Is RNP the Eventual Future of Air Navigation?

 - October 2, 2013, 12:55 AM
The Lhasa illustration shows three RF turns, each of different radii.

It is becoming more and more likely that in coming histories of aviation, the key major milestones will include the introduction of jet aircraft, the widespread adoption of satellite positioning and the arrival of required navigation performance (RNP). Jets and satnav are now irreplaceable elements that we take for granted.

In the 1980s the future of air traffic control was commonly summed up as “Rnav everywhere, and RNP where necessary.” Today, with the steady increase in aircraft numbers and their airspace demands, coupled with advances in avionics technology and miniaturization, the future ATC phrase will probably become “RNP everywhere”–but not just to accommodate more aircraft movements, as it unquestionably will. The driving force will be–in fact, in many cases already is–the cost benefits that RNP brings to aircraft operators in enhanced airport access, in addition to time, fuel and other savings.

Yet getting operators and regulators to even consider approving such a concept in the 1980s would have been a formidable task. “In IMC? Impossible!” would have been a common reaction. But eventually, after several years of investigations, analyses, massive amounts of data and trials involving aircraft manufacturers, avionics engineers, performance analysts, airline pilots, government regulators and other specialists, as well as the essential support of a handful of senior FAA personnel, the world’s first revenue RNP flight, descending IFR down Alaska’s Gastineau Channel into Juneau, was flown by an Alaska Airlines 737-400 in May 1996.

The Juneau procedure, and all that have followed it, was developed under the stringent FAA Terps or internationally formulated ICAO Procedures for Air Navigation Services-Aircraft Operations (Pans-Ops) that, along with a host of other requirements, demand a high level of planning expertise, meticulous documentation and rigid quality control throughout the entire process. In RNP, procedure development and approval is far more demanding than adding a new domestic air route to the NAS.

A Long Road to RNP Procedures

It is also a process that calls upon coordinated teams that include flight-path designers, technical pilots, ATC specialists, environmental specialists, data analysts and other support disciplines. At GE, which acquired RNP developer Naverus in 2009, the flight efficiency services department has a staff of approximately 100, with a dedicated core group of about 35 specialists based in Kent, Wash., headed by former Naverus chief technical officer Steve Fulton, now a senior technical fellow at GE. And before that, Alaska Airlines first officer Fulton sat in the 737’s right seat on the pioneering 1996 flight into Juneau, closely monitoring progress along the RNP route that he had developed. Today, Alaska captain Fulton shares his GE Fellowship commitments with his airline pilot schedules.

RNP procedure development is unlike anything designed earlier for civil operations. Operations leader for GE Aviation’s navigation services Matt Vacanti briefed AIN on the consecutive steps in the exacting process of translating a customer’s departure to destination need into a flight-checked RNP or RNP-AR (for authorization required) route approved by the customer’s regulatory authority.

An initial small group that includes the chief designer, a technical pilot, a quality-assurance specialist and other necessary experts conducts the pre-design phase. Their first question: is the requirement feasible, both from the viewpoints of the flight-path terrain and the aircraft proposed to fly it, and what are the potential options?

With that determined, all the relative aeronautical information data is collected and incorporated into the company’s planning tools. Against that background, an airspace analysis then narrows the feasibility options to reach the most efficient route that meets all the safety, performance and other requirements. In parallel, representatives of the customer and other stakeholders, such as air navigation service providers (ANSPs), are briefed to ensure a full understanding of the process and, conversely, for the representatives to advise the team of any local, possibly non-aviation, considerations that might have to be observed.

Following the decision on the optimum potential path, the project moves into the full design phase, where the initial small group expands to allocate specific tasks to individual specialist groups, with the leading objective being to refine the route to ensure that the proposed procedure complies with the applicable performance-based navigation (PBN) criteria. (A key point here: each procedure can be tailored specifically to the host aircraft’s performance characteristics, including energy management, human factors and its avionics capabilities. In RNP, one size definitely doesn’t fit all.) At the same time, the route is examined closely for its operational acceptability, including its flyability and, with inputs of ATC radar tracking data, its conformance with ATC in all traffic densities.

