Chapter 6
Introduction
Area Navigation (RNAV) systems, aeronautical applications,
and functions that depend on databases are widespread.
[Figure 6-1] Since the 1970s, installed flight systems
have relied on airborne navigation databases to support
their intended functions, such as navigation data used
to facilitate the presentation of flight information to the
flight crew and understanding and better visualization
of the governing aeronautical flight charts. With the
overwhelming upgrades to navigation systems and fully
integrated flight management systems (FMS) that are now
installed in almost all corporate and commercial aircraft,
the need for reliable and consistent airborne navigation
databases is more important than ever.
Airborne Navigation Databases
Figure 6-1. Area navigation (RNAV) receivers.
The capabilities of airborne navigation databases depend
largely on the way they are implemented by the avionics
manufacturers. They can provide data about a large
variety of locations, routes, and airspace segments for use
by many different types of RNAV equipment. Databases
can provide pilots with information regarding airports,
air traffic control (ATC) frequencies, runways, and special
use airspace. Without airborne navigation databases,
RNAV would be extremely limited. In order to understand
the capabilities and limitations of airborne navigation
databases, pilots must understand the way databases
are compiled and revised by the database provider and
processed by the avionics manufacturer. Vital to this
discussion is understanding of the regulations guiding
database maintenance and use.
There are many different types of RNAV systems certified
for instrument flight rules (IFR) use in the National Airspace
System (NAS). The two most prevalent types are GPS and
the multisensory FMS. [Figure 6-2] A modern GPS unit
accurately provides the pilot with the aircraft’s present
position; however, it must use an airborne navigation
database to determine its direction or distance from
another location. The database provides the GPS with
Figure 6-2. GPS with a flight route on display.
RNG
PULL SCAN
PUSH ON
BRT
PROC
CRSR
MSG
OBS
ALT
NRST
CLR
ENT
MENU
DTK
TK
. nm
LEG
VOR NDB INT USR ACT NAV FPL SET AUXAPT 1
SPC
BKSP
PFD
MFD
NAV
COM
PFD
MENU
FPL
PROC
CLR
ENT
SEL
DFLT MAP
SOFTKEY SELECT
EMERG
PUSH CRSR/PUSH 1-2
FMS/
NAV-COM
PUSH PAN
RANGE
+−
1 2 3
4 5 6
7 8 9
− 0 +
The display allows you to view information stored in the FMS.
Controls such as
buttons and knobs
allow you to make
entries into the FMS.
position information for navigation fixes so it may perform
the required geodetic calculations to determine the
appropriate tracks, headings, and distances to be flown.
[Figure 6-3]
Modern FMS are capable of a large number of functions
including basic en route navigation, complex departure
and arrival navigation, fuel planning, and precise vertical
navigation. Unlike stand-alone navigation systems, most
FMS use several navigation inputs. Typically, they formulate
the aircraft’s current position using a combination of
conventional distance measuring equipment (DME) signals,
inertial navigation systems (INS), GPS receivers, or other
RNAV devices. Like stand-alone navigation avionics, they
rely heavily on airborne navigation databases to provide
the information needed to perform their numerous
functions.
Airborne Navigation Database Standardization
Beginning in the 1970s, the requirement for airborne
navigation databases became more critical. In 1973,
Figure 6-3. FMS display.
National Airlines installed the Collins ANS-70 and AINS
70 RNAV systems in their DC-10 fleet, which marked the
first commercial use of avionics that required navigation
databases. A short time later, Delta Air Lines implemented
the use of an ARMA Corporation RNAV system that also
used a navigation database. Although the type of data
stored in the two systems was basically identical, the
designers created the databases to solve the individual
problems of each system, which meant that they were not
interchangeable. As the implementation of RNAV systems
expanded, a world standard for airborne navigation
databases was needed.
In 1973, Aeronautical Radio, Inc. (ARINC) sponsored the
formation of a committee to standardize aeronautical
databases. In 1975, the committee published the first
standard, ARINC Specification 424, which has remained the
worldwide accepted format for transmission of navigation
databases.
ARINC 424
ARINC 424 is the air transport industry’s recommended
standard for the preparation and transmission of data for
the assembly of airborne system navigation databases. The
data is intended for merging with the aircraft navigation
system software to provide a source of navigation
reference. Each subsequent version of ARINC 424
Specification provides additional capability for navigation
systems to utilize. Merging of ARINC 424 data with each
manufacturer’s system software is unique and ARINC 424
leg types provide vertical guidance and ground track for
a specific flight procedure. These leg types must provide
repeatable flight tracks for the procedure design. The
navigation database leg type is the path and terminator
concept.
ARINC 424 Specification describes 23 leg types by their path
and terminator. The path describes how the aircraft gets to
the terminator by flying direct (a heading, a track, a course,
etc.). The terminator is the event or condition that causes
the navigation computer system to switch to the next leg (a
fix, an altitude, an intercept, etc.). When a flight procedure
instructs the pilot to fly runway heading to 2000 feet then
direct to a fix, this is the path and terminator concept. The
path is the heading and the terminator is 2000 feet. The
next leg is then automatically sequenced. A series of leg
types are coded into a navigation database to make a flight
procedure. The navigation database allows an FMS or GPS
navigator to create a continuous display of navigational
data, thus enabling an aircraft to be flown along a specific
route. Vertical navigation can also be coded.
The data included in an airborne navigation database is
organized into ARINC 424 records. These records are strings
of characters that make up complex descriptions of each
navigation entity. ARINC records can be sorted into four
general groups: fix records, simple route records, complex
route records, and miscellaneous records. Although it is not
important for pilots to have in-depth knowledge of all the
fields contained in the ARINC 424 records, pilots should be
aware of the types of records contained in the navigation
database and their general content.
