Below is a list of generic checks that should be used when
checking the accuracy of the system prior to flight.
1. System initialization—pilots should confirm that
the navigation database is current and verify that the
aircrafts present position has been entered correctly.
2. Active flight plan check—the active flight plan
should be checked by comparing the aeronautical
charts, departure and arrival procedures, and other
applicable documents with the map display.
Figure 2-53. Fly-by and fly-over waypoints.
Fly-by waypoint
Fly-over waypoint
Alpha
Bravo
Bravo
Alpha
3. Prior to takeoff—ensure that the RNAV system is
available. If possible, check to see that the system
is updating when aircraft position is changing.
Note: While in flight, continue to verify system accuracy
by displaying bearing/range to a VOR/DME on the RNAV
system and compare it to the actual RMI reading of that
particular NAVAID.
Waypoints
Waypoints are predetermined geographical locations that
are defined in terms of latitude/longitude coordinates or
fixes, used to define an RNAV route or the flight path of
an aircraft employing RNAV. Waypoints may be a simple
named point in space or may be associated with existing
NAVAIDs, intersections, or fixes. A waypoint is most often
used to indicate a change in direction, speed, or altitude
along the desired path. Aviation RNAV procedures make
use of both fly-over and fly-by waypoints. A fly-over
waypoint is a waypoint that must be crossed vertically by
an aircraft. A fly-by waypoint is a waypoint that marks the
intersection of two straight paths, with the transition from
one path to another being made by the aircraft using a
precisely calculated turn that flies by but does not vertically
cross the waypoint. [Figure 2-53]
User-Defined Waypoints
Pilots typically create user-defined waypoints for use in
their own random RNAV direct navigation. They are newly
established, unpublished airspace fixes that are designated
geographic locations/positions that help provide positive
course guidance for navigation and a means of checking
progress on a flight. They may or may not be actually
plotted by the pilot on en route charts, but would normally
be communicated to ATC in terms of bearing and distance
or latitude/longitude. An example of user-defined
waypoints typically includes those generated by various
means including keyboard input, and even electronic map
mode functions used to establish waypoints with a cursor
on the display.
Another example is an offset phantom waypoint, which is
a point-in-space formed by a bearing and distance from
NAVAIDs, such as VORTACs and tactical air navigation
(TACAN) stations, using a variety of navigation systems.
When specifying unpublished waypoints in a flight plan,
they can be communicated using the frequency/bearing/
distance format or latitude and longitude, and they
automatically become compulsory reporting points unless
otherwise advised by ATC. All aircraft with latitude and
longitude navigation systems flying above FL 390 must use
latitude and longitude to define turning points.
Floating Waypoints
Floating waypoints, or reporting points, represent
airspace fixes at a point in space not directly associated
with a conventional airway. In many cases, they may be
established for such purposes as ATC metering fixes,
holding points, RNAV-direct routing, gateway waypoints,
STAR origination points leaving the en route structure,
and SID terminating points joining the en route structure.
In the top example of Figure 2-54, a low altitude en route
chart depicts three floating waypoints that have been
highlighted: SCORR, FILUP , and CHOOT. Notice that
waypoints are named with five-letter identifiers that are
unique and pronounceable. Pilots must be careful of
similar waypoint names. Notice on the high altitude en
route chart excerpt in the bottom example, the similar
sounding and spelled floating waypoint named SCOOR,
rather than SCORR. This emphasizes the importance
of correctly entering waypoints into database-driven
navigation systems. One waypoint character incorrectly
entered into your navigation system could adversely affect
your flight. The SCOOR floating reporting point also is
Figure 2-54. Floating waypoints.
depicted on a Severe Weather Avoidance Plan (SWAP) en
route chart. These waypoints and SWAP routes assist pilots
and controllers when severe weather affects the East Coast.
Computer Navigation Performance
An integral part of RNAV using en route charts typically
involves the use of airborne navigation databases.
