Alaska Air Tests RNP-Guided Descent, Sees 35 Percent Emissions Reduction

Since last summer, Alaska Air Group has been testing out its "Greener Skies" project at Sea-Tac. This week, they made eight landing approaches in a Boeing 737-700, using Required Navigation Performance (RNP) technology to make satellite- and GPS-optimized "continuous descents."

The results, with a 737-700 weighted as if carrying passengers, were eye-opening. During descent, the 737 used 400 pounds less fuel*, and cut emissions by 35 percent. Extrapolating those results to the Alaska/Horizon fleet at Sea-Tac, that's a savings of 2.1 million gallons annually--and not releasing the 22,000 metric tons of CO2 would be like taking 4,100 cars off the road.

How does it work? As you remember from your last trip in an airplane, descents usually involve a set of turns and stair-step drops in altitude, as the plane works its way down from cruising at about 30,000 feet. You might remember how the plane almost glided for a bit, then the engines powered up strongly as the plane advanced along the "flat" part of the stair step.

With an optimized descent profile (ODP), the plane descends from cruising altitude as in the illustration up top, with engines working much less. Instead of tower crews relaying the descent instructions in segments, the plane's path for landing is computed and cleared all in one go. Here's the FAA's description of ODP:

Operationally, for an OPD, the participating aircraft is issued a "Descend via" clearance from the Air Route Traffic Control Center allowing the aircraft pilot to select the top of descent point that provides the best operating efficiency for the arrival. Following the transfer of control to the Terminal Radar Approach Control facility, aircraft receive a clearance for the instrument landing approach. Subsequently, the aircraft is instructed to contact the tower outside the final approach fix. During this procedure, the aircraft throttles are at "flight idle" from the top of descent until just prior to configuring for landing on final. The reduction in radio communication as a result of OPD affords the controllers extra time to scan the operational environment and work other traffic.

That assumes an entirely different level of guidance precision, which is what Alaska Air brings to the table. Their news release gives the background on RNP:

Alaska Airlines pioneered RNP precision flight-guidance technology during the mid-1990s to help its planes land at some of the world's most remote and geographically challenging airports in the state of Alaska. RNP provides computer-plotted landing paths by using a combination of onboard navigation technology and the global positioning system (GPS) satellite network. It improves safety and reliability in all weather, and reduces reliance on ground-based navigation aids.

While the general approaches for landing wouldn't change, the optimized descent means that planes need less ground distance from the airport to execute a landing. That's the third benefit of RNP, besides fuel savings and emissions reduction. Currently planes line up for their approach from Elliott Bay all the way up to Paine Field, which means that whole stretch hears planes overhead.

If the new approach is simply to come in over Elliott Bay for a West Side Approach to Sea-Tac, then about 750,000 people will have much less jet noise to worry about. The Port is making no promises about ODP noise abatement for those closer in, but it seems possible to me that engine noise might be reduced.

The Port of Seattle hopes that within two to three years, 80 percent of Sea-Tac air traffic will use RNP and ODP. Right now the leaders in RNP preparedness at Sea-Tac are Alaska, Horizon, with Southwest Air making substantial progress, but major carriers like American, Delta, and Continental have seen the RNP future, too. (Boeing is delivering RNP as "standard" on all its commercial jets, but older planes need to be retrofitted.)

The FAA is committed to RNP technology--the benefits are clear. It's the integration with a national air traffic control system that needs to take into account both RNP-equipped and non-RNP aircraft that will take time. (The FAA's larger air traffic control upgrade, NextGen, will complete in 2025.)

*Boeing tells me that the 737 is a great plane to test ODP efficiency with because it accepts a lower flight idle speed than, say, the 747.