Boeing Co is making “steady progress” on the path to certifying a software update to the grounded 737 MAX and even has made the final test flight before a certification flight.

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

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. The software, according to the company’s chief executive, was installed and tested by technical and engineering leaders on board the aeroplane.

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

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 aeroplanes ever to fly.”

Boeing must deliver the update to international regulators for their review, which is expected to last about 90 days. In addition to software certification, international regulators must also decide on new pilot training. 

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.

All 737 MAX aircraft were grounded worldwide in March after two planes crashed:  Indonesia’s Lion Air on 29 October 2018 and Ethiopian Airlines on 10 March, which together killed all 346 on board. Boeing also had to freeze more than 4,600 orders for the 737 aircraft.