Because GPS receivers are basically “to-to” navigators,
they must always be navigating to a defined point. On
overlay approaches, if no pronounceable five-character
name is published for an approach waypoint or fix, it has
been given a database identifier consisting of letters and
numbers. These points appear in the list of waypoints in the
approach procedure database, but may not appear on the
approach chart. A point used for the purpose of defining
the navigation track for an airborne computer system (i.e.,
GPS or FMS) is called a Computer Navigation Fix (CNF). CNFs
include unnamed DME fixes, beginning and ending points
of DME arcs, and sensor final approach fixes (FAFs) on some
GPS overlay approaches.
To aid in the approach chart/database correlation process,
the FAA has begun a program to assign five-letter names
to CNFs and to chart CNFs on various National Oceanic
Service aeronautical products. [Figure 2-55] These CNFs
are not to be used for any ATC application, such as holding
for which the fix has not already been assessed. CNFs are
charted to distinguish them from conventional reporting
points, fixes, intersections, and waypoints. A CNF name is
enclosed in parenthesis, e.g., (MABEE) and is placed next
to the CNF it defines. If the CNF is not at an existing point
defined by means such as crossing radials or radial/DME,
the point is indicated by an X. The CNF name is not used in
filing a flight plan or in aircraft/ATC communications. Use
current phraseology (e.g., facility name, radial, distance) to
describe these fixes.
NOT FOR NAVIGATION
Figure 2-55. Computer navigation fix.
Many of the RNAV systems available today make it all
too easy to forget that en route charts are still required
and necessary for flight. As important as databases are,
they really are onboard the aircraft to provide navigation
guidance and situational awareness (SA); they are not
intended as a substitute for paper charts. When flying
with GPS, FMS, or planning a flight with a computer, it is
critical to understand the limitations of the system you are
using, for example, incomplete information, unloadable
procedures, complex procedures, and database storage
limitations.
Required Navigation Performance
Required navigation performance (RNP) is RNAV with
onboard navigation monitoring and alerting. RNP is also
a statement of navigation performance necessary for
operation within a defined airspace. A critical component
of RNP is the ability of the aircraft navigation system to
monitor its achieved navigation performance, and to
identify for the pilot whether the operational requirement
is, or is not being met during an operation. This onboard
performance monitoring and alerting capability;
therefore, allows a lessened reliance on ATC intervention
(via radar monitoring, automatic dependent surveillance-
broadcast (ADS-B), multilateration, communications),
and/or route separation to achieve the overall safety of
the operation. RNP capability of the aircraft is a major
component in determining the separation criteria to
ensure that the overall containment of the operation is
met.
The RNP capability of an aircraft varies depending upon
the aircraft equipment and the navigation infrastructure.
For example, an aircraft may be equipped and certified
for RNP 1.0, but may not be capable of RNP 1.0 operations
due to limited NAVAID coverage.
0.1 to 1.0
0.3 to 1.0
RNP AR Approach Segments
RNP Approach Segments
Terminal and En Route
En Route
0.1 to 1.0
0.3 to 1.0
1.0
2.0
RNP Level Typical Application Primary Route Width (NM) - Centerline to Boundary
Figure 2-56. U.S. standard RNP levels.
RNP Levels
An RNP level or type is applicable to a selected airspace,
route, or procedure. As defined in the Pilot/Controller
Glossary, the RNP level or type is a value typically expressed
as a distance in nautical miles from the intended centerline
of a procedure, route, or path. RNP applications also
account for potential errors at some multiple of RNP level
(e.g., twice the RNP level).
Standard RNP Levels
United States standard values supporting typical RNP
airspace are shown in Figure 2-56. Other RNP levels as
identified by ICAO, other states, and the FAA may also be
used.
Application of Standard RNP Levels
United States standard levels of RNP typically used for
various routes and procedures supporting RNAV operations
may be based on use of a specific navigational system
or sensor, such as GPS, or on multi-sensor RNAV systems
having suitable performance.
Note: The performance of navigation in RNP refers not only
to the level of accuracy of a particular sensor or aircraft
navigation system, but also to the degree of precision
with which the aircraft is flown. Specific required flight
procedures may vary for different RNP levels.
IFR En Route Altitudes
Minimum En Route Altitudes (MEAs), Minimum Reception
Altitudes (MRAs), Maximum Authorized Altitudes (MAAs),
Minimum Obstacle Clearance Altitudes (MOCAs), Minimum
Turning Altitudes (MTAs) and Minimum Crossing Altitudes
(MCAs) are established by the FAA for instrument flight
along Federal airways, as well as some off-airway routes.
