AOPA Pilot Magazine - December 2015 - 104

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ON INSTRUMENTS

the MDA and then have to fly the missed
approach procedure. Because you are now
past the legal missed approach point, any
turns you make can put terrain or obstacles
in your path.
LP (localizer performance) approaches.
As the name suggests, this is an approach
with conventional, localizer-like lateral
accuracy and no vertical guidance. It's a
WAAS nonprecision approach with a decision altitude (DA), published for airports
with terrain or obstructions that prohibit
the more precise, vertically guided LPV
approaches. Decision altitudes are missed
approach points described in terms of altitude above mean sea level-as opposed
to decision heights (DHs), which are referenced to heights above ground level as
measured at the touchdown zone.

If your sole GPS quits, you
may have no guidance
whatsoever! This is when
having a VOR receiver or
two, or even-gasp-an ADF
could save the day.
LNAV/VNAV (lateral navigation/
vertical navigation) approaches. These
approaches have both lateral and vertical guidance, with the vertical component
calculated either by a WAAS receiver's
internally generated glideslope, or barometric data from the airplane's altimeter or
flight management system's navigation and
air data system. With this barometric vertical navigation (Baro-VNAV) function, the
number of standard, WAAS-required GPS
satellites received is reduced from five to
four. With Baro-VNAV, a vertical path is also
computed between two waypoints along the
final approach course, or created as a single
angle from an originating waypoint along
the course. But because nonstandard temperatures create errors, there may be higher
minimums or other restrictions on baroaided LNAV/VNAV approaches, so pilots
should be aware of any temperature limitations-especially in cold conditions when
glideslopes decrease in altitude. LNAV/
VNAV approaches are published with DAs.
104 | AOPA PILOT December 2015

LPV (localizer performance with vertical guidance) approaches. This is the
most precise of all the WAAS approaches,
with both lateral and vertical guidance. By
design, their accuracy and course sensitivity
closely match those of ILS approaches, and a
decision altitude will be published. Typically,
LPV minimums are published with 200-foot
DAs and half-mile visibilities. So if there's
instrument weather-especially low IFR
conditions-at your destination, then an LPV
approach is your best bet.
One nice thing about WAAS approaches
is that WAAS GPS receivers do a final signal integrity test 60 seconds before the final
approach fix. If the test reveals less-thanoptimal signal quality, annunciations will
tell you-and you'll see an "approach downgraded-use LNAV minima" message. Now
you must fly the associated LNAV, nonprecision approach to the prescribed MDA.
LP approaches are not the fail-down mode
for an LPV approach. Typically, when an
approach downgrades it will do so to an
LNAV approach, meaning no glideslope.
If the signal test fails altogether, then
you'll get a red warning flag and a message such as "abort approach-navigation
lost." This is, of course, your cue to perform a missed approach immediately and
advise ATC of your problem. But you have
to ask yourself: Was it the GPS satellite network that's not up to snuff, or is it my receiver?
This is a major problem if you have just one
WAAS GPS receiver. With a two-receiver
setup you can compare one against the
other to find the answer. More trouble lurks
during the missed approach: With a single
GPS, you may have no guidance whatsoever!
This is when having a VOR receiver or two,
or even-gasp-an ADF could save the day.
This has been a general review of a
complicated topic. For specific information, consult your avionics manufacturer's
pilot's operating handbook. Another mustread is the FAA's Instrument Procedures
Handbook (FAA-H-8083-16). Still more
quality operational advice is available from
presentations by Andrew Knott of the TBM
Owners and Pilots Association and in Max
Trescott's GPS and WAAS Instrument
Flying Handbook.
AOPA
EMAIL tom.horne@aopa.org

http://www.AirFleetCapital.com

Table of Contents for the Digital Edition of AOPA Pilot Magazine - December 2015