WIND
°
ADF
HDG 015°
015°
24 2I
ADF
HDG 015°
015°
24 2I
ADF
HDG 350°
350°
21 15
2I I5
ADF
HDG 340°
340°
2I I5
ADF
HDG 330°
330°
3W
2I
I5 I2
ADF
HDG 330°
330°
3W
2I
I5 I2
°
Station
OBS
21 15
NAV
GS
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
Figure 9-8. ADF interception and tracking outbound.
° 180
°
°
Figure 7-10. VOR radials.
Magnetic North
Magnetic South
Figure 9-10. VOR radials.
°
75°
ADF
HDG 075°
075°
ADF
HDG 355°
355°
0 10 20 30 40 50 60 70 80
INBOUND
INTERCEPTION
°
°
30 24
30 24
2I
21 15
2I I5
Station
OBS
21 15
NAV
GS
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
Figure 9-9. Interception of bearing.
well as voice transmissions for communication and relay of
weather and other information.
VORs are classified according to their operational uses. The
standard VOR facility has a power output of approximately
200 watts, with a maximum usable range depending upon
the aircraft altitude, class of facility, location of the facility,
terrain conditions within the usable area of the facility, and
other factors. Above and beyond certain altitude and distance
limits, signal interference from other VOR facilities and a
weak signal make it unreliable. Coverage is typically at least
40 miles at normal minimum instrument flight rules (IFR)
altitudes. VORs with accuracy problems in parts of their
service volume are listed in Notices to Airmen (NOTAMs)
and in the Airport/Facility Directory (A/FD) under the name
of the NAVAID.
VOR Components
The ground equipment consists of a VOR ground station,
which is a small, low building topped with a flat white disc,
upon which are located the VOR antennas and a fiberglass
cone-shaped tower. [Figure 9-11] The station includes an
automatic monitoring system. The monitor automatically
turns off defective equipment and turns on the standby
transmitter. Generally, the accuracy of the signal from the
ground station is within 1°.
The courses oriented FROM the station are called radials. The
VOR information received by an aircraft is not influenced
by aircraft attitude or heading. [Figure 9-10] Radials can
be envisioned to be like the spokes of a wheel on which the
aircraft is on one specific radial at any time. For example,
aircraft A (heading 180°) is inbound on the 360° radial; after
crossing the station, the aircraft is outbound on the 180°
radial at A1. Aircraft B is shown crossing the 225° radial.
Similarly, at any point around the station, an aircraft can be
located somewhere on a specific VOR radial. Additionally,
a VOR needle on an RMI always points to the course that
takes you to the VOR station where conversely the ADF
needle points to the station as a RB from the aircraft. In
the example above, the ADF needle at position A would be
pointed straight ahead, at A1 to the aircraft’s 180° position
(tail) and at B to the aircraft’s right.
The VOR receiver measures and presents information to
indicate bearing TO or FROM the station. In addition to the
navigation signals transmitted by the VOR, a Morse code
signal is transmitted concurrently to identify the facility, as
OBS
21 15
FR
TO
OBS knob
CDI needle
Course index
Unreliable signal flag
FROM indicator
Approximately 2 degrees
in the VOR mode
TO indicator
Figure 9-12. The VOR indicator instrument.
Figure 7-11. VOR transmitter ground station. Figure 9-11. VOR transmitter (ground station).
VOR facilities are aurally identified by Morse code, or
voice, or both. The VOR can be used for ground-to-air
communication without interference with the navigation
signal. VOR facilities operate within the 108.0 to 117.95 MHz
frequency band and assignment between 108.0 and 112.0
MHz is in even-tenth increments to preclude any conflict
with ILS localizer frequency assignment, which uses the
odd tenths in this range.
The airborne equipment includes an antenna, a receiver, and
the indicator instrument. The receiver has a frequency knob to
select any of the frequencies between 108.0 to 117.95 MHz.
The ON/OFF/volume control turns on the navigation receiver
and controls the audio volume. The volume has no effect on
the operation of the receiver. You should listen to the station
identifier before relying on the instrument for navigation.
