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
Garmin’s recent upgrade to its popular GNS-430/530 navigators offers WAAS capability with vertical guidance.
Flight Planning For WAAS
For VFR pilots, WAAS offers a reliable backup to conventional navigation methods. Yes, you can put in your departure and destination and let the GPS give you a straight-line course between them, but unless it’s over ground you’re familiar with, you’d be wise to spend a little time with sectional charts. IFR pilots need to break out en route charts (or electronic flight-planning software) and figure out which airways to use; this is the first place where WAAS can pay off in a big way—with lower MEAs on some airways. In one example, WAAS-equipped pilots can expect to fly at 5,000 feet instead of 14,000 feet.

With the route worked out to your destination, it’s time to look at the approach plates and get a preflight briefing that includes a check of NOTAMs. In the unlikely event of a systemwide WAAS failure, an areawide NOTAM will be issued. It’s much more likely that you’ll see site-specific NOTAMs: “…GPS IS UNRELIABLE AND MAY BE UNAVAILABLE…” or “…WAAS LNAV/VNAV AND LPV MNM UNREL…” In the first case, you can’t count on using GPS in the specified area or for the specified time. If this affects you, you’ll need to use some other form of navigation or change your route. If you’re planning an instrument flight and your destination is in the affected area, you’ll need to plan for a non-GPS approach. The second case affects which minimums instrument pilots can use for an approach.

There’s one more preflight step you may need to take, depending on your equipment. Garmin supplies satellite-coverage prediction software that’s built into the GNS-430/530W trainer and is available separately for the GNS-480. It requires an Internet connection and analyzes satellite coverage over your planned flight path. It then advises you of any issues that could affect your use of a navigator as a primary form of navigation along that route. It’s required for any flight taking place more than 200 miles from your home airport. This means that on multiday trips you’ll need to carry a laptop computer (or electronic flight bag) with the prediction software loaded, and find a place where you can connect to the Internet before launching for the day.


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