GPS Vertical Guidance -
Understanding WAAS & Advisory Glideslopes
By Mark George, DPE & Jason Catanzariti,
Originally published in AOPA
Flight Training Magazine – August 2010
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
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
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
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
• 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.
This site is © Copyright Jason Catanzariti
2014, All Rights Reserved