Boeing acknowledged Saturday it had to correct flaws in its 737 MAX flight simulator software used to train pilots, after two deadly crashes involving the aircraft that killed 346 people.

"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," it said in a statement.

The company did not indicate when it first became aware of the problem and whether it informed regulators.

Its statement marked the first time Boeing acknowledged there was a design flaw in software linked to the 737 MAX, whose Maneuvering Characteristics Augmentation System (MCAS) anti-stall software has been blamed in large part for the Ethiopian Airlines tragedy.

According to Boeing, the flight simulator software was incapable of reproducing certain flight conditions similar to those at the time of the Ethiopian Airlines crash in March or the Lion Air crash in October.

The company said the latest "changes will improve the simulation of force loads on the manual trim wheel," a rarely used manual wheel to control the plane's angle. 

The planes have been grounded around the world, awaiting approval from US and international regulators before they can return to service.

Only Air Canada has a MAX simulator, industry sources told AFP.

Currently, there is only one flight simulator specific to the 737 MAX in the US, and it is owned by Boeing, according to Federal Aviation Administration documentation. 

US airlines train their pilots flying the MAX on a simulator built for the 737 Next Generation (NG), the version preceding the 737 MAX in the 737 aircraft family.

Southwest Airlines, a major 737 MAX customer with 34 of the aircraft in its fleet, said that's because during the certification process for the MAX, Boeing stressed that there were only minor differences with the NG and simple computer and online training could accommodate for the differences.

The FAA, the European Union Aviation Safety Agency and Canadian regulators had approved those recommendations, Boeing stresses.

However, the 737 NG lacks an MCAS, specially designed for the MAX in order to correct an aerodynamic anomaly due to its heavier motor and to prevent the plane from stalling.