Plane & Pilot
Sunday, July 1, 2007

WAAS Up?!


Can GPS replace ILS?


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.

Flying 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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