BOEING Co's 737 Max may return to service on a "phased" timetable if regulators around the world approve the grounded jet to fly at their own pace instead of closely following the lead of US officials.
"The principle schedule risk on that continues to be regulator alignment around the world," chief executive officer Dennis Muilenburg said on Wednesday after reiterating the company's estimate that the Max will be approved to return early in the fourth quarter. "A phased ungrounding among regulators around the world is a possibility."
The next few weeks will be critical for the planemaker as the flying ban on the Max approaches the six-month mark. Boeing engineers are wrapping up work on more than 500 queries from global regulators, who have delved into the jet's flight-control system beyond the software linked to two fatal crashes.
But it's unclear whether airworthiness officials around the world are prepared to move in lockstep with the Federal Aviation Administration, which has primary oversight because the Max is built in the United States.
The agency's counterpart in the European Union has recently stressed that it is carrying out an independent review that will include week-long flight tests of the Max, the newest version of the workhorse 737.
Boeing extended gains while Mr Muilenburg spoke at a Morgan Stanley conference. With some analysts predicting the Max crisis could drag into 2020, investors were relieved to hear Mr Muilenburg's assurance that Boeing is making "solid progress".
US regulators have established a certification management team for the Max, meeting regularly with counterparts from Europe, Canada and Brazil, the CEO said.
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, the planemaker has done a "second-wave evaluation" of the entire Max software and flight control systems with regulators, he said.
Even so, the European Union Aviation Safety Agency (EASA) has signalled its concerns with the jetliner along with plans to send its own pilots to the US to test redesigned systems.
Among other things, EASA is examining whether the Max's angle-of-attack sensors are sufficiently robust.
Both crashes - one in Indonesia in October, the other in Ethiopia in March - occurred after a malfunction of one of the sensors, which measure whether the plane's nose is pointed up or down relative to the oncoming air.
Mr Muilenburg acknowledged that EASA has expressed concern with Boeing's angle-of-attack architecture, which relies on two sensors while Airbus SE's A320 family uses three.
But he suggested that the matter could be addressed without modifying the Max with new hardware.
"I don't see those as divisive," Mr Muilenburg said of the EASA queries. "I just think those are questions that we need to answer as part of the process. Questions around things like angle of attack, system design, recognise that our architecture on Boeing airplanes is different than Airbus airplanes."
EASA told Bloomberg on Tuesday that having two vanes is considered "the bare minimum requirement to meet the safety objectives".
But the agency also said that the issue could be addressed through flight-crew procedures and training, design enhancements "or a combination of the two", EASA said.
Aviation executives have expressed worry that a widening split between regulators in the US and Europe will extend the grounding, potentially sowing confusion and anxiety as officials work to approve the resumption of commercial flights.
AerCap Holdings NV CEO Aengus Kelly and United Airlines Holdings Inc CEO Oscar Munoz were among executives sounding the alarm over the increasingly tenuous alliance last week. BLOOMBERG