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 and 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."
ALASKA PIONEERED
SeaTac-based Alaska Airlines, 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 in a kind of stair-step-style descent to the airport in which a controller instructs the pilots on 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.
BIG FUEL SAVINGS
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."
Following OPD procedures just for approaches to Sea-Tac that fly over Puget Sound between Vashon Island and the mainland, called the West Approach, could save the airlines that fly in Sea-Tac about 175,000 gallons of fuel a month or 2.1 million gallons annually, the airline calculates. Savings would multiply as optimized descent procedures were put into use for other approaches to the field. Emission reductions are commensurate with the fuel savings with a reduction of 22,400 metric tons of emissions annually, the amount emitted by 4,100 cars.
QUIET RIDES
Beyond the fuel and pollution reduction, the new approaches would cut the exposure of thousands of Puget Sound residents to aircraft noise, said Megan Lawrence, managing director of governmental and community relations for Alaska Airlines.
By the airline's calculations, 750,000 people in the Puget Sound area would hear less aircraft noise each year.
The benefit would come from more precise timing of arrivals in the airport vicinity.
What happens now, when multiple planes arrive in the Puget Sound area to land at roughly the same time, controllers extend the queue of planes lined up to land farther northward or southward depending on what direction the wind is blowing at the time.
When the line is long and planes are landing to the south, aircraft approaching from the south or west fly above Puget Sound northward, say as far as Lynnwood, before turning southward and joining the line of planes headed toward Sea-Tac runways. The same principle applies when landings are to the north with the line for landing extending well south of the airport.
Under a well-coordinated OPD system, aircraft would be instructed to slow down at cruise altitude where their fuel efficiency is at a maximum and noise reaching the ground is at a minimum to avoid reaching the area at the same time.
Using flight management computers, pilots can plan to arrive at a specific spot or waypoint at a precise time.
Adams said typically the computer manages to fly the plane to that specific spot with an error of less than five seconds.
Using that precise timing and an optimized descent path, aircraft would less frequently divert from the shortest path to the airport.
Once the FAA gives its blessing, the optimized descent plan would be published and available to any airline or aircraft owner who is equipped and trained to use it.
Alaska's Lawrence said a dozen or so civic organizations and political figures have endorsed the OPD plan so far.
By JOHN GILLIE
The Tacoma News Tribune