Story ID: 
RNP gains signal FANS breakthrough
By: Bernard Fitzsimons
Caption: The vision of a future air navigation system developed by the International Civil Aviation Organization (ICAO) at the beginning of the 1990s has taken a lo
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The vision of a future air navigation system developed by the International Civil Aviation Organization (ICAO) at the beginning of the 1990s has taken a long time to materialize. But the gradual execution of some of the main elements suggests the future may finally be at hand.

In Toulouse last October the European Organisation for the Safety of Air Navigation (Eurocontrol) and the U.S. Federal Aviation Administration (FAA) convened a workshop to examine the potential for implementation of required navigation performance (RNP), which represents one of the three pillars of the FANS-based communications, navigation and surveillance/ air traffic management concept. RNP promises to allow operators to use their extraordinarily accurate navigation systems to make flying safer and more efficient, and even to land on runways otherwise restricted to visual approaches in clear weather.

Over the last 10 years, RNP has facilitated several dramatic improvements in airport accessibility, starting with Alaska Airlines’ alternative approach to Juneau Airport in Alaska. U.S. specialist Naverus and Australian carrier Qantas developed one of the most recent–a precision approach to Queenstown in New Zealand.

Almost all the approaches developed so far have involved only a single operator, however. So Air New Zealand, for example, has been working with Naverus to develop its own approach to Queenstown, where approved Qantas airplanes and crews can follow an instrument procedure to a decision altitude of 250 feet but other aircraft cannot descend below 3,000 feet except in visual meteorological conditions.

RNP criteria usually are expressed in values of nautical miles. So, for example, Qantas Boeing 737s may use the Queenstown RNP approach to RNP 0.1. That means they will stay within 0.1 nautical mile of the nominal flight path 95 percent of the time, and within 0.2 nautical miles 99.9 percent of the time. Airbus has certified the A320 to RNP 0.15, and the new approach will enable those airplanes to follow a precision approach with a decision altitude of 270 feet.

A new lateral deviation display on the A320 enables the crew to monitor the flight path steering error directly on the primary flight display. Pilots used the display during demonstration flights in June, when they flew RNP procedures for arrivals on both runways, missed approaches and departures with both engines and with one engine out and the autopilot engaged.

Now the FAA, for one, has decided to end the approval of private procedures for single operators and focus on public approaches. Known as RNP approach procedures with special aircraft and aircrew authorization required (SAAAR), they follow the principles applied during the approval of Category II and III instrument landing system.

SAAAR procedures use an airspace containment measure of two times RNP, based on navigation, equipment and aircrew performance, with RNP values of 0.1 to 0.3 nautical miles. “If there is any error it is usually from aircrew performance,” said Don Pate, FAA flight procedure standards branch manager.

Missed approach RNP values can fall between 1 and 0.1 nautical miles, and pilots can use radius to fix (RF, equivalent to a turn with a fixed radius) turns on the final approach segment between the final approach fix and the decision altitude.

“RF turns allow you to fly in very restricted areas,” Pate observed. And while the procedures developed so far have generally focused on locations where rugged terrain requires highly precise navigation, the same criteria can apply in areas with multiple airports, where pilots must avoid arriving and departing traffic streams rather than mountains.

One of the desirable attributes of an aviation system is its global applicability; by the same token, the development of multiple standards with their own definitions has helped hinder the wider implementation of RNP procedures. So standardization of terminology and definitions has become one of the main thrusts in preparing the way for the wider application of RNP.

The FAA has presented Order 8260.52, for example, to the ICAO obstacle clearance panel, which accepted it and plans to reformat and republish it next year as a stand-alone ICAO RNP procedure design manual. Meanwhile, an ICAO study group has harmonized the classification of standards.

To start, the ICAO group classified specifications according to the need for containment integrity, or the probability that the navigation system will detect and alert the crew to the fact that the total system error is greater than the cross-track containment limit.

ICAO will designate those without containment as Rnav (for area navigation). The basic Rnav (BRnav) standard used in Europe since 1998 and the similar RNP-5 standard used in the Middle East will both become BRnav, with an RNP value of five nautical miles. The USRnav Type A standard, which has an RNP value of 2, will be known as continental Rnav (CRnav). And USRnav Type B and the precision Rnav standard used in Europe will both become terminal Rnav (TRnav) with an RNP value of 1.

In the future only operations that require containment integrity, notably those in terminal airspace and particularly approach operations, will use RNP. The measure should allow standardized approaches.

Right now approach operations are sensor specific, with separate design required for any of a variety of ground- or satellite-based navigation aids used to support the operation. Specifying the accuracy required rather than the means used to achieve it should eliminate that sort of duplication.