In an unprecedented commitment to Required Navigation Performance, Southwest Airlines will install RNP avionics and software on all 520 of its aircraft and use the procedures at the 63 airports it serves to cut fuel burn, emissions and noise.

RNP is a form of area navigation (RNAV) that contains an aircraft in a tight corridor of airspace, most often with GPS guidance.The aircraft's flight management computer has to be able to monitor progress and alert pilots if containment is no longer possible.

Alaska Airlines pioneered RNP procedures for the state's airports in difficult terrain in the 1990s, but Southwest's decision to equip all of its Boeing 737s will usher in a new era in the rest of the U.S., where RNP use is sparse so far. Delta Air Lines also is equipping 70% of its fleet (about 400 aircraft), and American Airlines and Continental are equipping some of theirs; but Southwest is the only carrier that has decided to engage a third party--Naverus Inc.--to develop 500-1,000 customized RNP procedures for its pilots to use.

All of the airlines getting set for RNP will also be using "public" RNP approaches and departures developed by the FAA. RNP uses GPS navigation to guide aircraft precisely to and from runways. It helps them stay well clear of obstacles, residential areas where noise is a nuisance, terrain, restricted airspace and any other area to be avoided. RNP can be used instead of VOR/DME and nondirectional beacon (NDB) approaches, and it provides vertical guidance that can be matched to an aircraft's performace capability. (Southwest will continue to rely on ILS for precision approaches.)

Other benefits include improved engine-out safety by aiding in obstacle avoidance, allowing Southwest aircraft to carry more weight when departing over mountainous terrain. Southwest flights will also have a better chance of being able to land at an airport in low visibility when ground-based navaids don't provide low enough minimums, according to Jeff Martin, senior director of flight operations. RNP enable Alaska Airlines to avoid nearly 1,000 diversions a year, for a huge savings.

Following pilot training, FAA certification, procedures development by Naverus and other tasks, the first RNP approach should be flown in late 2008, he says. Schedules for RNP rollout have not been defined beyond that, but the project could take five years, he adds.

All of the RNP approaches to runways customized for Southwest 737s will make use of continuous descent arrival (CDA) techniques as much as possible. Thus, they will be designed to bring Southwest 737s closer to the runway before beginning descent. By staying at or near idle power during descent, a significant amount of fuel will be saved while cutting emissions and noise. And RNP approaches and departures can take more direct routings.

Naverus, which was formed in 2003 by Alaska technical pilots, has developed hundreds of customized RNP approaches for WestJet, Qantas, Air New Zealand, Air China and other carriers. The advantage of using customized approaches, according to Martin, is that they can be matched to the performance capability of Southwest's 737s to achieve the best possible results. In the past, Naverus said it can cost hundreds of thousands of dollars to develop RNP procedures for one airport.

At a recent press conference with Delta Air Lines to announce progress on performance-based navigation, FAA Administrator Marion Blakey signaled that Southwest was planning to equip all of its aircraft for RNP. Delta is experimenting with CDA approaches at Atlanta and is saving $36 million a year in fuel with basic RNAV departures, another form of performance-based navigation.

Southwest will ready its 737-700 fleet first. The aim is to set the aircraft up to fly RNP 0.11 and 0.3 level-of-precision procedures. The 0.11 rating means aircraft will stay 0.11 naut. mi. left or right of centerline 95% of the time and 0.22 naut. mi. left or right 99.999% of the time. The 0.3 rating is 0.3 naut. mi. left or right 95% of the time and 0.6 naut. mi. 99.999% of the time.

About 58% of Southwest's fleet will be 737-700s by year-end after 39 more are delivered. The rest are classic -300 and -500 models, and these will be retrofitted later for RNP operations.

By using RNP everywhere--not just at the few airports in mountainous areas that Southwest serves--the airline aims to "unlock the tremendous capabilities of modern avionics," says Mike Van de Ven, chief of operations. The pilots are already using head-up guidance systems which will be employed along with head-down displays to help pilots visualize the RNP procedures.

Naverus has been talking with Southwest about this project for a year. When it's completed, the carrier will be the biggest single use of RNP worldwide, notes Eric Nordling, vice president of marketing for Naverus. (Southwest is the biggest single 737 operator, with about five times more of these aircraft than RNP pioneer Alaska Airlines.)

"The challenge ahead will be the production and approval rates of procedures, aircraft and flight crews," says aviation consultant Michael Harrison, a former senior FAA official. "On the procedural side, private contractors will be able to design and produce the approaches and departures and then seek FAA approval, instead of waiting in a long queue for the FAA to develop and publish the procedure. This FAA decision to allow third-party development of arrivals and departures has stimulated the market," he notes.

The advent of RNP will require airlines to work closely with the FAA to gain the required approvals, and with the agency's Air Traffic Organization that provides ATC services. RNP will also affect airspace design as the NextGen ATC system comes into being, according to Tim Tuttle, operational concepts lead for Boeing Advanced Air Traffic Management. Boeing is a strong backer of RNP.