Figure 16-28. VHF transmissions follow a line-of-sight course.
“A” and “B” signal
received
Neither “A” nor “B”
signal received
Only “A” signal received Only “B” signal received
VOR
station
“A”
VOR
station
“B”
• Item 8 is the route of flight. If the flight is to be direct,
enter the word “direct;” if not, enter the actual route to
be followed, such as via certain towns or navigation
aids.
• Item 10 is the estimated time en route. In the sample
flight plan, 5 minutes was added to the total time to
allow for the climb.
• Item 12 is the fuel on board in hours and minutes. This
is determined by dividing the total usable fuel aboard
in gallons by the estimated rate of fuel consumption
in gallons.
Remember, there is every advantage in filing a flight plan;
but do not forget to close the flight plan upon arrival. This
should be done via telephone to avoid radio congestion.
Ground-Based Navigation
Advances in navigational radio receivers installed in aircraft,
the development of aeronautical charts that show the exact
location of ground transmitting stations and their frequencies,
along with refined flight deck instrumentation make it
possible for pilots to navigate with precision to almost any
point desired. Although precision in navigation is obtainable
through the proper use of this equipment, beginning pilots
should use this equipment to supplement navigation by visual
reference to the ground (pilotage). This method provides the
pilot with an effective safeguard against disorientation in the
event of radio malfunction.
There are three radio navigation systems available for use
for VFR navigation. These are:
• VHF Omnidirectional Range (VOR)
• Nondirectional Radio Beacon (NDB)
• Global Positioning System (GPS)
Very High Frequency (VHF) Omnidirectional
Range (VOR)
The VOR system is present in three slightly different
navigation aids (NAVAIDs): VOR, VOR/distance measuring
equipment (DME)(discussed in a later section), and
VORTAC. By itself it is known as a VOR, and it provides
magnetic bearing information to and from the station. When
DME is also installed with a VOR, the NAVAID is referred
to as a VOR/DME. When military tactical air navigation
(TACAN) equipment is installed with a VOR, the NAVAID
is known as a VORTAC. DME is always an integral part of
a VORTAC. Regardless of the type of NAVAID utilized
(VOR, VOR/DME, or VORTAC), the VOR indicator
behaves the same. Unless otherwise noted in this section,
VOR, VOR/DME, and VORTAC NAVAIDs are all referred
to hereafter as VORs.
The prefix “omni-” means all, and an omnidirectional range
is a VHF radio transmitting ground station that projects
straight line courses (radials) from the station in all directions.
From a top view, it can be visualized as being similar to the
spokes from the hub of a wheel. The distance VOR radials are
projected depends upon the power output of the transmitter.
The course or radials projected from the station are referenced
to MN. Therefore, a radial is defined as a line of magnetic
bearing extending outward from the VOR station. Radials are
identified by numbers beginning with 001, which is 1° east
of MN and progress in sequence through all the degrees of a
circle until reaching 360. To aid in orientation, a compass rose
reference to magnetic north is superimposed on aeronautical
charts at the station location.
VOR ground stations transmit within a VHF frequency band
of 108.0–117.95 MHz. Because the equipment is VHF, the
signals transmitted are subject to line-of-sight restrictions.
Therefore, its range varies in direct proportion to the altitude
of receiving equipment. Generally, the reception range of
the signals at an altitude of 1,000 feet above ground level
(AGL) is about 40 to 45 miles. This distance increases with
altitude. [Figure 16-28]
VORs and VORTACs are classed according to operational
use. There are three classes:
• T (Terminal)
• L (Low altitude)
• H (High altitude)
The normal useful range for the various classes is shown in
the following table:
VOR/VORTAC NAVAIDS
Normal Usable Altitudes and Radius Distances
Distance
Class Altitudes ( M i l e s )
T 12,000' and below 25
L Below 18,000' 40
H Below 14,500' 40
H Within the conterminous 48 states
only, between 14,500 and 17,999' 100
H 18,000'—FL 450 130
H FL 450—60,000' 100
The useful range of certain facilities may be less than 50
miles. For further information concerning these restrictions,
refer to the Communication/NAVAID Remarks in the Chart
Supplement U.S.
The accuracy of course alignment of VOR radials is
considered to be excellent. It is generally within plus or minus
1°. However, certain parts of the VOR receiver equipment
deteriorate, affecting its accuracy. This is particularly true
at great distances from the VOR station. The best assurance
of maintaining an accurate VOR receiver is periodic checks
and calibrations. VOR accuracy checks are not a regulatory
requirement for VFR flight. However, to assure accuracy of
the equipment, these checks should be accomplished quite
frequently and a complete calibration should be performed
each year. The following means are provided for pilots to
check VOR accuracy:
• FAA VOR test facility (VOT)
• Certified airborne checkpoints
• Certified ground checkpoints located on airport
surfaces
If an aircraft has two VOR receivers installed, a dual VOR
receiver check can be made. To accomplish the dual receiver
check, a pilot must tune both VOR receivers to the same VOR
ground facility. The maximum permissible variation between
the two indicated bearings is 4°. A list of the airborne and
ground checkpoints is published in the Chart Supplement U.S.
