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or erroneous identification may be heard when a
steady false bearing is being displayed. Since ADF
receivers do not have a “flag” to warn the pilot when
erroneous bearing information is being displayed, the
pilot should continuously monitor the NDB’s
identification.
1−1−3. VHF Omni−directional Range (VOR)
a. VORs operate within the 108.0 to 117.95 MHz
frequency band and have a power output necessary to
provide coverage within their assigned operational
service volume. They are subject to line−of−sight
restrictions, and the range varies proportionally to the
altitude of the receiving equipment.
NOTE−
Normal 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
periods of maintenance, the facility may radiate a
T−E−S−T code (− −) or the code may be
removed.
AIM 2/19/04
1−1−2 Air Navigation Aids
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 omni−bearing
selector should read 0 degrees with the to/from
indication showing “from” or the omni−bearing
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
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AIM航空信息手册2004上(21)
