Features

April 2016 Issue




RNAV Approach Types

What you can fly depends on how youíre equipped, but ILS-like minimums based on GPS are common.

It wasn’t very long ago that GPS instrument approaches came in one basic flavor: A T-configured series of initial fixes feeding to a final approach course and final fix leading to the runway. There was no vertical guidance and the IFR-approved GPS navigator we used tightened up its tolerances from the en route and terminal to the approach phases. That was pretty much it: Despite the improved accuracy of GPS, we still flew an approach much the same way as a 1930s-era DC-3 crew flew an NDB procedure.

Much has changed with the advent of WAAS, the wide-area augmentation system that enhances standard GPS accuracy. Instead of one basic style of GPS approach designed to let the pilots manage the descent as it was aboard that DC-3 80 years ago, WAAS-enabled procedures and equipment now feature an electronic glideslope we can follow with our eyes or autopilot right down to the runway. Well, maybe. There now are several different types of GPS approaches baked into a standard RNAV procedure, each with their own nuances and minimums published on the same plate. What are their differences, and are there practical implications for how we fly them?

Growing Acceptance

Garmin aviation compass

The earliest approved uses of GPS for approaches came in 1994. At first, GPS could be used as a substitute for non-directional beacon (NDB) approaches via an overlay, and resulted in “NDB or GPS” in the procedure name. It wasn’t long before GPS also was approved to replace automatic direction finder (ADF) and distance measuring equipment (DME) in the IFR system.

Also in 1994, so-called stand-alone GPS approaches were developed and named “GPS RWY XX” approaches, where the XX was replaced by a runway number, or as “GPS X” procedures, where no straight-in minimums were published, only circling. The X was replaced by a letter, denoting the approach’s series, A, B, C, etc. In 2000, format changes brought us the current “RNAV (GPS) RWY XX” or “RNAV (TYPE) X RWY XX” naming, where TYPE is replaced by the equipment required, either GPS or RNP (required navigation performance; see the box on the opposite page) and the single X is replaced by Z or Y, denoting the series. That’s the basic naming convention we have now.

By 2003, the FAA began integrating WAAS-based enhancements into other parts of the ATC system and its procedures. Greater accuracy is achieved through ground-based WAAS ground reference stations, which monitor the GPS satellite constellation and sends corrections through WAAS master stations to two geosynchronous satellites. These satellites then transmit the corrections to WAAS-enabled GPS receivers.

By 2011, according to the FAA, the number of WAAS-enabled approaches equaled that of ILSes. As of early March 2016, there were 3628 WAAS-enabled localizer performance with vertical guidance (LPV) approach procedures in the U.S., serving 1762 airports. More than half of those airports—1016—do not have an ILS. The table in the sidebar on the opposite page lists the various types of RNAV (GPS) approaches available in the U.S. as of early March.

Both approach plates in the above right column are for RNAV procedures. One is expected to be flown with GPS while the other uses something called RNP, required navigation performance. RNP is an ICAO standard based on sophisticated flight management systems using a database of DME signals to generate RNAV guidance. An RNP 0.11 procedure offers +/- 0.11 nm accuracy. Since itís ground-based, RNP isnít good for overwater routes. Itís also rare in personal airplanes.

Breaking It Down

In that table, you may see some acronyms you kinda-sorta understand, plus maybe a few others you don’t. In fact, the different types of GPS-based RNAV approaches can be a bit bewildering. The acronyms mean different things of course, but it comes down to two features that distinguish them from each other: whether there’s vertical guidance (vertical navigation, or VNAV) and/or so-called localizer performance, where navigational sensitivity increases as the aircraft nears the runway. The sidebar accompanying the table includes explanations of the major differences between the various types, i.e, whether they provide localizer-like performance and/or vertical guidance.

What does all that mean in the real world? How can a pilot determine which procedures, if any, are legal and safe given the aircraft’s equipment and certifications? That’s something you basically should know before starting an engine, but you’ll find it in the airplane’s paperwork; look for a flight manual supplement.

If you’re driving a WAAS-enabled, TSO’d navigator like a Garmin GTN 750 or an Avidyne IFD540, and it’s correctly installed, you likely are approved for the lowest-available GPS approach minimums, LPV, which often are good to CAT I ILS standards of 200 feet agl. If those minimums aren’t available—perhaps due to an onboard barometric system casualty or poor signal strength—a TSO’d box will tell you, revert to the next-available level of service and let you know via an annunciator. When all else fails, expect to use the highest published minimums unless you can demonstrate you’re entitled to the lower ones.

With all that going on, the good news is LPV approaches to 200 feet agl are widely available—there are some 942 in the U.S., according to the FAA—and they offer a stable, constant-angle final, one that’s often smoother to fly than an actual ILS. And that’s a great thing.


Decoding RNAV (GPS) Approaches

- LPV: Localizer performance with vertical guidance. Offers localizer accuracy and a WAAS-based glideslope, like an ILS. Also like an ILS, lateral sensitivity increases as the aircraft approaches the runway. Minimums are published as a decision altitude (DA).

- LNAV/VNAV: Lateral/vertical navigation. Unlike an LPV procedure, lateral sensitivity does not increase as the aircraft nears the runway. Glideslope information is based on WAAS or barometric altitude. Minimums are published as a DA.

- LNAV: Lateral navigation. Similar to an LNAV/VNAV procedure except no glideslope is presented. Sensitivity does not increase as one nears the runway. Minimums are a minimum descent altitude (MDA), as with a VOR or localizer-only procedure.

- LP: Localizer performance. No vertical guidance provided. Sensitivity increases as the aircraft nears the runway, as with a traditional localizer. Minimums are an MDA.

- Stand-Alone GPS: The old-style GPS approach is basically an LNAV procedure but restricted to using only GPS. Offers an MDA.


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