Basically, these checks consist of verifying that the VOR
radials the aircraft equipment receives are aligned with the
radials the station transmits. There are not specific tolerances
in VOR checks required for VFR flight. But as a guide to
assure acceptable accuracy, the required IFR tolerances can
be used—±4° for ground checks and ±6° for airborne checks.
These checks can be performed by the pilot.
The VOR transmitting station can be positively identified
by its Morse code identification or by a recorded voice
identification that states the name of the station followed by
“VOR.” Many FSSs transmit voice messages on the same
frequency that the VOR operates. Voice transmissions should
not be relied upon to identify stations because many FSSs
remotely transmit over several omniranges that have names
different from that of the transmitting FSS. If the VOR is
out of service for maintenance, the coded identification is
removed and not transmitted. This serves to alert pilots that
this station should not be used for navigation. VOR receivers
are designed with an alarm flag to indicate when signal
strength is inadequate to operate the navigational equipment.
This happens if the aircraft is too far from the VOR or the
aircraft is too low and, therefore, is out of the line of sight
of the transmitting signals.
Using the VOR
In review, for VOR radio navigation, there are two
components required: ground transmitter and aircraft
receiving equipment. The ground transmitter is located at a
specific position on the ground and transmits on an assigned
frequency. The aircraft equipment includes a receiver with
a tuning device and a VOR or omninavigation instrument.
The navigation instrument could be a course deviation
indicator (CDI), horizontal situation indicator (HSI), or a
radio magnetic indicator (RMI). Each of these instruments
indicates the course to the tuned VOR.
Course Deviation Indicator (CDI)
The CDI is found in most training aircraft. It consists of an
omnibearing selector (OBS) sometimes referred to as the
course selector, a CDI needle (left-right needle), and a TO/
FROM indicator.
The course selector is an azimuth dial that can be rotated to
select a desired radial or to determine the radial over which
the aircraft is flying. In addition, the magnetic course “TO”
or “FROM” the station can be determined.
When the course selector is rotated, it moves the CDI or
needle to indicate the position of the radial relative to the
aircraft. If the course selector is rotated until the deviation
Figure 16-30. Horizontal situation indicator.
33
30
24
2II5
I2
6
3
GS GS
DC
NAV HDG
Lubber line
Compass warning flag
Glideslope deviation scale
NAV warning flag
Course deviation bar
Heading select knob
Course select pointer
Compass card
Symbolic aircraft
Heading select bug
Course select knob
To/From indicator Course deviation scale
Figure 16-29. VOR indicator.
OBS
N
E
S
W
333
24
21 15
12
30
6
TO
N
A
V
Unreliable signal flag
TO/FROM indicator
OBS knob
Course index
CDI needle
Approximately 2 degrees
in the VOR mode
needle is centered, the radial (magnetic course “FROM” the
station) or its reciprocal (magnetic course “TO” the station)
can be determined. The course deviation needle also moves
to the right or left if the aircraft is flown or drifting away
from the radial which is set in the course selector.
By centering the needle, the course selector indicates either
the course “FROM” the station or the course “TO” the station.
If the flag displays a “TO,” the course shown on the course
selector must be flown to the station. [Figure 16-29] If
“FROM” is displayed and the course shown is followed, the
aircraft is flown away from the station.
Horizontal Situation Indicator
The HSI is a direction indicator that uses the output
from a flux valve to drive the compass card. The HSI
[Figure 16-30] combines the magnetic compass with
navigation signals and a glideslope. The HSI gives the pilot
an indication of the location of the aircraft in relation to the
chosen course or radial.
In Figure 16-30, the aircraft magnetic heading displayed
on the compass card under the lubber line is 184°. The
course select pointer shown is set to 295°; the tail of the
pointer indicates the reciprocal, 115°. The course deviation
bar operates with a VOR/Localizer (VOR/LOC) or GPS
navigation receiver to indicate left or right deviations from
the course selected with the course select pointer; operating
in the same manner, the angular movement of a conventional
VOR/LOC needle indicates deviation from course.
The desired course is selected by rotating the course select
pointer, in relation to the compass card, by means of the
course select knob. The HSI has a fixed aircraft symbol
and the course deviation bar displays the aircraft’s position
relative to the selected course. The TO/FROM indicator is a
triangular pointer. When the indicator points to the head of the
course select pointer, the arrow shows the course selected. If
properly intercepted and flown, the course takes the aircraft
to the chosen facility. When the indicator points to the tail
of the course, the arrow shows that the course selected, if
properly intercepted and flown, takes the aircraft directly
away from the chosen facility.