Fix Records
Database records that describe specific locations on the
face of the earth can be considered fix records. Navigational
aids (NAVAIDs), waypoints, intersections, and airports are
all examples of this type of record. These records can be
used directly by avionics systems and can be included as
parts of more complex records like airways or approaches.
Another concept pilots should understand relates to how
aircraft make turns over navigation fixes. Fixes can be
designated as fly-over or fly-by, depending on how they
are used in a specific route. [Figure 6-4] Under certain
circumstances, a navigation fix is designated as fly-over.
This simply means that the aircraft must actually pass
directly over the fix before initiating a turn to a new course.
Conversely, a fix may be designated fly-by, allowing an
aircraft’s navigation system to use its turn anticipation
feature, which ensures that the proper radius of turn is
commanded to avoid overshooting the new course. Some
RNAV systems are not programmed to fully use this feature.
It is important to remember a fix can be coded as fly-over
and fly-by in the same procedure, depending on how the
fix is used (i.e., holding at an initial approach fix). RNAV or
GPS stand-alone IAPs are flown using data pertaining to
the particular IAP obtained from an onboard database
to include the sequence of all waypoints used for the
approach and missed approach, except that step down
waypoints may not be included in some TSO-C129 receiver
databases. Included in the database, in most receivers, is
coding that informs the navigation system of which WPs
are fly-over or fly-by. The navigation system may provide
guidance appropriately to include leading the turn prior
to a fly-by waypoint; or causing over flight of a fly-over
waypoint. Where the navigation system does not provide
such guidance, the pilot must accomplish the turn lead or
waypoint over flight manually. Chart symbology for the fly
by waypoint provides pilot awareness of expected actions.
Simple Route Records
Route records are those that describe a flightpath
instead of a fixed position. Simple route records contain
Flight plan path
Aircraft track
Waypoint
Waypoint
Fly-by
Fly-over
Figure 6-4. Fly-by-waypoints and fly-over-waypoints.
strings of fix records and information pertaining to how
the fixes should be used by the navigation avionics.
A Victor Airway, for example, is described in the database by
a series of en route airway records that contain the names
of fixes in the airway and information about how those
fixes make up the airway.
Complex Route Records
Complex route records include those strings of fixes that
describe complex flightpaths like standard instrument
departures (SIDs), standard terminal arrival routes (STARs),
and instrument approach procedures (IAPs). Like simple
routes, these records contain the names of fixes to be
used in the route, as well as instructions on how the route
is flown.
Miscellaneous Records
There are several other types of information that is coded
into airborne navigation databases, most of which deal
with airspace or communications. The receiver may contain
additional information, such as restricted airspace, airport
minimum safe altitudes, and grid minimum off route
altitudes (MORAs).
Path and Terminator Concept
The path and terminator concept is a means to permit
coding of terminal area procedures, SIDs, STARs, and
approach procedures. Simply put, a textual description of
a route or a terminal procedure is translated into a format
that is useable in RNAV systems. One of the most important
concepts for pilots to learn regarding the limitations of
RNAV equipment has to do with the way these systems
deal with the path and terminator field included in complex
route records.
The first RNAV systems were capable of only one type of
navigation; they could fly directly to a fix. This was not a
problem when operating in the en route environment
in which airways are mostly made up of direct routes
between fixes. The early approaches for RNAV did not
present problems for these systems and the databases
they used because they consisted mainly of DME/DME
overlay approaches flown only direct point-to-point
navigation. The desire for RNAV equipment to have the
ability to follow more complicated flightpaths necessitated
the development of the path and terminator field that is
included in complex route records.
Path and Terminator Legs
There are currently 23 different leg types, or path and
terminators that have been created in the ARINC 424
standard that enable RNAV systems to follow the complex
paths that make up instrument departures, arrivals, and
approaches. They describe to navigation avionics a path
to be followed and the criteria that must be met before
the path concludes and the next path begins. Although
there are 23 leg types available, none of the manufactured
database equipment is capable of using all of the leg types.
Pilots must continue to monitor procedures for accuracy
and not rely solely on the information that the database
is showing. If the RNAV system does not have the leg type
Figure 6-5. Initial fix.
Figure 6-7. Constant radius arc or RF leg.
Nextsegment
RFLEG
ARC
CENTER
FIX
Previoussegment
demanded by procedures, data packers have to select one
or a combination of available lleg types to give the best
approximation, which can result in an incorrect execution
of the procedure. Below is a list of the 23 leg types and
their uses that may or may not be used by all databases.
TF LEG
Figure 6-6. Track to a fix leg type.
Figure 6-8. Course to a fix or CF leg.
CF LEG
080°
Course is flown making adjustment for wind
DF LEG
Unspecified position
• Initial fix or IF leg—defines a database fix as a point
in space and is only required to define the beginning
of a route or procedure. [Figure 6-5]
• Track to a fix or TF leg—defines a great circle track
over the ground between two known database
fixes and the preferred method for specification of
straight legs (course or heading can be mentioned
on charts but designer should ensure TF leg is used
for coding). [Figure 6-6]
• Constant radius arc or RF leg—defines a constant
radius turn between two databases fixes, lines
tangent to the arc, and a center fix. [Figure 6-7]
• Course to a fix or CF leg—defines a specified course
to a specific database fix. Whenever possible, TF legs
Figure 6-9. Direct to a fix or DF leg.
Figure 6-10. Fix to an altitude or FA leg.
FA LEG
080°
Unspecified position
8,000'
FA leg is flown making adjustment for wind