Turbine Pilot: RNP primer

Understanding Next-Gen approaches

December 1, 2009

I’m flying an Alaska Airlines Boeing 737 down the Gastineau Channel, getting ILS-style guidance to a runway I could not see yet if it were clear and a million. The approach end of Juneau International Airport’s Runway 26 is at 11 o’ clock and seven miles, over there behind Douglas Island. Now, calling where I am a channel is a bit benign—it’s actually a fjord only a few hundred yards wide at the bottom, rising to parallel ridgelines more than 3,000 feet high and barely more than a mile wide in some places. And I’m not over the channel, I’m in it; the ridgelines are well above me on both sides of the airplane.

I’m making turns every few miles in the channel and my final turn to align with the runway is about 40 degrees, rolling wings level at about 500 feet agl—and I still can’t see the airport. I break out right at the minimums of 337 feet msl and a mile, on the centerline, and land. Sound unusual? This is a typical day flying a typical RNP (required navigational performance) approach in southeast Alaska—to a runway that has no other approach.

What is RNP?

Simply put, RNP is RNAV on steroids. In general terms, RNP is a statement of navigation performance necessary for a defined operation within a defined airspace. What sets RNP apart from RNAV is on-board monitoring and alerting, letting you know if the airplane’s actual navigation performance (ANP) meets approach requirements. It’s pretty simple—if ANP (actual) is greater than RNP (required) then you’re good.

Instrument approaches using RNP are classified as RNAV approaches, so the approach plate header bears the “RNAV (RNP)” label. But that’s where the similarity with RNAV (GPS) approaches ends. RNP operations earn their tighter tolerances and accuracies by adding assurances that go beyond on-board monitoring and alerting. These include:

  • Special aircraft and aircrew authorization required (SAAAR). This means the aircrew must have training in RNP operations, and airplane must have the proper equipment.
  • WAAS (Wide Area Augmentation System) capability. WAAS GPS receivers use both satellites and ground stations to come up with position accuracies up to five times greater than those displayed on non-WAAS GPS receivers. WAAS also permits glideslope-like vertical guidance on LNAV/VNAV and LPV (localizer performance with vertical guidance) approaches.
  • Dual GPS and FMS installations. This guards against any equipment-originated single point of failure that could cause a loss of guidance. The lowest RNP values require dual autopilots.
  • More accurate altimeters, and vertical navigation (VNAV) based on barometric altimetry.
  • An inertial reference unit (IRU). This serves as a backup in case a GPS satellite signal is lost. The backup IRU becomes vital, for example, if a missed approach is caused by loss of a GPS signal.

We typically define an RNP approach by its RNP “value,” which is normally expressed as a lateral distance in nautical miles from the centerline—as in “that’s an RNP 0.10 approach.” (There are also RNP 0.3 approaches.) The airplane is required to be within that value, meaning within .1nm or .3 nm either side of the course centerline, at least 95 percent of the time. However, data collected shows performance to be far more accurate than required and on centerline pretty much all the time.

Operation within that 1 times RNP containment is considered to be normal operation—pilots are required to execute a missed approach if they go beyond that. Outside of that, there is an additional buffer zone of 1 times RNP that separates aircraft from terrain. For example, normal operation for an RNP 0.10 approach path width would be 608 feet (1 times RNP) on each side of centerline and any terrain would be at least 1,216 feet (2 times RNP) away from centerline. To meet RNP qualifications, the airplane is required to be between the centerline and that 2 times RNP boundary 99.999 percent of the time.

RNP is not dependent upon a certain source, such as a VOR or a localizer, but is a measure of total system performance. In theory, no one cares how you got it but in reality RNP is based almost exclusively on GPS. There is no RNAV approximation of position using VOR radial/DME triangulation that is robust enough to provide the RNP for most terminal-based operations.

The bottom line is that RNP allows us to build approaches and departures we just couldn’t build before GPS. It offers the flexibility of varying glide paths and curved legs of varying radius and length, a huge benefit for procedure design. So, if a rock is in the way, just go around it.

RNP and WAAS

From a pilot’s perspective, both WAAS and RNP approaches look and fly like an ILS. There is a “localizer” (LNAV) and a “glideslope” (VNAV) and both use GPS as the primary navigation source.

