the pilot to make an immediate turn toward the Durango
VOR instead of flying the series of headings that terminate
at specific altitudes as dictated by the approach procedure.
[Figure 6-28] Pilots must be aware of their individual
systems Path and Terminator handling characteristics
and always review the manufacturer’s documentation to
familiarize themselves with the capabilities of the RNAV
equipment they are operating. Pilots should be aware that
some RNAV equipment was designed without the fly-over
capability which can cause problems for pilots attempting
to use this equipment to fly complex flightpaths in the
departure, arrival, or approach environments.
Role of the Database Provider
Compiling and maintaining a worldwide airborne
navigation database is a large and complex job. Within the
United States, the FAA sources give the database providers
information, in many different formats, which must be
analyzed, edited, and processed before it can be coded
into the database. In some cases, data from outside the
United States must be translated into English so it may be
analyzed and entered into the database. Once the data is
coded, it must be continually updated and maintained.
Once the FAA notifies the database provider that a change
is necessary, the update process begins. The change is
incorporated into a 28-day airborne database revision cycle
based on its assigned priority. If the information does not
reach the coding phase prior to its cutoff date (the date that
new aeronautical information can no longer be included
in the next update), it is held out of revision until the next
cycle. The cutoff date for aeronautical databases is typically
21 days prior to the effective date of the revision.
The integrity of the data is ensured through a process called
cyclic redundancy check (CRC). A CRC is an error detection
algorithm capable of detecting small bit-level changes in
a block of data. The CRC algorithm treats a data block as
a single, large binary value. The data block is divided by a
fixed binary number called a generator polynomial whose
form and magnitude is determined based on the level of
integrity desired. The remainder of the division is the CRC
value for the data block. This value is stored and transmitted
with the corresponding data block. The integrity of the
data is checked by reapplying the CRC algorithm prior to
distribution.
Role of the Avionics Manufacturer
When avionics manufacturers develop a piece of
equipment that requires an airborne navigation database,
they typically form an agreement with a database provider
to supply the database for that new avionics platform. It
is up to the manufacturer to determine what information
to include in the database for their system. In some cases,
the navigation data provider has to significantly reduce the
number of records in the database to accommodate the
storage capacity of the manufacturer’s new product, which
means that the database may not contain all procedures.
Another important fact to remember is that although there
are standard naming conventions included in the ARINC
424 specification, each manufacturer determines how
the names of fixes and procedures are displayed to the
pilot. This means that although the database may specify
EUGA <VOR 34 >APR
IAF / TRAN: D130N IF
Figure 6-29. Naming conventions of three different systems for the VOR 34 Approach.
the approach identifier field for the VOR/DME Runway
34 approach at Eugene Mahlon Sweet Airport (KEUG) in
Eugene, Oregon, as “V34, ” different avionics platforms
may display the identifier in any way the manufacturer
deems appropriate. For example, a GPS produced by one
manufacturer might display the approach as “VOR 34, ”
whereas another might refer to the approach as “VOR/DME
34, ” and an FMS produced by another manufacturer may
refer to it as “VOR34. ” [Figure 6-29]
These differences can cause visual inconsistencies between
chart and GPS displays, as well as confusion with approach
clearances and other ATC instructions for pilots unfamiliar
with specific manufacturer’s naming conventions. The
manufacturer determines the capabilities and limitations
of an RNAV system based on the decisions that it makes
regarding that system’s processing of the airborne
navigation database.
Users Role
Like paper charts, airborne navigation databases are
subject to revision. According to 14 CFR Part 91, § 91.503,
the end user (operator) is ultimately responsible for
ensuring that data meets the quality requirements for its
intended application. Updating data in an aeronautical
database is considered to be maintenance and all Part 91
operators may update databases in accordance with 14
CFR Part 91, § 43.3(g). Parts 121, 125, and 135 operators
must update databases in accordance with their approved
maintenance program. For Part 135 helicopter operators,
this includes maintenance by the pilot in accordance with
Figure 6-30. Database rolls.
14 CFR Part 43, § 43.3(h).
Pilots using the databases are ultimately responsible for
ensuring that the database they are operating with is
current. This includes checking Notices to Airmen (NOTAM)
type information concerning errors that may be supplied
by the avionics manufacturer or the database supplier. The
database user is responsible for learning how the specific
navigation equipment handles the navigation database.
The manufacturer’s documentation is the pilot’s best source
of information regarding the capabilities and limitations of
a specific database. [Figure 6-30]
Operational Limitations of Airborne Navigation
Databases
Understanding the capabilities and limitations of the
navigation systems installed in an aircraft is one of the
pilot’s biggest concerns for IFR flight. Considering the vast
number of RNAV systems and pilot interfaces available
today, it is critical that pilots and flight crews be familiar
with the manufacturer’s operating manual for each RNAV
system they operate and achieve and retain proficiency
operating those systems in the IFR environment.
Most professional and general aviation pilots are familiar
with the possible human factors issues related to flightdeck
automation. It is particularly important to consider those
issues when using airborne navigation databases. Although
modern avionics can provide precise guidance throughout
all phases of flight, including complex departures and
arrivals, not all systems have the same capabilities.
RNAV equipment installed in some aircraft is limited to
direct route point-to-point navigation. Therefore, it is
very important for pilots to familiarize themselves with
the capabilities of their systems through review of the
manufacturer documentation. Most modern RNAV systems
are contained within an integrated avionics system that
receives input from several different navigation and aircraft
system sensors. These integrated systems provide so much
information that pilots may sometimes fail to recognize
errors in navigation caused by database discrepancies or
misuse. Pilots must constantly ensure that the data they
enter into their avionics is accurate and current. Once
the transition to RNAV is made during a flight, pilots and
flight crews must always be capable and ready to revert to
conventional means of navigation if problems arise.
