AIM 

4/20/23

 

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Navigation Aids

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

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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 Section 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 checkpoints and airways.

3.

Certified checkpoints on the airport surface.

4.

If an airborne checkpoint is not available, select an established VOR airway. Select a prominent ground

point, preferably more than 20 NM from the VOR ground facility and maneuver the aircraft directly over the
point at a reasonably low altitude above terrain and obstructions.

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 service is
not provided by all radio repair stations. The aircraft owner or operator must determine which repair station in
the local area provides this service. A representative of the repair station must make an entry into the aircraft
logbook or other permanent record certifying to the radial accuracy and the date of transmission. The owner,
operator or representative of the repair station may accomplish the necessary checks in the aircraft and make a
logbook entry stating the results. It is necessary to verify which test radial is being transmitted and whether you
should get a “to” or “from” indication.

f.

Airborne and ground check points consist of certified radials that should be received at specific points on

the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport.

1.

Should an error in excess of plus or minus 4 degrees be indicated through use of a ground check, or plus

or minus 6 degrees using the airborne check, Instrument Flight Rules (IFR) flight must not be attempted without
first correcting the source of the error.

CAUTION

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No correction other than the correction card figures supplied by the manufacturer should be applied in making these
VOR receiver checks.

2.

Locations of airborne check points, ground check points and VOTs are published in the Chart Supplement

U.S.

3.

If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft,

one system may be checked against the other. Turn both systems to the same VOR ground facility and note the
indicated bearing to that station. The maximum permissible variations between the two indicated bearings is 4
degrees.

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5. Tactical Air Navigation (TACAN)

a.

For reasons peculiar to military or naval operations (unusual siting conditions, the pitching and rolling of

a naval vessel, etc.) the civil VOR/Distance Measuring Equipment (DME) system of air navigation was