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service volume. They are subject to line-of-sight
restrictions, and the range varies proportionally to the
altitude of the receiving equipment.
NOTENormal
service ranges for the various classes of VORs are
given in Navigational Aid (NAVAID) Service Volumes,
paragraph 1-1-8.
b. Most VORs are equipped for voice transmission
on the VOR frequency. VORs without voice
capability are indicated by the letter “W” (without
voice) included in the class designator (VORW).
c. The only positive method of identifying a VOR
is by its Morse Code identification or by the recorded
automatic voice identification which is always
indicated by use of the word “VOR” following the
range's name. Reliance on determining the identification
of an omnirange should never be placed on
listening to voice transmissions by the Flight Service
Station (FSS) (or approach control facility) involved.
Many FSSs remotely operate several omniranges
with different names. In some cases, none of the
VORs have the name of the “parent” FSS. During
AIM 2/14/08
1-1-2 Navigation Aids
periods of maintenance, the facility may radiate a
T-E-S-T code (--) or the code may be
removed.
d. Voice identification has been added to numerous
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 alignment
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.
1-1-4. VOR Receiver Check
a. The FAA VOR test facility (VOT) transmits a
test signal which provides users a convenient means
to determine the operational status and accuracy of a
VOR receiver while on the ground where a VOT is
located. The airborne use of VOT is permitted;
however, its use is strictly limited to those
areas/altitudes specifically authorized in the A/FD or
appropriate supplement.
b. To use the VOT service, tune in the VOT
frequency on your VOR receiver. With the Course
Deviation Indicator (CDI) centered, the omnibearing
selector should read 0 degrees with the
to/from indication showing “from” or the omnibearing
selector should read 180 degrees with the
to/from indication showing “to.” Should the VOR
receiver operate an RMI (Radio Magnetic Indicator),
it will indicate 180 degrees on any omni-bearing
selector (OBS) setting. Two means of identification
are used. One is a series of dots and the other is a
continuous tone. Information concerning an individual
test signal can be obtained from the local FSS.
c. Periodic VOR receiver calibration is most
important. If a receiver's Automatic Gain Control or
modulation circuit deteriorates, it is possible for it to
display acceptable accuracy and sensitivity close into
the VOR or VOT and display out-of-tolerance
readings when located at greater distances where
weaker signal areas exist. The likelihood of this
deterioration varies between receivers, and is
generally considered a function of time. The best
assurance of having an accurate receiver is periodic
calibration. Yearly intervals are recommended at
which time an authorized repair facility should
recalibrate the receiver to the manufacturer's
specifications.
d. Federal Aviation Regulations (14 CFR
Section91.171) provides for certain VOR equipment
accuracy checks prior to flight under instrument
flight rules. To comply with this requirement and to
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