Cover Story: RNP on Approach

By James W. Ramsey | August 1, 2007
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Considered by some as the ultimate breakthrough allowing the transition from ground-based to aircraft-based navigation, Required Navigation Performance (RNP) has enjoyed a rather slow, decade-long evolution. But recent developments in the United States show promise that this error-bounding solution to navigational performance has indeed taken hold.

Naverus, of Kent, Wash., this year became the first FAA-authorized company to provide RNP approach and departure procedures to airlines, airline operators and airports. In May, Naverus and Southwest Airlines announced they will partner on developing a tailored RNP program to be used by the entire Southwest 737 fleet.

Meanwhile, Pacific Northwest regional carrier Horizon Air, building on the decade-long experience of its affiliate Alaska Airlines, has become an industry leader in developing and proving RNP approaches into its Portland, Ore., hub, and it plans to add RNP approaches at six more airports. Horizon Air operates Bombardier CRJ-700 jets and is the largest North American operator of the Bombardier Q400 turboprop.

RNP is a type of area navigation (RNAV) that utilizes GPS with inertial reference system backup and sometimes DME (distance measuring equipment) navigation, allowing aircraft to fly predetermined paths loaded in their flight management computers. Accuracy is ensured through on-board performance monitoring, with a crew alerting system, if necessary.

Benefits from RNP are safer, more direct approaches — saving fuel and time — lower minimum approaches (increasing operational efficiency), and a reduction in airport noise and emissions. With RNP, aircraft can operate into terrain-challenged airports, fly precise ground tracks repeatedly and allow approaches to closely spaced parallel runways in inclement weather. RNP approaches also are used to avoid restricted airspace such as that of Reagan National Airport outside Washington, D.C., with its proximity to the White House. Alaska Airlines was one of the first carriers to fly the new FAA approach when the airport reopened after 9-11. The RNP procedure provides an instrument approach along the Potomac River that copies the VFR approach used in good weather.

Alaska Airlines pioneered RNP and was the first U.S. carrier to commit to equipping its entire fleet — a project that will be completed when the airline transitions to all Boeing 737s by the end of 2008. The carrier said it used more than 9,000 RNP approaches and departures in 2006, and "saved" some 1,300 flights from being diverted or canceled.

When designing RNP approaches, not only are terrain, obstructions and restricted areas factored in, but even changing winds. "Not only do we take into account airplane performance, but we ask, for that particular airport, ‘what are the rare and normal winds, and will the airplane be able to hold that course in a 100-knot crosswind?’ We make sure the airplane can stay on course even through extreme winds," said Eric Nordling, Naverus vice president of marketing.

RNP values, which designate the lateral performance requirement associated with a procedure, for most current approaches (including Portland’s RW 28R) are 0.3 nautical miles either side of the centerline. Naverus has designed a number of RNP 0.1 approaches, meaning the aircraft will stay within 0.1 nm of its intended track, but the operator must have regulatory approval to fly them.

As a cornerstone of performance-based navigation, RNP "is an important part of the FAA’s NextGen (Next Generation Air Transportation System) modernization plan," said FAA Administrator Marion Blakey. "RNP takes advantage of an airplane’s onboard navigation capability to fly a more precise flight path into an airport, and it increases airport access during marginal weather."

FAA’s roadmap for performance-based navigation is a three-phased transition. For the near-term (through 2010), the focus is on widescale RNAV implementation and the introduction of RNP for enroute, terminal and approach procedures. During the mid-term (2011-2015), there is a shift to predominately RNP operations for improving flight efficiency and airport access. The far-term (2016-2025) will concentrate on integrated RNP, Required Communications Performance (RCP), and other system enhancements.

FAA had authorized a total of 37 RNP procedures at 17 airports. In 2006, the agency published 28 Special Aircraft and Aircrew Authorization Required (SAAAR) RNP approaches at 14 airports; it planned to publish at least 25 more RNP approach procedures in FY 2007. In addition to Alaska and Horizon airlines, Delta and Continental have been approved to fly RNP approaches in the United States. Worldwide, RNP adopters include Canadian carrier WestJet, along with Air China, Air New Zealand, Qantas and Virgin Blue.

Avionics recently visited the Seattle area to talk with Naverus executives and to fly an RNP approach in Horizon Air’s Q400 simulator. With a recent order for 15 additional aircraft, Horizon Air will operate 48 of the 78-passenger Q400s.

Approach Architects

Founded by former Alaska Airlines pilots Steve Fulton and Hal Anderson, along with technology business executive Dan Gerrity, now CEO, Naverus blossomed in 2004 to become a full-service RNP provider. From its offices south of Seattle, the company helps airline customers navigate the difficult regulatory process, helping write their approval applications, and then establishes flight and dispatch procedures using FAA criteria. In fact, Naverus participated on committees that helped FAA draft Advisory Circular AC 90-101, which sets the regulatory basis for RNP.