VOR indicator instruments have at least the essential
components shown in the instrument illustrated in Figure 9-12.
Omnibearing Selector (OBS)
The desired course is selected by turning the omnibearing
selector (OBS) knob until the course is aligned with the
course index mark or displayed in the course window.
Course Deviation Indicator (CDI)
The course deviation indicator (CDI) is composed of an
instrument face and a needle hinged to move laterally across
the instrument face. The needle centers when the aircraft is
on the selected radial or its reciprocal. Full needle deflection
from the center position to either side of the dial indicates the
aircraft is 12° or more off course, assuming normal needle
sensitivity. The outer edge of the center circle is 2° off course;
with each dot representing an additional 2°.
TO/FROM Indicator
The TO/FROM indicator shows whether the selected course,
if intercepted and flown, takes the aircraft TO or FROM the
station. It does not indicate whether the aircraft is heading
to or from the station.
Flags or Other Signal Strength Indicators
The device that indicates a usable or an unreliable signal may
be an “OFF” flag. It retracts from view when signal strength
is sufficient for reliable instrument indications. Alternately,
insufficient signal strength may be indicated by a blank or
OFF in the TO/FROM window.
The indicator instrument may also be a horizontal situation
indicator (HSI), which combines the heading indicator
and CDI. [Figure 9-13] The combination of navigation
information from VOR/Localizer (LOC) with aircraft heading
information provides a visual picture of the aircraft’s location
and direction. This decreases pilot workload especially
with tasks such as course intercepts, flying a back-course
approach, or holding pattern entry. (See Chapter 5, Flight
Instruments, for operational characteristics.) [Figure 9-14]
Function of VOR
Orientation
The VOR does not account for the aircraft heading. It only
relays the aircraft direction from the station and has the same
indications regardless of which way the nose is pointing. Tune
the VOR receiver to the appropriate frequency of the selected
VOR ground station, turn up the audio volume, and identify
the station’s signal audibly. Then, rotate the OBS to center
the CDI needle and read the course under or over the index.
In Figure 9-12, 360° TO is the course indicated, while in
Figure 9-15, 180° TO is the course. The latter indicates that
the aircraft (which may be heading in any direction) is, at
this moment, located at any point on the 360° radial (line
from the station) except directly over the station or very
XPDR 5537 IDNT LCL10:12:34
INSET PFD CDI XPDR IDENT TMR/REF NRST ALERTS
VOR 1
270°
CRS 270° HDG 270°
TAS 106KT
OAT 6°C
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 270°
Heading
Heading select bugLubber line
Course select pointer
Course deviation scale
Course deviation indicator (CDI)
Compass card
Selected heading box
Figure 9-14. An HSI display as seen on the pilot’s primary flight display (PFD) on an electronic flight instrument. Note that only attributes
related to the HSI are labeled.
HDG
NAV
2I I5
Heading select bug
Compass warning flag
Heading select knob
NAV warning flag
Lubber line
Course deviation bar (CDI)
Course deviation scale
Course select pointer
Compass card
Symbolic aircraft
Glideslope pointer
Course select knob
Figure 9-13. A typical horizontal situation indicator (HSI).
CRS 180° HDG 215°
VOR
215°
CRS 180° HDG 330°
VOR
330°
VOR
VOR
270° 90°
°
°
260°
250°
240°
230°
220°
210°
200°
190°
340°
330° 320° 310° 300° 290°
280° 80°
70°
60° 50° 40° 30° 20°
160°
170°
150°
140°
130°
120°
110°
100°
ELECTRONIC FLIGHT
INSTRUMENTS ANALOG SYSTEMS
OBS
3 33
21 15
6 FR
OBS
3 33
21 15
TO
TO
FROM
Figure 9-15. CDI interpretation. The CDI as typically found on analog systems (right) and as found on electronic flight instruments (left).
close to it, as in Figure 9-15. The CDI deviates from side
to side as the aircraft passes over or nearly over the station
because of the volume of space above the station where the
zone of confusion exists. This zone of confusion is caused
by lack of adequate signal directly above the station due to
the radiation pattern of the station’s antenna, and because
the resultant of the opposing reference and variable signals
is small and constantly changing.
