WASHINGTON/CHICAGO (Reuters) - Boeing Co is making “steady progress” on the path to certifying a software update to the grounded 737 MAX and has made the final test flight before a certification flight, its chief executive said on Wednesday.

FILE PHOTO: Boeing Chairman, President and CEO Dennis Muilenburg speaks during a delivery celebration of the Boeing KC-46 Pegasus aerial refueling tanker to the U.S. Air Force in Everett, Washington, U.S., January 24, 2019. REUTERS/Lindsey Wasson/File Photo

Boeing’s newest 737 model, the MAX, was grounded worldwide in March following two fatal crashes, one on Lion Air in Indonesia on Oct. 29 and another on Ethiopian Airlines on March 10, which together killed all 346 on board.

Battling its biggest crisis in years, the world’s largest aircraft manufacturer is under pressure to convince global regulators that the aircraft can be safely recertified to fly again.

“We are making steady progress toward certification,” Boeing CEO Dennis Muilenburg said in a video on his Twitter account.

Standing in front of a 737 MAX plane at Boeing Field in Seattle, Muilenburg said the company had completed on Tuesday the official engineering flight test with the updated software with technical and engineering leaders on board the airplane.

“That was the final test flight prior to the certification flight,” he said.

Boeing is reprogramming 737 MAX software to prevent erroneous data from triggering an anti-stall system that came under scrutiny following the two deadly nose-down crashes.

Muilenburg said he had seen first-hand the new software in its final form operating in a range of flight conditions, adding that his team was committed to making the 737 MAX “one of the safest airplanes ever to fly.”

Boeing must deliver the fix to international regulators for their review, which is expected to last about 90 days.

Deliveries of its fastest-selling aircraft remain frozen and airlines have had to cancel thousands of flights heading into the busy Northern Hemisphere summer travel season.

Boeing has orders for about 4,600 of the fuel-efficient, single-aisle MAX models.

Of some 50 MAX customers and operators, Muilenburg said more than 85 percent had experienced the new software in simulator sessions and that 120 737 MAX test flights had been held with more than 230 hours of air time with the new software.

In addition to software certification, international regulators must also decide on new pilot training. Canadian Transport Minister Marc Garneau told Reuters on Wednesday that he thought pilots should receive simulator training rather than computer courses.

Boeing shares, which have lost about 11 percent of their value since the March Ethiopian crash, ended 1.1 percent lower at $377.52 on Wednesday.