Business & GA, Commercial

Horizon’s Push for RNP

By Charlotte Adams | May 1, 2002
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Horizon Air, a regional carrier in the northwestern United States, flies in a challenging environment and, therefore, has special needs. It flies into airports in mountainous terrain, and it seeks lower approach minima for these destinations. Some of Horizon’s hubs have closely spaced, parallel runways that, it hopes, will be able to support simultaneous instrument landings.

These circumstances have spurred Horizon to seek approval to fly required navigation performance (RNP) procedures–within a narrow, linear containment area–in the terminal and approach environment. Many observers consider these sorts of procedures to be RNP/RNAV, but technically they are not, since public RNP/RNAV standards do not yet exist. Horizon feels RNP will provide lower approach minima and greater airspace efficiency, in both difficult terrain and high-density airports, than is possible with microwave landing systems (MLS) or the Wide Area Augmentation System (WAAS). Horizon is equipping its new fleet of Bombardier CRJ 700s with a combination of flight management systems (FMS), GPS and inertial reference systems (IRS) to obtain its goal of a precision-like approach at every airport.

Raging Debate

Although numerous carriers want RNP/RNAV, WAAS backers say that the WAAS program (February 2002) can far outperform what RNP/RNAV will offer. A many-layered debate between the two sides is emerging, including both technical and political issues. People are asking what the new WAAS APV 1.5 (approach operations with vertical guidance) service level can provide, how RNP operators and prospective operators can use the service, and whether the focus on WAAS is affecting support for RNP/RNAV. The RNP/RNAV community could face a struggle to get what they want, when they want it: standardized, widespread, public-supported RNP/RNAV procedures with the advantages, but not the costs, of today’s "private procedures." Private procedures are developed, tested and maintained by the airlines rather than by the Federal Aviation Administration (FAA). The debate is complicated by the slippery nature of WAAS. Depending on who’s talking about it, WAAS could mean: 1. a sensor that supports any system, including FMS, as a position source; 2. various types of avionics that do not provide RNP/RNAV status, alerting or flight path capabilities; or 3. approaches, and these can sound synonymous with RNP/RNAV.

A key feature of RNP/RNAV approaches is that they will be flown in a linear lateral containment area, rather than in a funnel-shaped zone. This makes it possible to fly safely, but at lower minima, than would be the case with other navaids. The WAAS system’s new APV 1.5 service level will be able to surpass RNP 0.3 and even RNP 0.1, WAAS advocates claim.

FAA is trying to serve both parties. "I am committed to supporting all of the users of the National Airspace System (NAS)," says Kathy Abbott, FAA’s acting manager of the Flight Technologies and Procedures Division in the Flight Standards Service. "We are committed to RNP/RNAV procedures and to having them public." RNP/RNAV and WAAS are "parallel activities, and we are pursuing them in parallel," she says. "If anything, [RNP/RNAV] resources are growing." Abbott discussed WAAS in the context of providing "a sensor to input into the navigation system–as system position-fixing information, with integrity."

Regional Interest

Quite a few regionals are interested in RNP/RNAV procedures, although the technology is something that just a few major carriers and Horizon have invested in so far, says Scott Foose, vice president of the Regional Airline Association (RAA). Among operators approved to fly RNP approach procedures with linear containment is Alaska Airlines–Horizon’s sister company. Alaska has an approach to Juneau of RNP 0.15. The Terminal Area Operations Aviation Rulemaking Committee, or TAOARC, created by FAA to solve RNP/RNAV implementation, avionics and operational issues, includes representatives from regionals, such as Horizon, Continental Express, Atlantic Coast Airlines and Comair.

"All regional jet aircraft and the majority of regional turboprops have FMS onboard," Foose says. Ninety percent of regional jets are equipped with FMS, using GPS sensors. More than 6,000 aircraft worldwide have RNP-type avionics systems, adds Dave Nakamura, of Boeing, a participant in TAOARC and chairman of RTCA Special Committee 181. (That joint U.S./European industry group developed the specification that eventually will form the basis for an FAA RNP/RNAV avionics standard.)

RAA "wants FAA to define a system for the proliferation of instrument approaches, using RNP/RNAV in the public sector," Foose says. The airlines use more than 400 airports in the United States, and in many of them, RNP/RNAV would increase capacity. RNP/RNAV also would give commercial airports equipped with an instrument landing system (ILS) more than one instrument approach, he adds.