The CDI in Figure 9-15 indicates 180°, meaning that the
aircraft is on the 180° or the 360° radial of the station. The TO/
FROM indicator resolves the ambiguity. If the TO indicator is
showing, then it is 180° TO the station. The FROM indication
indicates the radial of the station the aircraft is presently on.
Movement of the CDI from center, if it occurs at a relatively
constant rate, indicates the aircraft is moving or drifting off
the 180°/360° line. If the movement is rapid or fluctuating,
this is an indication of impending station passage (the aircraft
is near the station). To determine the aircraft’s position
relative to the station, rotate the OBS until FROM appears
in the window, and then center the CDI needle. The index
indicates the VOR radial where the aircraft is located. The
inbound (to the station) course is the reciprocal of the radial.
If the VOR is set to the reciprocal of the intended course, the
CDI reflects reverse sensing. To correct for needle deflection,
turn away from the needle. To avoid this reverse sensing
situation, set the VOR to agree with the intended course.
A single NAVAID allows a pilot to determine the aircraft’s
position relative to a radial. Indications from a second
NAVAID are needed in order to narrow the aircraft’s position
down to an exact location on this radial.
Tracking TO and FROM the Station
To track to the station, rotate the OBS until TO appears,
then center the CDI. Fly the course indicated by the index.
If the CDI moves off center to the left, follow the needle by
correcting course to the left, beginning with a 20° correction.
When flying the course indicated on the index, a left deflection
of the needle indicates a crosswind component from the left.
If the amount of correction brings the needle back to center,
decrease the left course correction by half. If the CDI moves
left or right now, it should do so much more slowly, and smaller
heading corrections can be made for the next iteration.
Keeping the CDI centered takes the aircraft to the station.
To track to the station, the OBS value at the index is not
changed. To home to the station, the CDI needle is periodically
centered, and the new course under the index is used for the
aircraft heading. Homing follows a circuitous route to the
station, just as with ADF homing.
To track FROM the station on a VOR radial, you should
first orient the aircraft’s location with respect to the station
and the desired outbound track by centering the CDI needle
with a FROM indication. The track is intercepted by either
flying over the station or establishing an intercept heading.
The magnetic course of the desired radial is entered under the
index using the OBS and the intercept heading held until the
CDI centers. Then the procedure for tracking to the station
is used to fly outbound on the specified radial.
Course Interception
If the desired course is not the one being flown, first orient
the aircraft’s position with respect to the VOR station and the
course to be flown, and then establish an intercept heading.
The following steps may be used to intercept a predetermined
course, either inbound or outbound. Steps 1–3 may be omitted
when turning directly to intercept the course without initially
turning to parallel the desired course.
1. Determine the difference between the radial to be
intercepted and the radial on which the aircraft is
located (205° – 160° = 045°).
2. Double the difference to determine the interception
angle, which will not be less than 20° nor greater
than 90° (45° × 2 = 090°). 205° + 090° = 295° for
the intercept).
3. Rotate the OBS to the desired radial or inbound course.
4. Turn to the interception heading.
5. Hold this heading constant until the CDI center, which
indicates the aircraft is on course. (With practice in
judging the varying rates of closure with the course
centerline, pilots learn to lead the turn to prevent
overshooting the course.)
6. Turn to the MH corresponding to the selected course,
and follow tracking procedures inbound or outbound.
Course interception is illustrated in Figure 9-16.
VOR Operational Errors
Typical pilot-induced errors include:
1. Careless tuning and identification of station.
2. Failure to check receiver for accuracy/sensitivity.
3. Turning in the wrong direction during an orientation.
This error is common until visualizing position rather
than heading.
4. Failure to check the ambiguity (TO/FROM) indicator,
particularly during course reversals, resulting in reverse
sensing and corrections in the wrong direction.
