Aeronautical Information Manual (AIM), page 56

Index   55 -- Page 56 -- 57


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

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 Chart
Supplement U.S. 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


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

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

1-1-2 Navigation Aids

Page 56 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   55 -- Page 56 -- 57