Closed Indefinitely Airports
Some U.S. airports have been closed for up to several years,
with little or no chance that they will ever reopen; yet their
“indefinite” closure status – as opposed to permanent or
UFN closure, or abandonment – causes them to continue
to appear on both VFR and IFR charts and in airborne
navigation databases; and their instrument approach
procedures, if any, continue to be included – and still
appear to be valid – in the paper and electronic versions
of the United States Terminal Procedures Publication (TPP)
charts. Airpark South, 2K2, at Ozark, Missouri, is a case in
point.
Even though this airport has been closed going on two
years and, due to industrial and residential development
surrounding it, likely will never be reopened, the airport
is nonetheless still charted in a way that could easily lead
a pilot to believe that it is still open and operating. Even
the current U.S. Low Altitude En route chart displays a
blue symbol for this airport, indicating that it still has
a Department of Defense (DOD) approved instrument
approach procedure available for use.
Aircrews need to use caution when selecting an airport
in a cautionary or emergency situation, especially if the
airport was not previously analyzed suitable for diversion
during preflight. Aircrews could assume, based on charts
and their FMS database, the airport is suitable and perhaps
the only available diversionary or emergency option. The
airport however, could be closed and hazardous even for
emergency use. In these situations, Air Traffic Control may
be queried for the airport’s status.
Storage Limitations
As the data in a worldwide database grows, the required
data storage space increases. Over the years that panel-
mounted GPS and FMSs have developed, the size of the
commercially available airborne navigation databases has
grown exponentially.
Some manufacturer’s systems have kept up with this
growth and some have not. Many of the limitations of
older RNAV systems are a direct result of limited data
storage capacity. For this reason, avionics manufacturers
must make decisions regarding which types of procedures
will be included with their system. For instance, older GPS
units rarely include all of the waypoints that are coded into
master databases. Even some modern FMS equipment,
which typically have much larger storage capacity, do not
include all of the data that is available from the database
producers. The manufacturers often choose not to include
certain types of data that they think is of low importance
to the usability of the unit. For example, manufacturers
of FMS used in large airplanes may elect not to include
airports where the longest runway is less than 3,000 feet
or to include all the procedures for an airport.
Manufacturers of RNAV equipment can reduce the size of
the data storage required in their avionics by limiting the
geographic area the database covers. Like paper charts, the
amount of data that needs to be carried with the aircraft is
directly related to the size of the coverage area. Depending
on the data storage that is available, this means that the
larger the required coverage area, the less detailed the
database can be.
Again, due to the wide range of possible storage capacities,
and the number of different manufacturers and product
lines, the manufacturer’s documentation is the pilot’s best
source of information regarding limitations caused by
storage capacity of RNAV avionics.
Charting/Database Inconsistencies
It is important for pilots to remember that many
inconsistencies may exist between aeronautical charts
and airborne navigation databases. Since there are so
many sources of information included in the production
of these materials, and the data is manipulated by several
different organizations before it is eventually displayed
on RNAV equipment, the possibility is high that there will
be noticeable differences between the charts and the
databases. Because of this, pilots must be familiar with the
capabilities of the database and have updated aeronautical
charts while flying to ensure the proper course is being
flown.
Naming Conventions
Obvious differences exist between the names of procedures
shown on charts and those that appear on the displays of
many 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. 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. Some systems display procedure names
exactly as they are charted, but many do not.
The naming of multiple approaches of the same type to the
same runway is also changing. Multiple approaches with
the same guidance will be annotated with an alphabetical
suffix beginning at the end of the alphabet and working
backwards for subsequent procedures (e.g., ILS Z RWY
28, ILS Y RWY 28, etc.). The existing annotations, such as
ILS 2 RWY 28 or Silver ILS RWY 28, will be phased out and
replaced with the new designation.
NAVAIDs are also subject to naming discrepancies as well.
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 databases 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 6-31]
Using the VOR/DME Runway 34 approach at Eugene
Mahlon Sweet Airport (KEUG) in Eugene, Oregon, as
another example, which is named V34, may be displayed
differently by another avionics platform. For example, a
GPS produced by one manufacturer might display the
approach as VOR 34, whereas another might refer to the
approach as VOR/DME 34, and an FMS produced by another
manufacturer may refer to it as VOR34. These differences
can cause visual inconsistencies between chart and GPS
displays, as well as confusion with approach clearances
and other ATC instructions for pilots unfamiliar with specific
manufacturer’s naming conventions.
For detailed operational guidance, refer to Advisory Circular
(AC) 90-100, U.S. Terminal and En Route Area Navigation
(RNAV) Operations; AC 90-101, Approval Guidance for
Required Navigation Performance (RNP) Procedures with
Authorization Required (AR); AC 90-105, Approval guidance
for RNP Operations and Barometic Vertical Navigation
in the U.S. National Airspace System and in Oceanic and
Remote Continental Airspace; and AC 90-107, Guidance
for Localizer Performance with Vertical Guidance and
Localizer Performance without Vertical Guidance Approach
Operations in the U.S. National Airspace System.
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 United States, 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
and must not be used for conventional NAVAIDs or airports.
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 and
Figure 6-31. Manufacturer’s naming conventions.
incorrect ground track. 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 result in 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.