CRS 025°HDG 295°
VOR 1
295°
CRS 340°HDG 340°
VOR 1
340°
After needle centers
track inbound on
205° radial.
Maintain heading of
295°
Present position,
inboundon 160°
radial.
0 10 20 30 40 50 60 70 80
OBS
24 21
TO
24 2I
2I
I5 I2
OBS
24 21
TO
2I I5 I2
OBS
TO
°
°
°
295°
OBS
21 15
NAV
GS
Instrument view is from
the pilot’ s perspective,
and the movable card is
reset after each turn
As VOR needle
centers, lead the turn
to track inbound.
NOTES
CRS 025°HDG 025°
VOR1
025°
Figure 9-16. Course interception (VOR).
5. Failure to parallel the desired radial on a track
interception problem. Without this step, orientation
to the desired radial can be confusing. Since pilots
think in terms of left and right of course, aligning the
aircraft position to the radial/course is essential.
6. Overshooting and undershooting radials on
interception problems.
7. Overcontrolling corrections during tracking, especially
close to the station.
8. Misinterpretation of station passage. On VOR
receivers not equipped with an ON/OFF flag, a
voice transmission on the combined communication
and navigation radio (NAV/COM) in use for VOR
may cause the same TO/FROM fluctuations on the
ambiguity meter as shown during station passage.
Read the whole receiver—TO/FROM, CDI, and
OBS—before you make a decision. Do not utilize a
VOR reading observed while transmitting.
9. Chasing the CDI, resulting in homing instead of
tracking. Careless heading control and failure to
bracket wind corrections make this error common.
VOR Accuracy
The effectiveness of the VOR depends upon proper use and
adjustment of both ground and airborne equipment.
The accuracy of course alignment of the VOR is generally
plus or minus 1°. On some VORs, minor course roughness
may be observed, evidenced by course needle or brief flag
alarm. 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.
Certain propeller revolutions per minute (rpm) settings or
helicopter rotor speeds can cause the VOR CDI to fluctuate
as much as plus or minus 6°. Slight changes to the RPM
setting normally smooths out this roughness. Pilots are urged
to check for this modulation phenomenon prior to reporting
a VOR station or aircraft equipment for unsatisfactory
operation.
VOR Receiver Accuracy Check
VOR system course sensitivity may be checked by noting
the number of degrees of change as the OBS is rotated to
move the CDI from center to the last dot on either side. The
course selected should not exceed 10° or 12° either side. In
addition, Title 14 of the Code of Federal Regulations (14
CFR) part 91 provides for certain VOR equipment accuracy
checks, and an appropriate endorsement, within 30 days prior
to flight under IFR. To comply with this requirement and to
ensure satisfactory operation of the airborne system, use the
following means for checking VOR receiver accuracy:
1. VOR test facility (VOT) or a radiated test signal from
an appropriately rated radio repair station.
2. Certified checkpoints on the airport surface.
3. Certified airborne checkpoints.
VOR Test Facility (VOT)
The Federal Aviation Administration (FAA) VOT transmits
a test signal that 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.
Locations of VOTs are published in the A/FD. 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
flight service station (FSS.) 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.
To use the VOT service, tune in the VOT frequency 108.0
MHz on the VOR receiver. With the CDI centered, the
OBS should read 0° with the TO/FROM indication showing
FROM or the OBS should read 180° with the TO/FROM
indication showing TO. Should the VOR receiver operate an
RMI, it would indicate 180° on any OBS setting.
A radiated VOT from an appropriately rated radio repair
station serves the same purpose as an FAA VOT signal, and
the check is made in much the same manner as a VOT with
some differences.
The frequency normally approved by the Federal
Communications Commission (FCC) is 108.0 MHz;
however, repair stations are not permitted to radiate the
VOR test signal continuously. The owner or operator of the
aircraft must make arrangements with the repair station to
have the test signal transmitted. 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.
Certified Checkpoints
Airborne and ground checkpoints 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. Locations of these checkpoints are
published in the A/FD.