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SEATAC, Wash. - Long after dark one night last week when few planes were in the air, an Alaska Airlines Boeing 737 descended toward Seattle-Tacoma International Airport, its engines at idle power. Aboard were Alaska pilots and technicians and officials from the Federal Aviation Administration's Washington, D.C., headquarters.
The occasion was the latest test of an experimental landing procedure at the airport that promises to save millions of gallons of fuel, lessen residents' exposure to jet noise, cut pollution and save airlines' precious dollars.
The new procedure, dubbed Optimized Profile Descent by the airline industry and its regulators, is a measure that uses high technology satellite-guided navigation, the power of computerized aircraft flight management computers with the oversight of the FAA to change the decades-old procedures for bringing aircraft from cruising altitude to a landing on the runway.
The technology itself is 15 years old, but it has never been applied so completely in a complicated urban airport setting with all of its air space conflicts, political considerations and airborne traffic jams.
"There's nothing in the technology that we haven't used before," said Mike Adams, an Alaska Airlines technical captain who is helping establish the system at Sea-Tac, the airline's biggest hub airport "but the devil is in all of the details."
SeaTac-based Alaska, for instance, has used satellite-based navigation systems since 1994 to help it navigate its aircraft to landings at many of its terrain- and weather-challenged Alaska airports. Those global positioning navigation systems, for instance, allow the airline to fly between surrounding peaks following the curves of the Gastineau Channel into Juneau's airport even in bad weather.
Couple that navigation with computerized flight management systems, and aircraft can calculate and execute flight paths that will deliver an aircraft from a cruise altitude of, say, 39,000 feet to the runway threshold on a smoothly descending path that requires only minimal engine thrust.
Under the traditional landing procedure in use now, once an aircraft descends to 10,000 feet, it follows procedures dictated by air traffic controllers. Those procedures typically result is a kind of stair-step-style descent to the airport in which a controller instructs the pilots their heading, speed and altitude and brings them down toward the runway in a series of altitude changes.
That traditional procedure requires aircraft to descend and then level off several times in their drop from 10,000 feet. Each time the aircraft levels off, the pilots must increase power to maintain altitude.
Under the OPD procedure, the descent is a constant drop from altitude that can be accomplished in most cases without increasing engine power above idle power until immediately before landing.
In a step-down descent, a flight approaching Sea-Tac might be asked to level off three times before descending to the runway. Each three-mile segment of level flight uses about an extra 100 pounds of aviation fuel, Adams said. That's about 14 gallons of fuel.
The airline has estimated that leveling off and speed adjustments called for in the typical landing profile might consume as much as a 55- gallon barrel of jet fuel more than a constant, idle power descent.
"You can see that when you're making thousands of landings every year just how that adds up," Adams said. "It's very, very costly."
Expect the new procedure to begin use at Sea-Tac sometime next year, Adams said.