(Bloomberg) — Boeing Co. led industrial stocks higher after U.S. aviation regulators said they’d approved a critical set of test flights on the 737 Max to begin as soon as Monday, having reviewed the company’s safety assessment of fixes for the plane.

The Federal Aviation Administration confirmed the start of the multiday program in an email to congressional staffers on Sunday.

“Over the past several weeks the FAA has been reviewing the system safety assessment submitted by Boeing,” the agency said in the email. “The FAA’s Type Inspection Authorization Board has completed its review, clearing the way for flight certification testing to begin.”

Boeing jumped 5.2% to $178.78 at 9:38 a.m. in New York, the biggest gain on the Dow Jones Industrial Average. The stock had tumbled 48% this year through June 26, the sharpest decline on the blue-chip stock gauge.

The agency’s action signals that the government is finally comfortable with the multiple fixes that the planemaker has devised for the plane, which has been grounded for more than 15 months after two fatal crashes.

Two people briefed on the planning said earlier Sunday that the goal is to begin the tests Monday, but the start is still subject to last-minute delays. Such tests are one of the final stages by the government before it certifies an aircraft.

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The FAA will have one of its test pilots flying the plane alongside a Boeing pilot from Boeing Field in Seattle. They will be accompanied in the cockpit by an FAA flight-test engineer and a Boeing flight-test manager. Additional specialists will be in the cabin monitoring computerized instrumentation on the plane.

“We continue to work diligently on safely returning the 737 Max to service,” Boeing said in a statement Sunday. “We defer to the FAA and global regulators on the process.”

The Seattle Times reported Wednesday that the tests were expected as soon as Monday as the more than 15-month process of making fixes on Boeing’s best-selling plane nears completion.

The FAA said in its letter that a number of steps remain before the plane can resume carrying passengers in the U.S., and by extension, elsewhere around the world.

“It is important to note, getting to this step does not mean the FAA has completed its compliance evaluation or other work associated with return to service,” the agency said. “The FAA has not made a decision on return to service.”

It will take months for the agency to complete new pilot-training standards and issue regulations governing multiple software and hardware changes to the plane. Airline customers have been told that it could come in September if all goes well, though they still have to retrain pilots and perform maintenance on the fleets of planes that have been in storage before they enter service.

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The FAA added that it will retain the authority of inspecting each new plane to ensure that it meets all federal requirements.

In the past, the agency had largely delegated the responsibility to Boeing employees. Last November the agency said its own inspectors would take over that task. Inspections of undelivered planes after the accidents had found numerous instances in which debris from manufacturing had been left in areas such as fuel tanks.

The Max was grounded by FAA on March 13, 2019, after most of the rest of the world had already sidelined the plane following the second fatal crash involving a flight-control feature. The crashes — in October 2018 off the coast of Indonesia and in March 2019 near Addis Ababa — killed a total of 346 people.

An examination of the software that was driving down the plane’s nose repeatedly as a result of a malfunction prompted the discovery of other issues that required upgrades to improve safety on the plane. Adding redundancy to its flight-control computer took months and late last year Boeing discovered that the way it had installed wiring on the jet didn’t meet federal regulations.

The crashes have also prompted a reassessment of how aircraft manufacturers and regulators around the world assume pilots will react to certain emergencies.

The certification flights are scheduled to occur over three separate days. The agency, which worked closely with Boeing during the process of revising the plane, has a list of maneuvers that it will demonstrate on the plane to verify that alterations to its system function as designed.

Certification flights rarely result in surprises. In this case, devising the revisions to the Max have been one of the most scrutinized processes in history as outside panels of experts advised FAA and regulators in other nations also undertook reviews of the plane.

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