesday. In may, the agency's acting director, Daniel elwell, noted that it was unclear when the 737MAX would be lifted. Boeing CEO Dennis Muilenburg said last week that the plane is expected to return to service by the end of the year.