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GPS Vertical Guidance -
Understanding WAAS & Advisory Glideslopes


By Mark George, DPE & Jason Catanzariti, CFII

Originally published in AOPA Flight Training Magazine – August 2010

 

The Global Positioning System (GPS) has changed the way we fly, and sophisticated GPS receivers have become standard equipment, even in training airplanes. It falls to flight instructors to teach GPS operations in a comprehensive manner, which is especially crucial when preparing students for the instrument (IFR) rating.

GPS is a challenging subject because different manufacturers do things in their own ways, and the GPS system itself is evolving to add greater capabilities. With the implementation of the Wide Area Augmentation System (WAAS), vertical guidance has become a must-know topic for instrument pilots. WAAS enhances GPS accuracy enough to allow approaches with vertical guidance to airports with no ground-based navigational aids. In other words, a glideslope and minimums similar to an instrument landing system (ILS) become available, but without the associated infrastructure. Over 40,000 WAAS receivers have been sold as of fall 2008, and published approaches with WAAS minimums now outnumber ILS.

There are two critical issues about WAAS that instructors should make clear to instrument students. First, while you may have a WAAS receiver, you may not be executing an approach with WAAS vertical guidance and minimums. Second, some GPS receivers will provide a glideslope even when WAAS-enabled minimums aren’t authorized.

It’s important to understand the difference between having a WAAS receiver, and executing an approach with WAAS minimums. There are two ways to determine from an approach chart whether or not WAAS is available to those so equipped. The term “WAAS” will not appear in the title, where we usually see the type of equipment needed for the approach. Instead, it is in small letters on the upper left. The approach will also have “Localizer Performance with Vertical guidance” (LPV) minimums listed at the bottom. If those two items are not present, WAAS guidance is not available on that approach. Some approaches list LNAV / VNAV (Lateral navigation / Vertical navigation) minimums for barometrically corrected receivers and flight management systems, which are not common in training aircraft. A WAAS receiver will provide vertical guidance in this case, but not to LPV minimums.

A GPS approach having only LNAV (Lateral Navigation) minimums normally provides no vertical guidance at all. While you can fly this type of approach with a WAAS receiver, there are no LPV minimums and no WAAS vertical guidance. Pilots who mistakenly expect vertical guidance could put themselves in danger because an inactive glideslope needle will center itself. Failing to notice the off flag on the instrument could make them think they are flying a perfect glidepath when, in fact, they are receiving no vertical guidance at all.

Now comes the confusing part. Certain receivers will provide vertical guidance on some (but not all) non-WAAS enabled GPS approaches from the final approach fix. This feature is called an “advisory glideslope”. On a Garmin 430W for example, “LNAV + V” will appear in the unit’s display. Although only incorporated into WAAS receivers, the advisory glideslope does not “convert” an approach into a WAAS procedure. The advisory glideslope is just that - advisory only. This blurs the line between different types of GPS approaches, and creates a training quandary. Should instrument students be taught to follow an advisory glideslope?

Many instructors reason that glideslopes allow for more stable approaches, and so train their students to follow vertical guidance whenever it is provided. Although stable approaches are certainly desirable, the FAA has not yet taken a detailed position on how an advisory glideslope should, or should not, be utilized. That being the case, some examiners require checkride applicants to ignore advisory glideslopes, and instead follow the published stepdown procedures for the approach. Others may permit the use of the advisory glideslope, so long as the aircraft remains at or above the published minimum altitudes.

Despite questions about its proper usage, the advisory glideslope can be helpful, particularly in visual conditions. But it also has some drawbacks. These glideslopes are generally configured to intersect the missed approach waypoint (MAWP) at the minimum altitude permitted on the approach. This creates a sort of unofficial visual descent point which is not the MAWP, but at which you may not be able to land straight-in without already having the runway in sight. Descending promptly at the final approach fix may bring you out of the clouds earlier on the approach, and put you in a better position to land upon reaching the MAWP.

But following the stepdown instead of the advisory glideslope creates one other complication in training: the possibility of contradictory information from the instruments. Advisory glideslopes keep aircraft above the minimum descent altitude until reaching the MAWP. Descending promptly to the minimum altitude means you may see a low glideslope indication. More than one instrument pilot has become befuddled in this situation.

Understanding the nature of the advisory glideslope will help you and your students maintain situational awareness. If the glideslope doesn’t agree with what you’re doing on a non-WAAS enabled approach, verify that you’re flying the proper altitude for that segment of the approach and ignore the needle.

What should flight instructors teach instrument students about GPS vertical navigation? Some suggestions:

1. Know your equipment - in particular, do you have a WAAS receiver?
2. Recognize whether the type of approach you’re doing authorizes LPV minimums.
3. Recognize when vertical guidance is present, and whether it is WAAS or advisory in nature.
4. Utilize an advisory glideslope at your own risk.

If you haven’t yet instructed with a WAAS capable GPS unit in the panel, you will. It’s imperative to thoroughly understand your receiver’s capability and limitations before relying upon it in the IFR system. Flying an approach in low weather is no time to puzzle over why unexpected glideslope activity is giving you an indication you may not want to follow.

Resources:
• Garmin makes several manuals and PC simulators available for free on their web site.
• AOPA’s Air Safety Foundation offers online courses about GPS for VFR and IFR operations.
Elite Simulation Solutions now offers a Garmin 430 add-on for its simulators, which allows for realistic practice.
• The Aeronautical Information Manual has two sections that should be required reading for instrument students: 1-1-19 Global Positioning System, and 1-1-20 Wide Area Augmentation System.


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