Plane & Pilot
Sunday, July 1, 2007


Can GPS replace ILS?

waas upLately, several new acronyms have entered the GPS field; most notable among them is WAAS, which stands for Wide Area Augmentation System. To VFR pilots, WAAS is just a new level of GPS that’s more accurate and reliable, but to IFR pilots, it brings a confusing array of new options. Look at one of the new RNAV (GPS) approach plates, and you’ll see unfamiliar terms, especially in the minimums: LPV, LNAV and LNAV/VNAV. It’s enough to leave a pilot scratching his or her head, but in the next few pages, I’ll try to make sense of it for you.
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waas upFlying A WAAS Approach
Once the approach is activated, the navigator displays “TERM” to indicate terminal mode, either on the built-in display or on a separate annunciator (which may be required if the navigator’s built-in display is located too far outside the pilot’s normal field of vision), and guides you to the selected IAF. In “Vectors-to-Final” mode, it indicates deviation from the extended centerline. As you approach an IAF, the navigator prompts you that a turn is coming up. IAFs are fly-past waypoints; you’re not expected to fly over them, but to begin a standard-rate turn to the new course as you approach one. The navigator provides guidance through the turn. As you approach the FAF (when pre-WAAS navigators show APR to denote approach mode), the annunciation will change from TERM to LPV, LNAV/VNAV, LNAV+V or LNAV to indicate what minimums you’re authorized to use. You’ll note from the approach plate above that LPV minimums as low as 200 feet above the runway and with ½ mile visibility are possible at Duluth—but whether you can fly them depends on conditions at the airport and in the satellite constellation. When I simulated this approach at home (using Garmin’s GNS-530W trainer, which can be downloaded from a link at the end of this article), only LNAV+V (lateral navigation with advisory vertical guidance) minimums were available. If I had been flying this approach for real, that issue should have turned up in the preflight planning, either with a NOTAM or when I ran Garmin’s satellite-coverage prediction software.

As a practical matter, flying to LPV, LNAV/VNAV and LNAV+V minimums are all basically the same: You get both lateral and vertical guidance on your HSI or CDI. If you’re flying at the minimum authorized altitude for the segment from the initial fix to the FAF, you’ll intercept the computed glidepath at the FAF, and then follow it down an ILS-like glideslope (three degrees at Duluth) to the touchdown zone. For LPV and LNAV/VNAV approaches, the missed approach point is the decision altitude (DA), just as with an ILS—so you’re expected to follow the glidepath. For LNAV+V, you have the option to descend to the MDA any time after the FAF, as you would in a nonprecision LNAV or circling approach. In any case, the end of the runway is a flyover waypoint, and if you’re not in a position to make a landing by then, you’ll have to execute a missed approach. The navigator becomes a big help at that point, because once you tell it you’ve missed (by pressing the OBS button on a GNS-430/530, for example) it provides guidance through the missed-approach procedure, including any holding-pattern entry that may be required.


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