AIM
4/3/14
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Navigation Aids
removed. Some VOR receivers are capable of
identifying the VOR and will display the identifier of
the VOR if it has successfully done so. However, it is
still the pilot’s responsibility to verify the identity of
the VOR by conventional methods.
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.
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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−bear-
ing 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
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 Sec-
tion 91.171) provides for certain VOR equipment
accuracy checks prior to flight under instrument
flight rules. To comply with this requirement and to
ensure satisfactory operation of the airborne system,
the FAA has provided pilots with the following means
of checking VOR receiver accuracy:
1.
VOT or a radiated test signal from an
appropriately rated radio repair station.
2.
Certified airborne check points.
3.
Certified check points on the airport surface.
e.
A radiated VOT from an appropriately rated
radio repair station serves the same purpose as an
FAA VOR signal and the check is made in much the
same manner as a VOT with the following
differences:
1.
The frequency normally approved by the
Federal Communications Commission is
108.0 MHz.
2.
Repair stations are not permitted to radiate the
VOR test signal continuously; consequently, the
owner or operator must make arrangements with the
repair station to have the test signal transmitted. This