RNP/RNAV backers are impatient with their progress. "About 12 years ago, FAA promoted the installation of RNAV equipment," Foose says. But RNAV-capable operators have not been able to take advantage of it." There are some in the industry who question where FAA’s preferences have been, Nakamura says. "There is a perception that RNP/RNAV procedures are not already public and being actively deployed because they would take away from WAAS and LAAS [Local Area Augmentation System] activity." There’s the appearance of a "competition" between RNP/RNAV and WAAS–reflected in FAA–because they are driven by different aviation communities focused on limited government resources, Nakamura says.

"There’s a concern that has been discussed between FAA and industry–the commercial airlines–regarding whether the focus FAA has had on getting WAAS up is taking resources and support away from RNP/RNAV," Foose adds.

Horizon’s Strategy

Horizon, like Alaska, is a bit of a special case. WAAS won’t meet its needs now. WAAS does not provide 99 percent or better APV 1.5 availability in the northwestern tip of the lower 48, and WAAS missed approach procedures (MAP) are inadequate for the airline’s destinations, says Perry Solmonson, Horizon’s manager of flight operations-technical.

Despite the uncertainty of when public standards will become available, Horizon is forging ahead with linear-containment RNP, which it estimates will pay off through increased approach completions and greater terminal area throughput over a 10-year period. "We’ve resigned ourselves to the fact that the only way to get started is with special authorizations," Solmonson says. "But these advanced approaches have to become public and be widely adopted to have any effect on airspace efficiency.

"We are trying to find a way, not only to fly larger airplanes to outlying airports in difficult terrain, but to apply [precision-like] operations to all of our airports, where we don’t have ILS approaches," Solmonson adds.

Horizon’s proposed approaches will reduce exposure to controlled flight into terrain (CFIT) accidents in difficult terrain because of the linear (rather than trapezoidal) lateral protection area and FMS-based flight path guidance inbound and outbound. This results in the lowest possible approach minima to certain airports.

Actually, Horizon is already flying what are essentially RNP arrivals and departures, relying on onboard systems rather than ground-based navaids. The carrier received FAA approval two years ago to take off and land when local ground-based navaids are out of service. The authorization covers Horizon’s 28 Dash 8 Q200 and 15 Q400 turboprops, equipped with dual FMS, GPS receivers and an onboard IRS, which calculate the aircraft’s position and movement even when navaids are not working. The approval applies to any airport in Horizon’s route structure.

Because the position of the navaid has been programmed into the FMS, Horizon crews can take off from an area where there are no operative navaids, Solmonson says. On the approach side, Horizon Q200s and Q400s are permitted to fly into airports where there is an inoperable supporting navaid, such as an NDB. Horizon’s North Bend, Ore., airport, for example, is equipped with ILS. But if a pilot misses an approach, he has "to come back to NDB hold." If the NDB is out, the approach is effectively shut down. Again, Horizon can land in this situation because it has programmed the location of the NDB into its FMS database, "creating a virtual NDB," Solmonson explains.

"Horizon remains the only air carrier to have this ‘navsub’ authorization to fly within the departure and arrival terminal area while all navaids are out of service, except for the approach primary navaid," he says. "With the Universal FMS, we can operate outside any navaid service volume while flying in the terminal area. As long as certain GPS requirements are met, the FMS keeps track of where the airplane is much better than VOR or DME can."

Horizon has ordered 30 CRJ 700 regional jets, which it is equipping with dual Collins 4200 FMS, dual Collins 4000A GPS systems, and dual Litton LT 101 IRS. The CRJs also will feature Flight Dynamics (Collins) headup guidance systems (HGS)–for Cat III approaches–and Collins CMU 900 aircraft communications addressing and reporting systems (ACARS).

Horizon hopes to fly its Dash 8 Q400s and Q200s with linear-containment RNP approach procedures, as well. The Q400s are being equipped with dual Universal UNS-1E multisensor FMS, GPS and a single Litton LT-101 IRS. Horizon has flown test RNP/RNAV 0.3 approaches with its Dash 8 Q200s to Wenatchee, Wash., and Sun Valley, Idaho.

With avionics that are approved for linear-containment RNP, an aircraft’s actual navigation performance, or ANP, can be the same linear path 30 miles away from the runway as it is at the runway threshold, unlike ILS, MLS, VOR, NDB, GPS and WAAS, Solmonson says. And the flight path can be curved to avoid high terrain or noise-sensitive areas.

