NEW YORK (NYTIMES) - The return of Boeing's 737 Max appears to be slipping. Again.

The plane has been grounded since March after two deadly crashes that killed 346 people. It has disrupted the global aviation industry and plunged Boeing into the biggest crisis the aerospace giant has ever faced.

Yet today, after more than eight months of intensive work by Boeing and aviation regulators, the timing of the return of the 737 Max appears more precarious than ever.

While Boeing has said publicly that it expects the Federal Aviation Administration (FAA) to begin the process of ungrounding the plane this year, that now appears unlikely, according to a government official familiar with the process.

Instead, it is increasingly likely that the grounding will continue into 2020, given the series of tests Boeing must complete before the regulator clears the plane to fly.

FAA officials believe that it could take until late January for the agency to lift the grounding and approve training requirements for pilots. It would then take weeks for airlines to prepare Max jets to operate commercial routes.

Meanwhile, Congress, following a hearing last month where it grilled Boeing's chief executive Dennis Muilenburg, is planning to hold a hearing next month at which it expects FAA officials to testify about whether there are other problems with the Max that Boeing hasn't addressed.

The nagging uncertainty over the plane's future has cast a pall over Boeing, the largest manufacturing exporter in the United States and has shaved 15 per cent off the company's value since March.

"It's becoming increasingly clear that they're not going to get it sorted out this year," said Mr Scott Hamilton, managing director of the Leeham Co, an aviation consultancy. "I would be completely flabbergasted if the airplane does not get recertified, but I've been flabbergasted on more than one occasion."

Boeing said it was following the lead of the FAA and global regulators.

"They will determine when key milestones are achieved and when the fleet and training requirements are certified so the Max can safely return to service," Mr Gordon Johndroe, a company spokesman, said in a statement.

Any further delays could have profound consequences for Boeing and its customers, which have ordered some 5,000 Max jets.

Earlier this year, Boeing executives suggested that if the grounding persisted beyond this year, the company would consider shutting down the Max production line. Such a move would have enormous economic consequences, most likely resulting in sweeping job losses at Boeing and many of its suppliers.

Yet even if the Max does return to the skies early next year and Boeing avoids shutting down the production line, the grounding's effect will linger for years. Airlines have had to cancel routes and slow expansion plans. Boeing has a backlog of hundreds of planes to deliver, a process that could take more than a year.

And global aviation regulators, which have historically deferred to the FAA, are already exerting more independence. The European Union Aviation Safety Agency recently said it would take a more proactive role in evaluating Boeing's next jet, the 777X.

Lawmakers in Washington are also continuing their investigation into Boeing and the FAA.

The House transportation committee plans to hold another hearing on the regulator's approval of the Max in early December, calling the head of the FAA, Mr Stephen Dickson, and potentially other senior leaders at the regulator to testify, according to three government officials familiar with the plans.

Members of the committee plan to grill the officials on how they initially determined the plane's safety and what actions they took between the first fatal crash, off the coast of Indonesia in October 2018, and the second, in Ethiopia in March.

They will also ask about issues unrelated to the software that contributed to both accidents, including the design of rudder cables on the Max.

In July, The New York Times reported that before the Max was approved in 2017, managers at the FAA had sided with Boeing over their own safety experts, who wanted the manufacturer to make the cables more redundant to avoid a potentially catastrophic failure. Boeing argued that a failure was so unlikely that a change was unnecessary.

On Tuesday, the FAA informed Boeing that the agency would inspect every Max and determine whether each plane is ready for flight. In the past, Boeing has been able to make that determination without the FAA reviewing the jets individually.

It was the latest setback for Boeing in a halting monthslong effort to return the Max to service.

"We welcome and embrace this decision by the FAA," said Mr Johndroe of Boeing. "Safety is our No 1 priority."

The company had been pushing the regulator to speed the approval process, requesting that the agency conduct simulator tests of the plane with pilots before the FAA completed its audit of new flight control software that Boeing has developed for the Max.

Last month, Mr Dickson, the FAA chief, released a video acknowledging the effort to rush the process and instructing his employees to resist pressure to lift the grounding quickly.

Still, some inside global regulators remain skeptical that Boeing should install the software update to MCAS at all. This month, a manager at Canada's aviation regulator wrote an email to officials at the FAA, the European Union Aviation Safety Agency and Brazil's National Civil Aviation Agency arguing that Boeing should remove MCAS from the 737 Max before the plane is cleared to fly again.

The agencies did not officially endorse or rebuke the manager's views. But the fact that such a fundamental discussion was happening so late in the process adds a further sense of unpredictability.

"It is amazing that somebody at Transport Canada and the FAA says that MCAS shouldn't have been on the airplane," Mr Hamilton said.

Boeing is facing multiple challenges beyond the Max crisis as well. Its next widebody jet, the 777X, is delayed.

The 787 Dreamliner programme is plagued by production problems at its North Charleston, South Carolina, factory. And the KC-46, a refueling tanker made for the military, is well behind schedule, drawing the ire of lawmakers.

For now, most industry observers still believe the Max will be flying commercially by the spring. Yet after eight months of delays, setbacks and increasingly contentious congressional hearings, nothing is guaranteed.

"There's so many things that could happen," said Mr Richard Aboulafia, an aviation analyst at the Teal Group. "It's not in Boeing's hands."