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AIM 

6/17/21 

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

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

 

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

altitude en route charts and are 

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

NOTE

 

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

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 

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Navigation Aids