The main difference between WAAS and RNP is the GPS signal itself. WAAS enhances the signal through differential correction (using an additional satellite) to a typical accuracy of about three meters, while RNP uses unenhanced GPS, with a typical accuracy of about 15 meters. This results in a WAAS airplane having a more accurate fix on its location than an RNP airplane.

While WAAS is more accurate, RNP has more design flexibility. Generally, if terrain is not a consideration, WAAS will usually result in lower minima than RNP to a given runway. If terrain is a consideration, RNP will usually result in lower minima to a given runway than WAAS—if a WAAS approach could even be built.

WAAS uses an electronic glidepath calculated by GPS position, where RNP uses a barometric (sometimes called BARO VNAV) glidepath that is affected by temperature and altimetry. That is why RNP procedures require current altimeter settings and sometimes have temperature limits.

WAAS sees more widespread use than RNP. In addition to the 11 RNP procedures that Alaska Airlines developed, there are now more than 100 public RNP approaches in the United States, and the FAA is developing about 35 new RNP approaches a year. Most RNP approaches have minima lines associated with several different RNP values for days when satellite performance is less than stellar. With a full constellation of satellites—and if their owner, the Department of Defense, keeps accuracy high by leaving selective availability off—those days are rare.

By contrast there are almost 1,400 WAAS LPV approaches. For the most part, RNP is flown by the airlines, as well as recent-model high-end business jets. WAAS is flown by a much larger, ever-increasing group of general aviation airplanes equipped with WAAS-approved GPS receivers. WAAS-enabled approach capability is quickly becoming mainstream in airplanes ranging from the smallest piston singles to the biggest business jets. One important distinction, at least for the moment, is that RNP approaches can be done anywhere, while WAAS approaches are limited to the range of the WAAS satellites.

Required equipment

FAA Advisory Circular AC90-101 outlines equipage, operational, and training requirements for RNP approaches. You only need one approved box to fly WAAS approaches; for RNP you need dual flight management computers, inertial reference systems, GPS receivers, air data computers, a radio altimeter, and more. You might be wondering if you get any relief from RNP equipage for being WAAS-equipped—but you don’t, at least not yet.

A more “basic” RNP is being discussed that would lower the barrier of entry while hopefully not removing all of its utility. Beyond that, public RNP departure criteria are being developed, along with studies to examine the viability of dual-stream RNP approaches to parallel runways and decision altitudes in turns. As the voice of general aviation, AOPA is involved in these efforts on your behalf.

The pilot’s view

“Anchorage Center, Alaska Seven-Six over Annette Island at Flight Level 350.”

“Alaska Seven-Six, Anchorage Center. Good afternoon, maintain 7,000 feet until on a published portion of the approach, cleared for the RNP Runway 26 approach to the Juneau Airport. Contact Juneau Tower over MARMN, have a nice day.” That and a landing clearance are all I will hear between 35,000 feet and the runway.

The first time I heard that I nearly choked. What do you mean, cleared for the approach? How do I get from 35,000 feet to the ground on my own without ATC? Well, with RNP, the airplane has a defined path all the way from cruise to the runway, so we rely less on ATC. This reduces communications workload, as well as frequency congestion. And because of an RNP approach path’s higher level of accuracy, I get to fly to minimums that are far lower than those available with more traditional approach procedures

If I were not flying RNP to Juneau, chances are I’d be flying an NDB, VOR, or LDA nonprecision approach and choosing between a tailwind or circling to land on a short runway with no safety area in ugly weather. And, if I miss or get unstable and have to go around, my lateral guidance may be provided by an NDB that is swinging like an ape from a tree.

Instead of worrying about these issues, with RNP, I just fly. Sometimes I have course-deviation indicators like those you see when you fly RNAV or ILS approaches. Sometimes I don’t, which means I have to pick up lateral and vertical deviation in textual form off the flight management system display by my knee. I see lateral deviation in hundredths of miles (each hundredth is 60 feet) left or right of centerline and I see vertical deviation in feet above or below path. It’s a little different in the beginning, but you get used to it.

