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 up
On instrument approaches, Garmin's GNS-480 (the first WAAS navigator certificated to TSO-C146a) can provide ILS-like vertical and horizontal guidance.
Planning A WAAS Approach
Any approach-approved GPS navigator, with or without WAAS, can fly to nonprecision straight-in lateral navigation (LNAV) or circling minimums—typically getting you down to 500 feet above the runway and requiring a mile or more visibility. If you have an approach-approved navigator with WAAS or a barometric input, you may be able to use slightly lower lateral navigation with vertical guidance (LNAV/VNAV) minimums. This looks like an ILS localizer and glideslope presentation, but may be less accurate (or have a smaller protected area at the airport you’re approaching) and thus requires higher minimums—on the order of 400 or 500 feet and one mile. The most desirable approach, with the lowest minimums is localizer precision with vertical guidance (LPV), which can be as low as a category-1 ILS: 200 feet and half a mile, though 250 feet and one mile are currently more common.

In most cases, a local NOTAM will warn you if satellite coverage (or other issues) preclude certain approach minimums. However, some airports with WAAS approaches aren’t included in the NOTAM reporting service. On FAA National Chart Office (NACO) plates, this is indicated by a white “W” on a black background in the briefing strip just below the WAAS channel number box. Jeppesen’s presentation is a bit different but provides the same information. In these cases, you’re required to plan for nonprecision LNAV or circling minimums, though lower minimums may be available when you arrive.

With the preflight briefing complete and a flight plan on file, it’s time to check whether the GPS navigator’s database is up to date—it will tell you during the power-up sequence. Assuming the database is okay, you can now enter your flight plan. The exact steps vary from unit to unit, but generally you’ll define the departure and arrival airports and waypoints between them. If you’re flying IFR, you’ll have the option to select departure and arrival procedures (it might be wise to call clearance delivery first—they don’t always give you what you ask for). Once that’s done, it’s time to call ground control for taxi clearance and complete your run-up checks. Then you can launch.

En route, the GPS (with or without WAAS), gives you navigation signals similar to those from a VOR. Deviation off course is shown on the HSI or CDI—though with GPS navigation, deviation is displayed linearly (with full-scale deflection indicating five miles off course) rather than in angle. You can also monitor your flight on the navigator’s moving-map display (or on an MFD if you have one).

For IFR pilots, the big difference between TSO-129a (pre-WAAS) and TSO-C146a (WAAS) navigation only shows up on area navigation (RNAV) approaches. For example, look at the approach plate for the RNAV (GPS) Runway 9 approach to Duluth International Airport on page 66. The WAAS channel number is shown in the upper left corner. The plan view shows a “T-Bone” layout typical of RNAV (GPS) approaches, with two initial approach fixes (IAFs) on either side of a long straight-in final approach course. If ATC clears you for the approach without specifying a fix, you’re expected to navigate directly to whichever IAF is closest to your route and then make a turn direct to the initial fix (IF). From there, you’ll turn to the final-approach course, and from the final-approach fix (FAF), you’ll descend to whichever set of minimums you’re using.


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