The next step is validation of the candidate procedure. This is initially performed in a Level C or D flight simulator, followed by flight validation by the technical pilots, usually flown in an appropriate ANSP aircraft that accommodates design team observers and special recording equipment. In addition to conducting further flyability assessments, the team members inspect the terrain topography and significant obstacles on either side of the route, in some cases having to assess the effects of recent local tectonic plate shifts when using the WGS 84 datum with periodic surveys and multiple-coordinate systems. An important aspect of both these validations is the constant assessment of en route “escape” routes, in the event of engine failure or other emergencies. Next, the company performs a flight operations safety assessment, conducted by a regulator-provided technical pilot with full knowledge of RNP procedures, familiarity with operations in the customer’s part of the world, and type rated in the customer’s aircraft used in the assessment. The flight includes the planned departure, en route and arrival procedures, as well as a safety assessment of the go-around procedure.

This then leads to operational implementation of the planned route. Important factors here are an environmental impact study, including noise and emissions, and completion of the design documentation, including verification of the customer’s data, all of which are required before regulatory approval is granted. In several countries, however, GE has delegated design documentation approval. What then follows are essential support activities, such as refining the procedure’s coding and charting, and coordination with the regulator’s navigation database cycle and other related tasks. Once the whole process is complete, the customer is ready to make the first passenger boarding call.

For a relatively straightforward RNP procedure, the process takes three or four months. But for a complex -AR route over challenging terrain, it could take up to a year.

For the procedure production team, however, the job still isn’t over. Their commitment continues with providing procedure maintenance and flight path optimization. In most cases, optimization caters to permissible minor adjustments requested by the customer, but they can sometimes involve major changes. At Queenstown, New Zealand, reconfiguration of the local airways structure required significant rerouting of the RNP procedure, which earlier wended its way around the local mountains to reach the tourist resort’s airport.

RNP Lexicon

As with all new aviation developments, RNP has brought with it new terms that some may not be familiar with.

Containment describes the extent of the lateral and, in certain advanced applications, the vertical boundaries of an RNP route. RNP approvals are given as RNP-x, where x is the demonstrated navigation accuracy of the aircraft’s navigation system. An RNP-0.1 means that the aircraft can maintain its prescribed path within +/- 0.1 nautical miles. RNP-2 means that it can stay within +/- 2 nautical miles of the path. Containment widens the boundary by doubling the accuracy statement to provide an additional safety buffer, based on 95 percent of the aircraft’s total system error, or TSE. Correspondingly, an RNP-0.1 containment expands the permitted path limits to +/- 0.2 nm and, in the RNP-2 case, expands the limits to +/- 4 nm.

RNP-AR is analogous to the FAA’s Cat III approval of pilots and aircraft. For RNP-AR, pilots require prior approval, based on both RNP procedures and avionics system knowledge, understanding and training, before being granted an FAA endorsement. RNP replaces the special aircraft and aircrew authorization required (SAAAR).

The RF Turn is a key element in advanced RNP operations. Bear in mind that with the exception of the takeoff and landing, an advanced RNP procedure is flown throughout under FMS and autopilot control. Manual control is permitted only with the flight director in certain circumstances. RF Turns are built into the FMS as turns of appropriate radii, centered on stored GPS positions. Where steeper than normal turns are required, the FMS computer may be pre-programmed to reduce speed, should that be necessary.

In summary, RNP has brought reliable IFR flight operations to airports that were previously inaccessible, other than in potentially risky visual conditions. And while investments in avionics, crew training and the customized procedures may be still high, several airlines are now seeing valuable payoffs in time, fuel burn and engine and airframe hours, as are many corporate operators too. Equally important, deskbound bureaucrat regulators no longer say “That’s impossible.”