Yes, there are nice days when I turn all the automation off and just hand-fly a pattern to a runway—and I love that. But those opportunities are few and far between in Alaska. So, I am glad to have a tool in my aviation toolbox that will get me to the runway threshold stable, on speed, on centerline, and ready to land. It’s also immensely helpful to have an RNP-defined missed approach procedure with both lateral and vertical guidance should something not go as planned. But missing an approach due to low weather is less likely with RNP. RNP approaches have lower minimums, so what would have been a “miss” with a plain-Jane RNAV GPS, VOR, or other nonprecision approach will often be what’s called an “RNP save” arrival.

Alaska Airlines pioneered it in the mid-1990s, and RNP is now in use in many nations. But an expansion of its capabilities and uses is in the offing, beginning with more RNP approaches in the United States. The FAA’s Next Generation Air Transportation System (NextGen) timetable calls for RNP to be used for reducing traffic separation on oceanic routes starting in 2010. Reduced domestic en route separation using RNAV and RNP is planned for implementation in 2013. By 2020, the goal is to realize international harmonization of RNP and satellite-based communications under a larger plan called the Future Air Navigation System (FANS). FANS was adopted by the International Civil Aviation Organization (ICAO), with the goal of boosting capacity and reducing workload by integrating communications, navigation, and surveillance functions. There’s a lot to gain. From a capacity point of view, RNP doesn’t define approach path or airway lateral dimensions in terms of nautical miles; it defines them in tenths of miles. This means each airplane needs less separation space. The result: more airplanes can safely navigate within a given volume of airspace. But FANS promises more. Automatic position reporting, and reporting of critical flight parameters using Automatic Dependent Surveillance technology will be possible. So will controller-pilot datalink communications, whereby clearances and other communications are conducted using text messages, which will reduce pilot and controller workload—and potential misunderstandings.

It seems we are nibbling at the edges of a revolution in flying. Those of us lucky enough to fly using RNP should fly more precisely, efficiently, and safely—and it shouldn’t be long in coming.

Marc K. Henegar is a captain for a national airline.

RNP in sight

Avionics manufacturers have wasted no time in keeping up with RNP technology. The latest strategies have been to merge RNP capability with increasingly sophisticated primary flight displays (PFDs). The result is a wealth of graphic information, right before the pilot’s eyes. Both Honeywell’s Primus Epic and Rockwell Collins’ Pro Line Fusion avionics suites, for example, now can merge synthetic vision imagery with traditional flight guidance data. The Fusion adds infrared imagery to its PFDs, and the Epic will soon add that feature as well.

It may be night, or you might be in clouds, but with synthetic vision on the PFD, it’s always day-VFR, complete with terrain depiction. And there are plenty more capabilities with Honeywell’s latest upgrades.

The Primus Epic platform is used in Gulfstream’s G350, G450, G500, and G550. It’s also in the Cessna Sovereign, the Hawker 4000, the Emivest SJ30 (formerly the Sino-Swearingen SJ30), and in the Falcons 900EX, 2000EX, and 7X. Gulfstream calls its customized version of the Epic “PlaneView.” Falcon Jet Corporation calls its latest Epic suite the “EASy II.” Bombardier calls the ProLine Fusion “Global View” in its Global Express XRS and Global 5000 large-cabin, long-range business jets.

The list of features is too lengthy to detail in this space, but the EASy II package adds several significant upgrades, some of which are optional. Synthetic vision (Falcon Jet calls it “SmartView”) tops the list, but the addition of RNP 0.3 capability as standard is a big leap forward, as is SmartRunway (a runway awareness and advisory system [RAAS] with aural cuing to warn against approaches that are too high, low, or if the airplane’s configuration is improper), and autothrottle arming on the ground.

RNP 0.1 capability is another option. This provides a more accurate flight path on approach; a fully-deflected course deviation indicator needle amounts to just .1 nm--or about 608 feet--from the course centerline. WAAS LPV and ADS-B capability are other options, as is an automatic descent mode (ADM), which commands an emergency descent in case of depressurization. XM WX datalink weather and WAAS LPV are also available, and so are electronic charts.

There are even provisions for adding Future Air Navigation System-1A (FANS-1A) functions, such as controller-pilot datalink communications (CPDLC). Under CPDLC, communications between cockpit and controllers is via text messages.

—Thomas A. Horne