The company designs RNP procedures — it has developed more than 350 "tailored" procedures — and "leases" them to its customers. FAA-developed RNP procedures are deemed "Public Approaches," available to a broad range of operators.

"The FAA has been very deliberate. That’s a daunting challenge that requires procedures tailored for ‘very capable aircraft and flight crews be harmonized with the public procedures," said Anderson, Naverus vice president of flight operations.

Naverus also helps regulatory agencies establish standards and assists aircraft and avionics manufacturers with equipment certification. The company believes it has developed a winning product.

"This is one of those rare opportunities in aviation where something that represents a significant enhancement in safety also is a significant economic benefit," said Anderson. "And it uses a latent capability that is already on the airplane. GPS, FMS — these are things that have been on planes for 10 to 20 years, and certainly every production Airbus and Boeing airplane has an RNP capability on it now."

Drawing on their experience, which involved the use of RNP for a network of airports in southeast Alaska in the 1990s, the Naverus team realized "there was a tremendous need for this, because what we confronted in Alaska was not uncommon for much of the rest of the world," Anderson said.

"These terrain-challenged, limited infrastructure airports, while they represent only a small percentage of an airline’s total operations, are what create the largest disruptions of the airline from a scheduling standpoint. The whole training program at an airline might be driven by one or two airports."

Naverus points to the RNP procedures it has designed for Qantas and Air New Zealand at Queenstown and Brisbane, Australia, and for Air China at Lhasa and Linzhi.

Likewise, Alaska Airlines and Horizon Air have developed RNP approaches for Sun Valley (served by the Hailey, Idaho, airport), and for Palm Springs, Calif. Alaska already flies RNP approaches into Palm Springs, and Horizon was planning to soon.

Calgary-based WestJet was Naverus’ first customer in 2004, and is currently using Naverus-designed approaches for 18 of the 22 airports the airline serves in Canada. WestJet, which operates Boeing 737NGs in areas of mountainous terrain, opted for a network solution for consistency of operation, creating an RNP procedure to every runway end in its system.

If the ground-based navaid used for an instrument approach at a runway end was out of service, the airline could still operate to the airport. Many airports, even in simple terrain environments, still have runways that don’t have instrument approaches to them, Anderson explained. For others, ILS systems are sometimes inoperative because of runway or airport construction and may be out of commission for periods of time.

Why would Southwest Airlines, which operates into fewer terrain-challenged airports than a WestJet, want RNP for its entire system? "We discovered how tailored RNP can be used for more than just terrain-challenged airports," said Mike Van de Ven, Southwest executive vice president and chief of operations. "By using it everywhere we’ll unlock the tremendous capabilities of modern avionics, maximizing our investment in new aircraft and equipment."

"Southwest is doing RNP because, among the many benefits it offers, it will save millions of dollars in fuel costs," said Nordling. "Operational reliability has an impact, as does the environment, and then there is safety — a big thing."

Turboprop RNP

Horizon Air is following the path of Alaska Airlines in pioneering the use of leading-edge avionics technology, including head-up displays (HUD), RNAV and RNP.

As the lead carrier at Portland International Airport, one of five airports identified for initial RNP implementation by a FAA/industry task force, Horizon helped develop and test RNP procedures. Since they were approved by FAA for operation in August 2006, Horizon has been flying the procedures in revenue service on its Q400s into Portland.

Horizon at this writing was developing its Palm Springs approach, which requires a software update for the aircraft’s FMS. Other candidate airports that Horizon serves are Medford and North Bend, Ore., and Butte, Missoula, and Helena, Mont.

Horizon began testing RNP in 1997 when, according to Perry Solmonson, the airline’s manager of flight operations-technical, "most of the FAA we spoke with didn’t understand what RNP was, let alone how to approve it."

Horizon was flying earlier-model Q200s, equipped with an Electronic Flight Instrument System (EFIS) that could not display a curved path, as used in RNP. Learning from experimental RNP approaches into the Hailey, Idaho, and Wanatchee, Wash., airports helped Horizon better define and equip new aircraft for RNP. With delivery of the Dash 8-400 (now called the Q400), and experience FAA had gained, the airline was able to establish a viable RNP plan.

Key to the Q400s ability to perform RNP are dual FMS with the latest software, dual GPS, a single Litton IRS, and Thales EFIS displays that can show curved path RNP approaches. Perry credits Universal Avionics Systems Corp., which provides its UNS-1E flight management system for the Q400, and Bombardier, the aircraft OEM, with helping make RNP a reality.

