Boeing's effort to resume flights for its grounded fleet of 737 Max airplanes now faces new hurdles, as regulators outside the United States said they will give independent scrutiny to the company's plans before allowing the planes to fly again.

Internally, meanwhile, Boeing on Tuesday appointed its head of engineering, John Hamilton, to lead the company's efforts in the twin investigations into the Lion Air and Ethiopian Airlines crashes.

While officials in the United States are hoping that the Federal Aviation Administration (FAA) will approve a software update by the end of this month, leaders in Europe said they won't allow the aircraft to fly until they have done a separate review. Likewise, officials in Canada said they'll want to conduct their own assessment of Boeing's software fix.

Those decisions demonstrate a fracturing regulatory environment for Boeing, which has historically won approvals with foreign agencies following the lead of the FAA. And it suggests that Boeing's effort to get the Max flying again may take longer, even as manufacturing continues at its plant in Renton, Wash.

Peter Goelz, a former managing director for the National Transportation Safety Board, said it's unusual for those foreign regulators to spend extra time scrutinizing something after the FAA. He said that could create problems for Boeing, as the company is looking toward the light at the end of the tunnel but may end up with approval to fly again in the United States while planes remain grounded elsewhere.

Goelz said Boeing and the FAA are both going to have to do a lot to regain their credibility over the next year.

"It's certainly something that the FAA is going to have to address and work on," Goelz said. "They are going to have to regain the confidence of other regulatory and safety organizations, to say that the gold standard has been restored."

Chao intervenes

Along with skepticism around the globe, Boeing also faces scrutiny in the United States. Transportation Secretary Elaine Chao said Tuesday she has asked the department's inspector general to conduct a formal audit of the certification process for the Max.

On Tuesday, President Donald Trump nominated former Delta Air Lines executive Steve Dickson to lead the FAA. The FAA has been led by an acting administrator, Daniel Elwell, for more than a year.

Rep. Rick Larsen, D-Wash., whose congressional district covers Boeing facilities in the Everett area, has said he expects Max planes to return to the skies by the end of April.

Canada re-examines

But on Tuesday, Air Canada announced that it does not intend to fly its 24 Max aircraft for at least a few more months. Canada's transport minister, Marc Garneau, said on Monday that the agency is re-examining the validation it gave the Max, according to the CBC. Garneau also said Transport Canada would do its own examination of Boeing's proposed software fix "even if it's certified by the FAA."

In Europe, a leader at safety agency EASA said it will look "very deeply" at the Max updates and not allow the aircraft to fly "if we have not found acceptable answers to all our questions," according to Reuters. The FAA and EASA have historically followed each other's lead, with the FAA certifying Boeing planes and EASA focusing on Airbus ones.

A Seattle Times story over the weekend, along with other news agencies, detailed how FAA managers pushed its engineers to delegate more of the certification process to Boeing itself. The Times story also detailed flaws in an original safety analysis that Boeing delivered to the FAA.

Boeing CEO Dennis Muilenburg, in a public message and accompanying video, said Boeing will soon "release a software update and related pilot training for the 737 Max that will address concerns discovered in the aftermath of the Lion Air Flight 610 accident."

The software change is focused on the Maneuvering Characteristics Augmentation System, also known as MCAS, which was added to the plane so that it would behave in a similar way to older 737 models. MCAS is designed to push the plane's nose down if a sensor determines that the plane is ascending at too steep an angle, putting it at risk of a stall.

In the wake of the Lion Air crash in October, which killed 189 people, investigators have said that a false sensor reading repeatedly triggered MCAS to push the nose of the plane down. Investigators haven't released a report from this month's crash in Ethiopia, which killed 157 people, but officials say there are "clear similarities" with the Lion Air crash.