Required Navigational Performance: Coming to an Airport Near You

Air Safety Week, Jan 30, 2006

Well satisfied now with the technique and several years of experimentation, Alaska Air [ALK] and the FAA are expanding the use of Required Navigational Performance (RNP), an onboard guidance system that helps pilots steer their approaches and departures through bad weather or around natural obstacles. The concept has been around for a few years, but the FAA is just beginning to make it available to other airlines.

In 2005 alone, Alaska Air's pilots conducted about 6,100 RNP approaches or departures, and about 850 of those were considered direct RNP "saves," says Kevin Finan, Alaska Air's executive vice president of operations. A "save" would indicate any arrival or departure that would not have happened using conventional navigational systems in instrument conditions.

Just concerning Juneau Int'l Airport (JNU), the carrier averages about two saves out of an average total of 20 daily arrivals and departures, Finan adds. It makes a "huge difference" for the local community to have air service that is increasingly reliable and consistent, as well as safe.

Back in 1996, Alaska Air became the first airline to use RNP, starting at Juneau. Since then the carrier has expanded its FAA-approved approaches into six more airports inside Alaska, as well as several more outside the state, including at Ronald Reagan Washington National (DCA). In the near future, the carrier plans to add RNP approaches to three more Alaskan airports.

Meanwhile, in late December, the FAA said it would allow additional carriers to use the RNP approach into DCA's Runway 19 along the Potomac river, a route chosen for noise abatement as well as for safety. Finan explains to Air Safety Week that all the other RNP approaches that Alaska Air has been using are special instrument approach procedures, which are specific for use by Alaska Air only. But the DCA approach is a "public approach," meaning that other carriers with the correct equipment and pilot training will be able to use it.

By Sept. 30, the end of fiscal year 2006, the FAA also says it plans to develop RNP procedures for use at an additional 13 U.S. airports, some of which will have multiple RNP approaches, an agency spokeswoman tells Air Safety Week. This will include 25 public procedures and six special instrument approach procedures.

Precursors of RNP came online in the early 1990s with such things as GPS and inertial reference systems. At the same time, there were officials associated with Alaska Air who realized that you could combine these technologies into one system, and get a defined flight path both horizontally and vertically, Finan says. The initial challenge centered on Juneau, which is surrounded by a challenging terrain and gets plenty of bad weather. The goal was to get a tailor-made, specific flight path into the airport, one that could be used over and over, while providing better operational reliability and lower weather minimums.

That is just what RNP has done. And those minimums for Juneau, for example, have come way down, Finan adds, from a cloud ceiling minimum of 1,260 ft. with 2 miles of visibility to a minimum ceiling of 337-ft. and one mile of visibility.

Conventional navigation has relied on signals sent to aircraft from a ground station, and is based as much as possible on flying a straight line. It's a "pretty constraining" technique, Finan adds.

The main approach into Juneau follows the Gastineau Channel for about 15 miles between mountainous terrain. Instead of a straight line, a pilot wants to follow a curved route over the channel, while also maintaining a constant rate of descent, of course. A pilot who can get a clear picture of the runway and the surrounding terrain in visual conditions can make a safe landing. But when that's not possible, RNP is needed.

In essence, RNP provides a charted course for the pilot to reference during approach or departure. A special data code and database is loaded into the aircraft's flight management computers. The pilot can then select either an on-screen readout of the guidance information, or hook the whole system into the autopilot.

Besides providing precise guidance for approach and departure routes, the technology also gives pilots very good situational awareness through terrain maps that include the airport and the surrounding area. Because RNP also lessens the likelihood of having to enter holding patterns in bad weather, it saves on fuel.

There also is "tremendous redundancy" built into the system at Alaska Air, Finan says. Each craft has two flight management computers, two GPS receivers, two inertial reference systems, as well as an enhanced ground proximity warning system.

But the vast majority of Alaska Air's approaches and departures do not use RNP, and only 70 percent of the carriers' fleet is equipped with the system. Most of the airports served by the carrier have instrument landing systems, which provide good minimums and vertical and horizontal guidance, Sarah Dalton, Alaska Air's director of airspace and technology, tells Air Safety Week. Moreover, while the utility of RNP is maximized at a location such as Juneau, it is not needed everywhere.