Four passenger flights into Seattle-Tacoma International Airport on Monday evening will launch a Federal Aviation Administration (FAA) test project that promises to leave Seattle's skies quieter and greener.

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Shortly after 11 p.m. Monday evening, Alaska Airlines flight 432 from Los Angeles will be the first passenger flight into Seattle-Tacoma International Airport guided down to the runway by signals from a satellite.

Three other flights will follow before midnight, marking the start of a Federal Aviation Administration (FAA) test project that could lead to routine use next year of a navigation technology that promises to leave Seattle’s skies quieter and greener.

Flying along invisible but precise pathways, jets in the trial will smoothly descend with engines at idle until near the runway, saving fuel and reducing emissions.

The FAA’s co-lead on the project, Doug Marek, compares it to sliding down a banister instead of taking the stairs. Planes today are guided in by air traffic controllers in a step-by-step descent that takes more time, and more fuel.

Another change: Jets arriving from the south, if they need to bypass the airport and turn for a landing toward the south, will turn over Elliott Bay instead of over North Seattle residential areas.

The so-called “Greener Skies Over Seattle” flight trials will continue for up to six months, involving some of the passenger flights flown by Alaska Air, Horizon Air, US Airways and SkyWest.

The aircraft-navigation technology was pioneered by Alaska Airlines, which in 2009 used the system on late-night test flights into Sea-Tac — without paying passengers.

The FAA project is part of a massively complicated and expensive revamp of the U.S. air traffic control system — known nationally as NextGen — that must be implemented airport by airport.

The new technologies and procedures have to work alongside the current ground-based aircraft-navigation system. And they must be introduced without a pause, much less a disruption, to the immense flow of daily air traffic in U.S. skies.

JetBlue Chief Executive Dave Barger, in Seattle last month to chair a NextGen federal-advisory-committee meeting, said Seattle will benefit as one of the “first movers” to implement the systems.

Barger praised “the collaboration between the FAA, Boeing, Alaska, the Port of Seattle, working it over the last four years.” JetBlue isn’t participating in the trial flights, but Barger said he’ll be asking how quickly it can join the project.

Elements of NextGen have been implemented separately and piecemeal at various airports around the country.

Phoenix, for example, has the smooth-glide descents in place. Alaska Airlines pilots already fly satellite-guided tracks into smaller airports surrounded by difficult terrain in Alaska.

But because Seattle’s air space is complex — and yet not saturated as in, say, New York — it is a perfect proving ground to introduce NextGen procedures on a bigger scale.

David Suomi, the FAA’s local deputy regional administrator, said that if airlines here save fuel — and money — with the system, competing carriers will be eager to follow at airports around the country.

“Seattle is uniquely qualified to help bootstrap NextGen going forward,” Suomi said.

“A clear, decisive, successful application of NextGen anywhere is going to incentivize airports and airlines to move forward.”

After Alaska’s successful test flights three years ago, the FAA took over the project in 2010 with a commitment to spend $1.5 million to $2 million to implement it.

So far, the project has devised new satellite-guided procedures only for planes arriving from the western U.S. or western Canada, or from across the Pacific Ocean.

More than 260 air traffic controllers at three regional facilities have been trained.

The FAA’s Marek said the purpose of the trial flights is to refine procedures to ensure air traffic controllers can integrate the satellite-guided arrivals seamlessly into the flow of other plane traffic.

For the first week, only late-night arrivals will be involved, with little other air traffic around. In the second week, the trials will begin on the day shift.

With the current system, a controller instructs a pilot to descend in several steps, like going down a giant staircase — say from 12,000 feet to 10,000, then to 6,000. Each time the plane reaches its assigned altitude, the engines power up to hold at that height until the next descent instruction.

With the new system, the step-down disappears.

At a distance of about 40 miles from Sea-Tac, a controller tells the pilot what arrival route he should take. Once the pilot enters that into the flight computer, the autopilot will do the rest, following a continuous and smooth trajectory to the runway.

Because a plane always lands into the wind, jets make their final approach into Sea-Tac sometimes from the north, sometimes from the south.

Planes will save either 14 or 26 miles of flying, depending on their approach route, said Alaska Airlines spokesman Paul McElroy.

He said the turn over Elliott Bay rather than farther north should reduce overflights and jet noise for 750,000 Seattle residents in the northern neighborhoods.

The shorter routes and smooth, optimized descents together should reduce noise, fuel burn and carbon emissions.

Federal law requires completion of a formal environmental assessment of the impact of the changed flight paths on people underneath. That will be done after the flight trials are completed.

Marek said once that’s successfully done, the FAA will proclaim the new arrival routes and approaches usable by any airplane with the latest avionics equipment coming into Sea-Tac from the south, west or north.

He said 80 percent of the jets flying into Sea-Tac are equipped to use the new routes, which he expects to open up in the first half of next year.

Dominic Gates: 206-464-2963 or