The altitudes are established after it has been determined
that the NAVAIDs to be used are adequate and so oriented
on the airways or routes that signal coverage is acceptable,
and that flight can be maintained within prescribed route
widths.
For IFR operations, regulations require that pilots operate
their aircraft at or above minimum altitudes. Except when
necessary for takeoff or landing, pilots may not operate an
aircraft under IFR below applicable minimum altitudes, or
if no applicable minimum altitude is prescribed, in the case
of operations over an area designated as mountainous, an
altitude of 2,000 feet above the highest obstacle within a
horizontal distance of 4 NM from the course to be flown. In
any other case, an altitude of 1,000 feet above the highest
obstacle within a horizontal distance of 4 NM from the
course to be flown must be maintained as a minimum
altitude. If both a MEA and a MOCA are prescribed for a
particular route or route segment, pilots may operate an
aircraft below the MEA down to, but not below, the MOCA,
only when within 22 NM of the VOR. When climbing to
a higher minimum IFR altitude (MIA), pilots must begin
climbing immediately after passing the point beyond
which that minimum altitude applies, except when ground
obstructions intervene, the point beyond which that higher
minimum altitude applies must be crossed at or above the
applicable MCA for the VOR.
If on an IFR flight plan, but cleared by ATC to maintain VFR
conditions on top, pilots may not fly below minimum en
route IFR altitudes. Minimum altitude rules are designed to
ensure safe vertical separation between the aircraft and the
terrain. These minimum altitude rules apply to all IFR flights,
whether in IFR or VFR weather conditions, and whether
assigned a specific altitude or VFR conditions on top.
Minimum En Route Altitude (MEA)
The MEA is the lowest published altitude between radio
fixes that assures acceptable navigational signal coverage
and meets obstacle clearance requirements between those
fixes. The MEA prescribed for a Federal airway or segment,
RNAV low or high route, or other direct route applies to the
entire width of the airway, segment, or route between the
radio fixes defining the airway, segment, or route. MEAs
for routes wholly contained within controlled airspace
normally provide a buffer above the floor of controlled
airspace consisting of at least 300 feet within transition
areas and 500 feet within control areas. MEAs are established
based upon obstacle clearance over terrain and manmade
objects, adequacy of navigation facility performance, and
communications requirements.
RNAV Minimum En Route Altitude
RNAV MEAs are depicted on some IFR en route low altitude
charts, allowing both RNAV and non-RNAV pilots to use the
same chart for instrument navigation.
Minimum Reception Altitude (MRA)
MRAs are determined by FAA flight inspection traversing
an entire route of flight to establish the minimum altitude
the navigation signal can be received for the route and for
off-course NAVAID facilities that determine a fix. When the
MRA at the fix is higher than the MEA, an MRA is established
for the fix and is the lowest altitude at which an intersection
can be determined.
Maximum Authorized Altitude (MAA)
An MAA is a published altitude representing the maximum
usable altitude or flight level for an airspace structure
Figure 2-57. Maximum authorized altitude (MAA).
Figure 2-58. Minimum obstacle clearance altitude (MOCA).
Minimum obstacle clearance altitude
or route segment. [Figure 2-57] It is the highest altitude
on a Federal airway, jet route, RNAV low or high route,
or other direct route for which an MEA is designated at
which adequate reception of navigation signals is assured.
MAAs represent procedural limits determined by technical
limitations or other factors, such as limited airspace or
frequency interference of ground-based facilities.
Minimum Obstruction Clearance Altitude
(MOCA)
The MOCA is the lowest published altitude in effect between
fixes on VOR airways, off-airway routes, or route segments
that meets obstacle clearance requirements for the entire
route segment. [Figure 2-58] This altitude also assures
acceptable navigational signal coverage only within 22 NM
of a VOR. The MOCA seen on the en route chart may have
been computed by adding the required obstacle clearance
(ROC) to the controlling obstacle in the primary area or
computed by using a TERPS chart if the controlling obstacle
is located in the secondary area. This figure is then rounded
to the nearest 100 foot increment (i.e., 2,049 feet becomes
2,000, and 2,050 feet becomes 2,100 feet). An extra 1,000
MTA
V330 E to V520 W 16000
V465 NE to 330 W or V520 W 16000
Figure 2-59. Minimum turning altitude (MTA).
