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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
are now capable of direct flight over long distances
with increasing precision. Furthermore, RNAV
(RNP) instrument approach procedures are now
capable of precision curved flight tracks. [Figure A13] The availability of RNAV equipment has reached
all facets of commercial, corporate, and general aviation. Airborne navigation databases have played a
large role in this progress.
Although database providers have implemented a standard for airborne navigation databases, pilots must
understand that RNAV is an evolving technology.
Information published on current aeronautical charts
must be used in cases where discrepancies or uncertainties exist with a navigation database. There are many
variables relating to database, manufacturer, and user
limitations that must be considered when operating with
any RNAV equipment. Manufacturer documentation,
aeronautical charts, and FAA publications are the pilot’s
best source of information regarding these capabilities
and limitations.
Figure A-13. Example of an RNAV (RNP) RF Leg Segment and Associated FMS Control Display Unit.
SETOC
FONVI
JUBOL
WIRSO
LEGEND
RNP 2 = RNP Value of 2.00 NM
2 RNP = 2 Times the RNP Value (2 x RNP)
2 x RNP
2 x RNP
RNP Segment
Width (4 x RNP)
RF Leg (Constant
Radius Arc)
Course Centerline
(RF Flight Track)
Arc Ending Point
(Segment Terminating Fix)
Next
Segment
Radius 1.5 NM
(Example)
Arc Center Point
(Arc Center Fix)
Previous
Segment
Arc Initial Point
(Segment Initial Fix)
RNP 0.11
(Example)
RNP 0.11
RNP 0.11
4 RNP
2 RNP
2 RNP
P V
LE
c Ce
nter
nter Fix
Poin
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本文链接地址：
Instrument Procedures Handbook (IPH)仪表程序手册上(174)
