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A-11
sources of information included in the production of
these materials, and the data is manipulated by several
different organizations before it eventually is displayed
on RNAV equipment, the possibility is high that there
will be noticeable differences between the charts and
the databases. However, only the inconsistencies that
may be built into the databases are addressed in this
discussion.
NAMING CONVENTIONS
As was discussed earlier in this appendix, obvious differences exist between the names of procedures shown
on charts and those that appear on the displays of many
Figure A-10. ILS/DME Runway 2 in Durango, Colorado.
A-12
RNAV systems. Most of these differences can be
accounted for simply by the way the avionics manufacturers elect to display the information to the pilot. It is
the avionics manufacturer that creates the interface
between the pilot and the database, so the ARINC 424
naming conventions do not really apply. For example,
the VOR 12R approach in San Jose, California, might
be displayed several different ways depending on how
the manufacturer designs the pilot interface. [Figure
A-11] Some systems display procedure names exactly
as they are charted, but many do not.
Although the three different names shown in Figure
A-11 identify the same approach, the navigation system manufacturer has manipulated them into different
formats to work within the framework of each specific
machine. Of course, the data provided to the manufacturer in ARINC 424 format designates the approach as
a 132-character data record that is not appropriate for
display, so the manufacturer must create its own naming conventions for each of its systems.
NAVAIDs are subject to naming discrepancies. This
problem is complicated by the fact that multiple
NAVAIDs can be designated with the same identifier.
VOR XYZ may occur several times in a provider’s
database, so the avionics manufacturer must design a
way to identify these fixes by a more specific means
than the three-letter identifier. Selection of geographic
region is used in most instances to narrow the pilot’s
selection of NAVAIDs with like identifiers.
Non-directional beacons (NDBs) and locator outer
markers (LOMs) can be displayed differently than they
are charted. When the first airborne navigation data-
bases were being implemented, NDBs were included in
the database as waypoints instead of NAVAIDs. This
necessitated the use of five character identifiers for
NDBs. Eventually, the NDBs were coded into the database as NAVAIDs, but many of the RNAV systems in
use today continue to use the five-character identifier.
These systems display the characters “NB” after the
charted NDB identifier. Therefore, NDB ABC would
be displayed as “ABCNB.”
Other systems refer to NDB NAVAIDs using either the
NDB’s charted name if it is five or fewer letters, or the
one to three character identifier. PENDY NDB located
in North Carolina, for instance, is displayed on some
systems as “PENDY,” while other systems might only
display the NDBs identifier “ACZ.” [Figure A-12]
ISSUES RELATED TO MAGNETIC VARIATION
Magnetic variations for locations coded into airborne
navigation databases can be acquired in several ways.
In many cases they are supplied by government
agencies in the “Epoch Year Variation” format.
Theoretically, this value is determined by government
sources and published for public use every five years.
Providers of airborne navigation databases do not use
annual drift values; instead the database uses the
“Epoch Year Variation” until it is updated by the appropriate source provider. In the U.S., this is the National
Oceanic and Atmospheric Administration (NOAA).
In some cases the variation for a given location is a
value that has been calculated by the avionics system. These “Dynamic Magnetic Variation” values can
be different than those used for locations during aeronautical charting.
Figure A-11. Three Different Formats for the Same Approach.
It is important to remember that even though ARINC
standard records for airways and other procedures contain the appropriate magnetic headings and radials for
routes, most RNAV systems do not use this information
for en route flight. Magnetic courses are computed by
airborne avionics using geodesic calculations based on
the latitude and longitude of the waypoints along the
route. Since all of these calculations are based on
true north, the navigation system must have a way to
account for magnetic variation. This can cause many
　
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