When Horizon received its Q400s, "we trained our flight crews in RNAV procedures, just like they were RNP, knowing that one day, when RNP came along they could fly them with no additional training," Solmonson said.

RNAV provides aircraft with better access and flexibility for point-to-point operations. FAA has authorized 155 RNAV procedures at 38 airports and planned to publish an additional 42 procedures by the end of FY 2007.

"The real difference between RNAV and RNP is the integrity and continuity of the procedure," Solmonson said. "Integrity is the certainty that the system is functioning properly and providing good information. Comparing integrity requirements of the two, RNP SAAR operations are 100 times more stringent than RNAV operations."

Continuity is the ability to continue an operation even though a failure may occur. "With RNAV there is no continuity — if a RNAV procedure is dependent upon GPS, and GPS fails, the only option is to ask for radar vectors," Solmonson said.

With RNP SAAAR, the aircraft must be able to continue navigation along the flight path. GPS is required to initiate the approach, but if it subsequently fails, a missed approach must be initiated (if the runway is not in sight) and the aircraft will continue to navigate along the RNP path using inertial navigation.

"Each aircraft must be able to navigate solely on inertial navigation to a point where it is safely above terrain, obstacles or conflicting airspace," Solmonson said. DME could be used as a substitute for inertial to maintain continuity, but potential terrain masking (the aircraft must have line-of-sight with the DME station) could be a problem.

The "standard" RNP SAAR value is 0.3 n.m. either side of the center line. However, Alaska Airlines is flying an RNP 0.1 procedure at Reagan National Airport. The only aircraft to fly RNP approaches less than 0.3 in the United States are B737s with Smiths Aerospace (now GE Aviation) flight management systems.

To date, RNP is being used for approaches, but it can work well for departures as well, Solmonson said.

However, criteria for RNP-based departures has not yet been developed. He believes these criteria will be available within five years.

Alaska Airlines and WestJet have been flying RNP approaches in Boeing 737NGs, using Smiths’ flight management systems. When Horizon gained final FAA approvals to fly Q400 RNP approaches, "it was the first time a non-737, non-Smiths FMS airframe platform had ever been approved to do RNP," Solmonson said.

WestJet, the Calgary, Alberta-based carrier that contracted with Naverus, is another RNP pioneer, having started with a development project at Kelowna, B.C. The airline provides service across Canada, and plans to have a fleet of 69 737NGs by year’s end.

"The original reason for RNP was primarily flight safety, to get away from the ‘dive and drive’ non precision approaches and develop more precision approaches, which is what RNP provides," said Darcy Granley, WestJet director of flight technical operations. But RNP also brought improved operational capability.

"It allowed us to develop approaches to runways that did not have approaches before. And we were able to design transitions to airports that save track miles and reduce fuel burn. Other things came out of it: improved on-time performance, reduced log times and greater aircraft utilization. Right now, across our route structure, we are using RNP about 35 percent of the time," Granley said.

Portland Approach

In developing the Portland RNP instrument approaches, Horizon Air took advantage of the existing VFR approach that routes aircraft along the Columbia River, thus alleviating noise problems for Portland residents. This helped elicit support and cooperation of the Port of Portland Commission, which operates the airport.

Flying Horizon’s RNP approach to Portland’s Runway 28R from the Q400 simulator in Seattle involved coming in from the north on a heading of 181 degrees.

The pilots, with the aircraft descending, then fly curved legs, making an arc first to the left, then to the right, with the autopilot coupled, reaching a certain select altitude at each waypoint. Every one of those altitudes was precise when the aircraft arrived there. The power adjustment was very slight on the part of the pilot to maintain the required airspeed. The Q400 rolled out on a 279-degree heading lined up precisely with Runway 28R.

"One of the things you will notice with RNP approach procedures is not a lot of heavy maneuvering. The banked turns tend to be very gentle, typically 11 to 13 degrees of bank. It gets rid of more aggressive maneuvering that is required with traditional ground-based navaid approaches," said Horizon pilot Steve Bush.

In the back of the simulator, Perry Solmonson, Horizon Air’s manager of flight operations-technical, charted the approach, and the track line was nearly identical to preceding approaches. As of mid-May, some 160 of these RNP procedures had been flown into Runways 28R and 28L at Portland in revenue service. Horizon uses the same type of Zip disc — used to load updated nav data into the FMS every 28 days — to record flight path information from the FMS as it flies the RNP approaches.

"We can look at the GPS performance, at the aircraft lateral and vertical tracking performance — actually generate a data base to show how well the aircraft behaves in reference to the defined path," Solmonson said. "As we get more data, the intent is to get credit with the FAA so we can reduce the separation, meaning lower (landing) minimums over time." — James Ramsey

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