background image

AIM

10/12/17

1−1−2

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−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