Centerline
3.6°
4.5°
Fix displacement area
En route facility
Facility providing intersection radial
Primary area
Secondary area
Figure 2-60. Turning area at the intersection fix with NAVAID distance less than 51 NM.
feet is added in mountainous areas, in most cases.
ATC controllers have an important role in helping pilots
remain clear of obstructions. Controllers are instructed to
issue a safety alert if the aircraft is in a position that, in their
judgment, places the pilot in unsafe proximity to terrain,
obstructions, or other aircraft. Once pilots inform ATC of
action being taken to resolve the situation, the controller
may discontinue the issuance of further alerts. A typical
terrain/obstruction alert may sound like this: “(Aircraft call
sign ), Low altitude alert. Check your altitude immediately.
The MOCA in your area is 12,000. ”
Minimum Turning Altitude (MTA)
Minimum turning altitude (MTA) is a charted altitude
providing vertical and lateral obstruction clearance based
on turn criteria over certain fixes, NAVAIDs, waypoints,
and on charted route segments. [Figure 2-59] When a
VHF airway or route terminates at a NAVAID or fix, the
primary area extends beyond that termination point.
When a change of course on VHF airways and routes is
necessary, the en route obstacle clearance turning area
extends the primary and secondary obstacle clearance
areas to accommodate the turn radius of the aircraft. Since
turns at or after fix passage may exceed airway and route
boundaries, pilots are expected to adhere to airway and
route protected airspace by leading turns early before a
fix. The turn area provides obstacle clearance for both turn
anticipation (turning prior to the fix) and flyover protection
(turning after crossing the fix). This does not violate the
requirement to fly the centerline of the airway. Many factors
enter into the construction and application of the turning
area to provide pilots with adequate obstacle clearance
protection. These may include aircraft speed, the amount
of turn versus NAVAID distance, flight track, curve radii,
MEAs, and MTA. [Figure 2-60]
Due to increased airspeeds at 10,000 feet MSL or above, an
expanded area in the vicinity of the turning fix is examined
to ensure the published MEA is sufficient for obstacle
clearance. In some locations (normally mountainous),
terrain/obstacles in the expanded search area may obviate
the published MEA and necessitate a higher minimum
altitude while conducting the turning maneuver. Turning
fixes requiring a higher MTA are charted with a flag along
with accompanying text describing the MTA restriction.
[Figure 2-59]
An MTA restriction normally consists of the ATS route
leading to the turning fix, the ATS route leading from the
turning fix, and an altitude (e.g., MTA V330 E TO V520
W 16000). When an MTA is applicable for the intended
route of flight, pilots must ensure they are at or above the
charted MTA prior to beginning the turn and maintain at
Airway
number
or route
V330 *9500E
# MTA
* 13400WJAC 10
300 MTN ROC RED
DEL MCA ATIDA
COME ADD MCA
AT OSITY DEC MOCA
INC MCA PRECIP
TER DEC MOCA
MEA CARDINAL ALT
JAC R-251 UNUSABLE
BYD 10 # CHART:
MTA V330 E TO
VS20W 16000
DEL directional MEA
MEA CARONIAL ALT
Date Office Title Signature
From
To
Idaho Falls, ID VOR/DME
*Osity, ID
Osity, ID
# Jackson, WY VOR/DME
Routine
or docket
number
GNSS
MEA
Change
over point
Fix
MRA/MCA Remarks
Flight
inspection
dates
Controlling @
terrain/Obstruction
and coordinates
Tree 6177 @
432912.00N/1114118.00W
Terrain 6077
432912.00N/1114118.00W
AAO 12138 (SEC) @
434118.30N/1104858.30W
Terrain 11132
433900.00N/1105057.00W
MRA
MOCA
MAA
MEA
--7900-
--13600-
Transmittal of Airways/Route Data
AJW-3773 Manager Ray Nussear
Figure 2-61. Minimum turning altitude information located in the remarks section of FAA Form 8260-16 Transmittal of Airways/Route Data.