New Approaches

Horizon now uses MLS to obtain the lowest possible approach minima at Sun Valley and Wenatchee. The airline has a "special" MLS approach to Wenatchee, an area surrounded by mountains. MLS provides Horizon a 4.5-degree approach angle, 537-foot decision height and 1.25-nautical mile (nm) runway visibility. An RNP 0.3 approach, however, promises a 288-foot decision height, 0.5-nm runway visibility, and an approach angle of 3 degrees.

The MLS approach to Wenatchee is flown at a steeper angle than a normal 3-degree ILS approach. "It’s not something you can do with larger aircraft, such as the Q400, and certainly not with the CRJ 700," Solmonson says. During a missed approach, moreover, MLS does not provide course guidance. Instead a heading is flown. Flying a heading, rather than following a course, requires huge obstacle clearance distances on a missed approach. Initial approach minima, therefore, have to be that much higher. "Most of the time, in mountainous terrain, minima are determined, not because of obstacles on the approach side, but because of obstacles on the missed approach side."

"FAA is capable of designing WAAS APV 1.5 MAP procedures," says Bob Jackson, manager of business development for WAAS developer, Raytheon. There is only a small subset of airports where MAP is the controlling constraint on approach minima. And WAAS MAP guidance "is simply a matter of operators asking for it in the [FMS] database," Jackson says.

Horizon wants to achieve RNP 0.3 approaches at its Wenatchee and Sun Valley destinations. But the carrier needs RNP 0.1 for its Seattle hub, in order to fly simultaneous IFR approaches to closely spaced, parallel runways. Small RNP values can permit these operations where runways are too close together to permit simultaneous ILS approaches.

RNP/RNAV: The Debate Continues

The expansion of linear-containment, required navigation performance (RNP) procedures into the public arena has been slower than desired. Some blame the Wide Area Augmentation System (WAAS) program, a seven-year-old, estimated $3.2-billion, life cycle-cost system that competes for FAA resources. WAAS has gained even more attention with its new APV 1.5 service level, which WAAS developer, Raytheon, claims will be able to outperform RNP/RNAV by a factor of 10 if full advantage is taken of its capabilities after WAAS is commissioned next year.

"FAA is committed to support RNP/RNAV, but [those procedures] are being rapidly overtaken by the service you can get out of WAAS," claims Bob Jackson, Raytheon’s manager of business development. FAA might even be reluctant to permit public RNP/RNAV approaches, as they require a high level of sophistication and training, Jackson says.

WAAS proponents tout the system’s APV 1.5 capability. Whereas APV 1–like RNP 0.3–provides a lateral protection distance of one-third of a mile, WAAS APV 1.5 reduces the lateral protection limit to 130 feet (40 meters), the equivalent of RNP 0.025, Jackson says. "The bottom line is that WAAS is an order of magnitude more accurate than RNP 0.3," he asserts. "As air carriers become more aware of WAAS capability, there will be rapid acceptance of that service."

But the 0.025 number for WAAS indicates positioning accuracy, rather than the total system accuracy requirement, points out Dave Nakamura, of Boeing, chairman of RTCA Special Committee 181, which has developed the specification that eventually will form the basis for an RNP/RNAV avionics standard. "Even with WAAS, RNP will not be an order of magnitude better, when aircraft flight technical error [FTE] is included." For air transport, adding WAAS "can only get you to RNP 0.25 [from 0.3] unless there is a means to reduce FTE." FTE might be reduced in a system where the autopilot is tracking the path, using WAAS as a reference. Even so, it’s not clear whether or how that could be achieved or whether the additional performance would be worth the cost of equipage to air carriers.

The RNP/RNAV side claims it isn’t anti-WAAS. They want to fit WAAS into FMS-based procedures to produce even greater navigational accuracy. The Regional Airline Association (RAA) has asked FAA whether the WAAS signal can be used as a sensor input to FMS for linear-containment RNP approaches, says Scott Foose, RAA vice president.

"Regional airlines and majors fly in highly congested airspace, and we need technology allowing us to better define and narrow paths and increase the capacity of terminal and en-route airspace." In fact, the use of WAAS as a sensor input to FMS for RNP/RNAV is anticipated in DO-229C, the WAAS Minimum Operation Performance Standards (MOPS) document, Jackson says.

The best of all possible worlds would be using WAAS as a position signal input to the FMS, agrees Perry Solmonson, Horizon’s manager of flight operations-technical. The trouble is that WAAS "is being packaged, not only as a position sensor, but as a navigation path." But certain "beta-class" WAAS receivers will support whatever navigation path has been programmed into the FMS database, Jackson says.

