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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
discrepancies between the charted values and the
values derived by the avionics. Some navigation
receivers use the magnetic variation, or station declination, contained in the ARINC data records to make
calculations, while other systems have independent
ways of determining the magnetic variation in the
general area of the VOR or waypoint.
Discrepancies can occur for many reasons. Even when
the variation values from the database are used, the
resulting calculated course might be different from the
course depicted on the charts. Using the magnetic variation
for the region, instead of the actual station declination, can
result in differences between charted and calculated
courses. Station declination is only updated when a
NAVAID is “site checked” by the governing authority that
controls it, so it is often different than the current magnetic variation for that location. Using an onboard
means of determining variation usually entails coding
some sort of earth model into the avionics memory.
Since magnetic variation for a given location changes
predictably over time, this model may only be correct
for one time in the lifecycle of the avionics. This means
that if the intended lifecycle of a GPS unit were 20
years, the point at which the variation model might be
correct would be when the GPS unit was 10 years old.
The discrepancy would be greatest when the unit was
new, and again near the end of its life span.
Another issue that can cause slight differences between
charted course values and those in the database occurs
when a terminal procedure is coded using “Magnetic
Variation of Record.” When approaches or other procedures are designed, the designers use specific rules to
apply variation to a given procedure. Some controlling
government agencies may elect to use the Epoch Year
Variation of an airport to define entire procedures at
that airport. This may cause the course discrepancies
between the charted value and the value calculated
using the actual variations from the database.
ISSUES RELATED TO REVISION CYCLE
Pilots should be aware that the length of the airborne
navigation database revision cycle could cause discrepancies between aeronautical charts and information
derived from the database. One important difference
between aeronautical charts and databases is the length
of cutoff time. Cutoff refers to the length of time
between the last day that changes can be made in the
revision, and the date the information becomes effective. Aeronautical charts typically have a cutoff date of
10 days prior to the effective date of the charts.
Figure A-12. Manufacturers Naming Conventions.
A-13
A-14
EVOLUTION OF RNAV
The use of RNAV equipment utilizing airborne navigation databases has significantly increased the
capabilities of aircraft operating in the NAS. Pilots
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Instrument Procedures Handbook (IPH)仪表程序手册下(169)
