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The Federal Aviation Administration (FAA) is expected to approve the Boeing 787 Dreamliner’s return to passenger service as early as Friday afternoon.

Barring unforeseen last-minute hitches, Boeing’s new airplane will finally be allowed back in the air — three months after the fleet was grounded worldwide in the wake of two incidents in January in which the plane’s lithium-ion batteries overheated.

A person familiar with the details said federal officials were preparing late Thursday to issue the go-ahead. That means the FAA approves of the Dreamliner with Boeing’s new safety-enhanced lithium-ion battery system installed.

Since Boeing completed a series of 20 preapproved tests of its new battery system April 5, the aviation business has been buzzing with speculation as to when the FAA would arrive at a decision.

Some in the industry believed the agency would not act before a National Transportation Safety Board (NTSB) hearing scheduled for next Tuesday and Wednesday.

That hearing arises out of an NTSB investigation of a battery fire in January aboard a Japan Airlines 787 on the ground in Boston. It will discuss how the original battery passed muster at Boeing then made it through the FAA’s original certification process.

In forthright briefings on its investigation over the past three months, the NTSB has made clear it is still seeking the root cause of the overheating and fire.

But because the fire destroyed the evidence, the root cause may ultimately prove elusive, and so Boeing designed a fix it believes will cover all bases.

Its solution, according to the person with knowledge of the details, is good enough to satisfy the FAA, which acts independently of the NTSB.

The battery fix includes, for each of the two large batteries on board, a steel containment box that can withstand an explosion equivalent to the maximum energy the battery can hold.

And from each battery a titanium tube connects to the fuselage skin and will vent any gases from an explosion safely outside the aircraft.

Various internal modifications to the battery — chiefly electrical and thermal insulation between the battery’s eight cells — are designed to stop any overheating from spreading cell to cell, minimizing any chance of a so-called “ thermal runaway.”

Technically, the FAA green light will apply only to the sole U.S. operator of the 787, United Airlines, which has taken delivery of six Dreamliners.

However, FAA approval of Boeing’s battery fix clears the way for civil-aviation authorities in other countries to follow suit and allow their carriers to fly again.

Typically, other regulators promptly follow the FAA’s lead.

Procedurally, the airplanes cannot fly until the FAA issues a new Airworthiness Directive to supersede the one that grounded the planes Jan. 16.

That likely won’t happen Friday and may take a couple of days, just because of the time involved to publish a move in the Federal Register.

However, that won’t hold anyone back in practical terms.

As soon as the FAA makes its approval public, United Airlines will be free to start modifying its aircraft and installing the new battery system on its jets.

The work will take several days per airplane.

United said earlier this month it would resume 787 passenger service May 31.

A few planes may be flying before then, but it may take until June before some airlines regain full regular service.

After the batteries are installed, flight crews will undergo some retraining, and routes that have been canceled must be reactivated and tickets sold.

Boeing has teams of mechanics ready, some already deployed across the globe, to begin the work as soon as the FAA gives the green light.

The 50 Dreamliners in the worldwide in-service fleet are scattered across the globe, in Chile, Ethiopia, India, Japan, Poland, Qatar as well as the United States.

The response of the Japanese Civil Aviation Board will be crucial, for All Nippon Airways and Japan Airlines operate 24 of the 50 grounded jets.

Boeing’s teams are poised to fan out and get to work over the weekend.

The FAA’s decision was signaled Thursday when the agency granted Boeing permission to fly routine pre-delivery check flights, without passengers.

Boeing flew 787 Dreamliner No. 83, complete with its new battery system, on such a check flight Thursday from Paine Field in Everett to Moses Lake and back.

Dominic Gates: 206-464-2963 or