Boeing's grounded 737 Max may return to the skies on a "phased" schedule if global aviation regulators approve the jet for commercial flights on their own timetable rather than follow US guidelines, amid a rift among government bodies, the manufacturer's chief executive said. Boeing believes the 737 Max is expected to return to service during the fourth quarter of 2019, Boeing's Dennis Muilenburg said at a Morgan Stanley investor conference in Laguna Beach, California late Wednesday. "The principle schedule risk on that continues to be regulator alignment around the world,” he said, according to Bloomberg. “A phased un-grounding amongst regulators around the world is a possibility.” The troubled aircraft has been grounded since mid-March after two fatal crashes within the span of five months that collectively killed 346 people. The next few months are crucial for Boeing, facing the worst crisis in its 103-year history, as it approaches the six-month mark since its planes were banned from commercial flight globally. A growing divide between the European Union Aviation Safety Agency (EASA) and the US Federal Aviation Administration may extend the grounding. “We’re making good, solid progress on a return to service," Mr Muilenburg assured his audience. Boeing shares extended their gains during the executive's speech, advancing as much as 3.4 per cent in New York, after the assurance brought some relief to investors. Barclays analyst David Strauss predicted the Max is unlikely to return to service until earl 2020 as regulators in the US and Europe remain divided and Boeing has yet to submit its finalised software fix planned for this month. Alexandre de Juniac, who heads global airline trade group International Air Transport Association, earlier said he was “worried and disappointed” by the lack of unity among regulators. Mr Muilenburg said the US has set up a certification management team for the Max, meeting regularly with counterparts from Europe, Canada and Brazil. Boeing has completed 600 flights testing its redesign of the system, known as MCAS, that has been linked to the two accidents. Since mid-year, Boeing has done a “second-wave evaluation” of the entire Max software and flight control systems with regulators, the CEO said, according to Bloomberg. EASA earlier expressed its worries with the Max along and plans to send its own pilots to the US to test redesigned systems. EASA is also examining whether the Max’s angle-of-attack sensors, which measure the angle of the plane compared with oncoming air, are sufficiently robust. Mr Muilenburg played down the possibility that this could mean potentially expensive hardware changes to the airplane in addition to Boeing's planned software upgrade, the Seattle Times newspaper reported. Referring to the fact that the Airbus A320 - the main rival to the 737 - has three Angle of Attack sensors, he said that “our architecture on Boeing airplanes is different than Airbus airplanes,” and added “that doesn’t necessarily mean hardware changes”. Addressing other topics, Mr Muilenburg also touched on the impact of the ongoing US-China trade war on the world's biggest planemaker. The escalating spat between the two world powers is creating uncertainty into Boeing’s wide-body production and deliveries, particularly the 787 Dreamliner and the larger 777 that are popular with Chinese airlines, Mr Muilenburg said. "We’re still hopeful that a trade deal will be accomplished and that airplanes will be part of that,” he said. “But a lack of a trade deal does add risk to our wide-body skyline. So we’re paying close attention.”