(CNN) — No one knows when the Boeing 737 Max will return to service. But it won’t be any time soon.

Boeing eagerly awaits the US Federal Aviation Administration’s approval to allow the grounded plane to fly again. But the FAA refuses to provide a specific timeframe, saying only that it will make a decision sometime this year.

“We continue to work with other international aviation safety regulators to review the proposed changes to the aircraft,” said the FAA’s most recent statement on the plane, issued on January 9. “Our first priority is safety, and we have set no timeframe for when the work will be completed.”

The plane has been grounded since March because of two fatal crashes that killed 346 people. Boeing had initially hoped a fix for the safety system, known as MCAS, could be approved in a matter of weeks after the second crash. But it has missed target date after target date for completion of the process.

Aerospace analysts believe regulatory approval could come in the spring. Cai von Rumohr of Cowen now forecasts March. Richard Aboulafia of the Teal Group says he’s thinking April or perhaps May. But they both say that is only their best guess at this point.

Embarrassing revelations, intense scrutiny

“It’s no longer about MCAS. It’s about things that might be discovered,” said Aboulafia. “This is going to be the most scrutinized certification in history.”

Recently, Boeing was embarrassed after it released internal communications showing that some of its own employees questioned the 737 Max’s design and safety during the original certification process. One employee wrote the plane was “designed by clowns, who in turn are supervised by monkeys.” Two other employees agreed they wouldn’t let their family members fly on the plane.

When Boeing made the documents public, the FAA said, “While the tone and content of some of the language contained in the documents is disappointing, the FAA remains focused on following a thorough process for returning the Boeing 737 Max to passenger service.”

But the process to getting final approval has become an excruciatingly slow one, with every minor glitch getting far more attention than it would during a normal certification process.

Recent questions about wiring inside the plane, which wasn’t a known issue until recently and is not believed to have played any role in the crashes, shows that the process is going at a very slow pace, Aboulafia noted. During a recent technical review of the 737 Max, Boeing observed an issue with the plane’s flight computers that affected the booting up of the computer when it was on the ground, a source familiar with the matter told CNN on Friday.

“We are making necessary updates and working with the FAA on submission of this change, and keeping our customers and suppliers informed,” said Boeing in response to the computer issue. “Our highest priority is ensuring the 737 Max is safe and meets all regulatory requirements before it returns to service.”

The questions about the wiring and the computer show the risk that more issues like this could come up as investigators comb through every nook and cranny of the plane. And Boeing will have to resolve each one before approval.

Approval to fly won’t immediately put passengers on the plane

It’s also unclear whether aviation regulators around the world will follow the FAA’s lead and approve the plane to fly when it is cleared to fly in the United States. More than 80% of the planes are in foreign countries. One of the reasons the FAA has emphasized its work with other regulators is the agency hopes they will quickly follow its lead. But the process of getting that consensus is also causing delays, according to industry experts.

After the 737 Max is approved to return to service, it will be at least weeks, if not months, before any of them are carrying paying passengers once again.

Boeing and the FAA have agreed that as part of the return to service 737 Max pilots will first complete simulator training. Pilots were previously permitted to fly the 737 Max without learning the specifics of the aircraft on a simulator. The new trainings won’t start until the certification process is complete. That will slow the time it will take to have the teams of pilots necessary to handle the flights.

The airlines that own the planes have canceled thousands of flights because of the grounding, trimming their capacity to carry passengers. But they probably won’t rush to get the plane back in service. Because of all the bad publicity about the plane, airlines probably will be cautious about re-introducing the plane into service.

“Is anyone going to want to put the Max into their schedule and be the Lone Ranger flying the plane? I don’t think so,” said von Rumohr. “I think they’re going to want to wait until everyone is ready to fly the plane.”

What no one really knows how passengers will react to its return. All the airlines say they will be as flexible as possible handling passenger concerns if they don’t want to fly on a Max after they already bought a ticket.

“If all the regulators say its good, the airlines and pilots all say it’s good, I think passengers will come. But initially it could be an issue,” said von Rumohr. That could also cause airlines to move more slowly reintroducing the planes, he said.

Another reason airlines won’t need to rush to get the plane back in service is once they miss the spring break spring travel rush, they won’t need the additional capacity until the summer travel season begins after Memorial Day. Southwest and American Airlines have both announced they have canceled flights until early June, at the start of the summer travel season.

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