AIM

8/15/19

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Navigation Aids

removed. Some VOR equipment decodes the
identifier and displays it to the pilot for verification
to charts, while other equipment simply displays the
expected identifier from a database to aid in
verification to the audio tones. You should be familiar
with your equipment and use it appropriately.  If your
equipment automatically decodes the identifier, it is
not necessary to listen to the audio identification.

d.

Voice identification has been added to numer-

ous VORs. The transmission consists of a voice
announcement, “AIRVILLE VOR” alternating with
the usual Morse Code identification.

e.

The effectiveness of the VOR depends upon

proper use and adjustment of both ground and
airborne equipment.

1. Accuracy.

The accuracy of course align-

ment of the VOR is excellent, being generally plus or
minus 1 degree.

2. Roughness.

On some VORs, minor course

roughness may be observed, evidenced by course
needle or brief flag alarm activity (some receivers are
more susceptible to these irregularities than others).
At a few stations, usually in mountainous terrain, the
pilot may occasionally observe a brief course needle
oscillation, similar to the indication of “approaching
station.” Pilots flying over unfamiliar routes are
cautioned to be on the alert for these vagaries, and in
particular, to use the “to/from” indicator to determine
positive station passage.

(a)

Certain propeller revolutions per minute

(RPM) settings or helicopter rotor speeds can cause
the VOR Course Deviation Indicator to fluctuate as
much as plus or minus six degrees. Slight changes to
the RPM setting will normally smooth out this
roughness. Pilots are urged to check for this
modulation phenomenon prior to reporting a VOR
station or aircraft equipment for unsatisfactory
operation.

f. The VOR Minimum Operational Network

(MON).

 As flight procedures and route structure

based on VORs are gradually being replaced with
Performance

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Based Navigation (PBN) procedures,

the FAA is removing selected VORs from service.
PBN procedures are primarily enabled by GPS and its
augmentation systems, collectively referred to as
Global Navigation Satellite System (GNSS). Aircraft
that carry DME/DME equipment can also use RNAV
which provides a backup to continue flying PBN

during a GNSS disruption. For those aircraft that do
not carry DME/DME, the FAA is retaining a limited
network of VORs, called the VOR MON, to provide
a basic conventional navigation service for operators
to use if GNSS becomes unavailable. During a GNSS
disruption, the MON will enable aircraft to navigate
through the affected area or to a safe landing at a
MON airport without reliance on GNSS. Navigation
using the MON will not be as efficient as the new
PBN route structure, but use of the MON will provide
nearly continuous VOR signal coverage at 5,000 feet
AGL across the NAS, outside of the Western U.S.
Mountainous Area (WUSMA).

NOTE

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There is no plan to change the NAVAID and route structure
in the WUSMA.

The VOR MON has been retained principally for IFR
aircraft that are not equipped with DME/DME
avionics. However, VFR aircraft may use the MON
as desired. Aircraft equipped with DME/DME
navigation systems would, in most cases, use
DME/DME to continue flight using RNAV to their
destination. However, these aircraft may, of course,
use the MON.

1. Distance to a MON airport.

 The VOR MON

will ensure that regardless of an aircraft’s position in
the contiguous United States (CONUS), a MON
airport (equipped with legacy ILS or VOR
approaches)  will be within 100 nautical miles. These
airports are referred to as “MON airports” and will
have an ILS approach or a VOR approach if an ILS
is not available. VORs to support these approaches
will be retained in the VOR MON. MON airports are
charted on low

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altitude en route charts and are

contained in the Chart Supplement U.S. and other
appropriate publications.

NOTE

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Any suitable airport can be used to land in the event of a
VOR outage. For example, an airport with a DME

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re-

quired ILS approach may be available and could be used
by aircraft that are equipped with DME. The intent of the
MON airport is to provide an approach that can be used by
aircraft without ADF or DME when radar may not be
available.

2. Navigating to an airport.

 The VOR MON

will retain sufficient VORs and increase VOR service
volume to ensure that pilots will have nearly
continuous signal reception of a VOR when flying at
5,000 feet AGL. A key concept of the MON is to
ensure that an aircraft will always be within 100 NM