There’s also a potential avionics problem for linear-containment RNP operators using WAAS. If FAA should require that WAAS APV 1.5 information be incorporated into existing avionics displays, carriers would face expensive software changes, such as linear-to-angular display scaling, and have to recertify equipment, Solmonson says. Where is the incentive to do that when carriers’ approaches already have been approved?

Jackson doesn’t foresee these problems with beta-class WAAS receivers, which could feed into FMS. "The receiver simply uses the increased accuracy and tighter protection [limits] output by the WAAS GPS sensor to derive a smaller RNP value, which can be used with existing display scaling and databases," he says.

"Provided that WAAS is added without changing the functions or capabilities of the FMS, no additional changes would be necessary," says Bruce DeCleene, FAA’s navigation program manager-aircraft certification. However, some FAA officials have said that aircraft systems must indicate which sensors are being used or must deliberately be de-selected, according to Nakamura. Any change in system displays or system de-selection can be viewed as a change in capability.

FAA already is building APV 1.5 procedures at five airports, which will benefit general aviation, says an agency official. These new WAAS approaches provide trapezoidal-shaped obstacle clearance areas, DeCleene says, as the containment area needs to be "consistent with total system performance, including manual pilotage."

In some cases, coexistence between the two types of approaches may be impossible, Nakamura maintains. "WAAS and RNP/RNAV are inherently different"–WAAS is angular and RNP/RNAV, linear. So, in selected areas, where the runways are close together, you can’t mix the two because they won’t fit well, he says. "It’s difficult to mix and match straws and cones." In other words, in this type of environment, "an angular obstacle protection area cannot overlap with a linear one," as this "will affect air traffic management and traffic flow."

RNP/RNAV: What’s in a Name?

Area navigation, or RNAV, is the ability to fly between three-dimensional points in space rather than between fixed-base navaids on the ground. RNP, or required navigation performance, refers to the accuracy with which RNAV can be flown. In this context, the approach RNP number of 0.3 nm means that an aircraft must fly within 0.3 nm from the centerline of the flight path with a 95 percent probability of that accuracy.

Strictly speaking, the combined term, RNP/RNAV, denotes RNP values of 2.0 or lower (in the U.S. only), with system integrity of 10-5 and system continuity of 10-4, according to RTCA. Boeing 737-400s, -700s and -900s contain navigation systems and flight systems that have been demonstrated to provide a capability for RNP/RNAV operations down to an RNP value of 0.11 nm.

It is generally acknowledged, however, that RNP/RNAV approaches will not truly exist until there are public RNP/RNAV standards to guide regulators, airlines and aircraft/systems manufacturers. These standards for equipment and procedures are expected to emerge in a year or so. Meanwhile, this article uses the phrase, "RNP with linear containment" to indicate the type of procedures that Alaska Airlines is flying today, down to RNP 0.15, and "RNP/RNAV" to indicate the eventual public procedures. Observers hope the differences between the two will turn out to be negligible.

A Step Toward the Public Domain

Required Navigation Performance (RNP) approaches with linear containment areas typically are designed, tested and maintained by a single carrier, as a "special procedure." But that will begin to change, if the Federal Aviation Administration (FAA) approves an innovative approach to Reagan Washington National Airport (DCA). This approach–involving multiple carriers and multiple RNP values–is the logical "next step along the continuum toward eventual public approaches," says Pam Hamilton, manager of air traffic and navigation technology for US Airways, the lead carrier on the project. The airline hopes to obtain approach approval this year.

Seven major airlines are involved in the project under a partnership program between FAA and industry. They hope to have the approach classified as RNP 0.3, RNP 0.2 and RNP 0.11. The level flown by each carrier will depend upon its level of equipment, certification and training. Alaska Airlines is the technical lead, designing the procedures for the approach.

Today US Airways flies a special RNP 0.3 approach to DCA’s runway 19. At the RNP 0.3 level, the new, multicarrier approach to runway 19 will allow a reduction in minima of about 140 feet, Hamilton says. Additional minima reductions can be achieved for lower RNP values.

Reagan National’s new approach "provides the opportunity to take advantage of our advanced navigation capability to maximize the value of what we already have invested in," Hamilton says. The new procedure, however, is "very much a work in progress," she cautions. "There are a number of human factors challenges that will have to be resolved."

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