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AIM

10/12/17

1−1−4

Navigation Aids

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

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 representat-

ive 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−

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.

1−1−5. 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 considered unsuitable for

military or naval use. A new navigational system,

TACAN, was therefore developed by the military and

naval forces to more readily lend itself to military and

naval requirements. As a result, the FAA has

integrated TACAN facilities with the civil VOR/

DME program. Although the theoretical, or technical

principles of operation of TACAN equipment are

quite different from those of VOR/DME facilities, the

end result, as far as the navigating pilot is concerned,

is the same. These integrated facilities are called

VORTACs.

b. TACAN ground equipment consists of either a

fixed or mobile transmitting unit. The airborne unit in

conjunction with the ground unit reduces the

transmitted signal to a visual presentation of both

azimuth and distance information. TACAN is a pulse

system and operates in the Ultrahigh Frequency

(UHF) band of frequencies. Its use requires TACAN

airborne equipment and does not operate through

conventional VOR equipment.

1−1−6. VHF Omni−directional

Range/Tactical Air Navigation (VORTAC)

a. A VORTAC is a facility consisting of two

components, VOR and TACAN, which provides

three individual services: VOR azimuth, TACAN

azimuth and TACAN distance (DME) at one site.

Although consisting of more than one component,