Boeing has reportedly admitted for the first time that there was a flaw in its 737 MAX flight simulators.

“Boeing has made corrections to the 737 MAX simulator software and has provided additional information to device operators to ensure that the simulator experience is representative across different flight conditions,” the manufacturer told the AFP news agency in a statement.

Boeing acknowledged that the flight simulators were incapable of reproducing the kind of flight conditions that occurred at the time of the Ethiopian Airlines crash in March or the Lion Air crash in October.

The planes have been grounded worldwide since the two crashes, which killed 346 people. Both accidents were blamed on a defect in the anti-stall system.

Boeing did not say when it became aware of the simulator problem.

Boeing had already acknowledged earlier this month it knew a safety alert on 737 MAX planes was not working correctly.