A flight-navigation system using the Global Positioning System and inertial reference in airplanes is starting to catch the attention of U.S. airlines, which are beginning to see it as a means to save money. The system, known as RNP for "required navigational performance," permits planes to make the most efficient approaches to airports even in weather that would cause flights guided by more traditional instrument systems to be diverted.

The plane follows flight path instructions programmed into its flight computer. Instruments onboard the aircraft monitor the plane's position in the air by inertial reference and by keeping in touch with global positioning satellites. Altitude is monitored by barometric pressure. The system keeps the pilot informed of any deviation from the flight path, and of the certainty of a calculation of location.

Taking a new course: Southwest Airlines plans to work with a consulting company, Naverus, to implement required navigational performance, a highly accurate flight-guidance system, throughout its fleet.

RNP was developed in the 1990s by pilots at Alaska Airlines to increase the number of flights they could make into some challenging airports. Landing or leaving Juneau, for instance, requires flight over rugged, high terrain. The accuracy of the Global Positioning System as a guide to aircraft made it possible to negotiate tight flight paths through mountains in weather that would have prohibited flights using older means of guidance.

The system was the subject of a feature in the July 2002 issue of Mechanical Engineering. Two of the authors of that article, Hal Andersen and Steve Fulton, were Alaska Airlines pilots who developed the RNP system and have since become founders of a company called Naverus Inc., which provides a variety of services connected with the technology.

RNP is a form of what the U.S. Federal Aviation Administration calls performance-based navigation. Another form is area navigation, known as RNAV. It uses various navigational aids, including GPS and must be externally monitored. Naverus argues that RNAV is less precise than RNP.

According to the FAA administrator, Marion C. Blakey, "RNAV procedures create more lanes in limited airspace. Pilots can reroute around weather. RNAV helps aircraft avoid longer routes." Earlier this year, at a press conference in May, she reported that RNAV "is saving about 8.5 million dollars per year annually" at the Dallas/Fort Worth International Airport.

Basil Barimo, vice president for operations and safety at the airlines' trade group, the Air Transport Association, described RNAV as a "3-D flight path through space." Before departure, the pilot enters way points including longitude, latitude, and altitude into the flight control computer. The computer can guide the plane.

According to Blakey, RNAV departures have permitted "between 11 and 20 additional operations per hour" at Dallas/Fort Worth.

"RNP is making the same kind of headway," she said. "RNP increases airport access during marginal weather. Alaska Airlines reported more than 980 'diversion saves' in 2006 due to RNP. That's when an airplane normally would have to be diverted because of weather."

The FAA expects to have more than 70 RNP approaches in place at several airports by the end of this year. It will have almost 200 RNAV procedures in place.

Before an airline can fly using RNP guidance, its crews and aircraft must receive FAA approval. Alaska Airlines and its subsidiary, Horizon, are flying RNP procedures in the United States. Delta and Continental have been approved, but their use of RNP has been limited. According to the FAA, other airlines have applied.

One of the services that Naverus sells is RNP crew training. The company, based in Kent, Wash., also says that all Required Navigational Performance procedures are not alike.

Eric Nordling, the company's vice president of marketing, points out that the RNP procedures published by the FAA must take a general approach. That is, they are public procedures designed to be flown by any airplane. Naverus offers a service in which it tailors RNP procedures to the specific aircraft that a carrier may be flying.

Tailored procedures take into account such factors as glide and climb profiles of planes. The shortest possible path takes advantage of the plane's characteristics and also of how an operator flies, Nordling said.

Southwest Airlines has signed a contract with Naverus to develop tailored RNP procedures. According to a spokesman at Southwest, development of policies, procedures, and training materials will occupy the next eight to 12 months.

"After that, we expect the FAA application process to take a few months," he said. "With approval in hand, all of our pilots will go through RNP training, which can take anywhere from 18 to 24 months."

RNP was developed by fliers from state-of-the-art technology to extend the abilities of an airline to serve customers in rugged terrain. The Gastineau Channel approach to Juneau, for instance, has been cited as an example of the challenges facing commercial pilots in Alaska.

The approach is a tight path between high mountains. The accuracy of the GPS and inertial reference guidance gets aircraft through, even in weather that would prohibit the approach under older flight rules.

Naverus has tailored RNP procedures for carriers outside the U.S., in New Zealand and Canada, for instance. Work that it carried out for Air China effectively has opened an airport in Tibet to commercial aviation. The Linzhi Airport was essentially inaccessible to large commercial planes because of its location at about 10,000 feet above sea level in a winding river valley of the Himalaya range.

According to Nordling, Naverus worked out a total of about 800 nautical miles of approach and departure sequences for a Boeing 757 to follow at Linzhi. The longest procedure stretched for about 95 miles. According to Nordling, the inaugural RNP flight into the airport marked the first time a commercial airliner touched the runway there.

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