When the NAV warning flag appears, it indicates no reliable
signal is being received. The appearance of the HDG flag
indicates the compass card is not functioning properly.
Radio Magnetic Indicator (RMI)
The RMI is a navigational aid providing aircraft magnetic
or directional gyro heading and very high frequency
omnidirectional range (VOR), GPS, and automatic direction
Figure 16-31. Radio magnetic indicator.
33
30
24
21
15
12
6
3
S
W
E
N
HDG
A
D
F
A
D
F
NAVNAV
Figure 16-32. Tracking a radial in a crosswind.
N
E
S
W
333
24
21 15
12
30 6
TO
OBS
33
30
24
2I I5
I2
6
3
N
E
S
W
333
24
21 15
12
30 6
TO
OBS
33
30
24 2I
I5
I2
63
N
E
S
W
333
24
21 15
12
30 6
TO
OBS
33
30
24 2I I5
I2
6
3
N
E
S
W
333
24
21 15
12
30 6
TO
OBS
33
30
24 2I I5
I2
6
3
N
E
S
W
333
24
21 15
12
30 6
TO
OBS
33
30
24
2I I5
I2
6
3
N
E
S
W
333
24
21 15
12
30 6
FROM
OBS
33
30
24
2I I5
I2
6
3
N
E
S
W
333
24
21 15
12
30 6
FROM
OBS
33
30
24
2I I5
I2
6
3
N E
SW
3
33
24 21
15
12
30
6
FROM
OBS
33
30
24 2I
I5
I2
63
3
2
1
4
5
6
7
8
0
18
27
6
BRAVO
BRA 115.0
WIND
finder (ADF) bearing information. [Figure 16-31] Remote
indicating compasses were developed to compensate for
errors in and limitations of older types of heading indicators.
The remote compass transmitter is a separate unit usually
mounted in a wingtip to eliminate the possibility of magnetic
interference. The RMI consists of a compass card, a heading
index, two bearing pointers, and pointer function switches.
The two pointers are driven by any two combinations of a
GPS, an ADF, and/or a VOR. The pilot has the ability to select
the navigation aid to be indicated. The pointer indicates the
course to the selected NAVAID or waypoint. In Figure 16-31,
the green pointer is indicating the station tuned on the ADF.
The yellow pointer is indicating the course to a VOR or GPS
waypoint. Note that there is no requirement for a pilot to
select a course with the RMI. Only the selected navigation
source is pointed to by the needle(s).
Tracking With VOR
The following describes a step-by-step procedure for tracking
to and from a VOR station using a CDI. Figure 16-32
illustrates the procedure.
First, tune the VOR receiver to the frequency of the selected
VOR station. For example, 115.0 to receive Bravo VOR.
Next, check the identifiers to verify that the desired VOR
is being received. As soon as the VOR is properly tuned,
the course deviation needle deflects either left or right.
Then, rotate the azimuth dial to the course selector until the
course deviation needle centers and the TO-FROM indicator
indicates “TO.” If the needle centers with a “FROM”
indication, the azimuth should be rotated 180° because, in
this case, it is desired to fly “TO” the station. Now, turn the
aircraft to the heading indicated on the VOR azimuth dial or
course selector, 350° in this example.
If a heading of 350° is maintained with a wind from the right
as shown, the aircraft drifts to the left of the intended track.
As the aircraft drifts off course, the VOR course deviation
needle gradually moves to the right of center or indicates the
direction of the desired radial or track.
Figure 16-33. Time-distance check example.
=Time in seconds between bearings
Degrees of bearing change
Minutes to station
= 12 minutes to the station
Time-Distance Check Example
For example, if 2 minutes (120 seconds) is required to fly a
bearing change of 10 degrees, the aircraft is—
120
10
To return to the desired radial, the aircraft heading must be
altered to the right. As the aircraft returns to the desired track,
the deviation needle slowly returns to center. When centered,
the aircraft is on the desired radial and a left turn must be
made toward, but not to the original heading of 350° because
a wind drift correction must be established. The amount of
correction depends upon the strength of the wind. If the wind
velocity is unknown, a trial-and-error method can be used
to find the correct heading. Assume, for this example, a 10°
correction for a heading of 360° is maintained.
While maintaining a heading of 360°, assume that the course
deviation begins to move to the left. This means that the wind
correction of 10° is too great and the aircraft is flying to the
right of course. A slight turn to the left should be made to
permit the aircraft to return to the desired radial.
When the deviation needle centers, a small wind drift
correction of 5° or a heading correction of 355° should be
flown. If this correction is adequate, the aircraft remains
on the radial. If not, small variations in heading should be
made to keep the needle centered and consequently keep the
aircraft on the radial.
As the VOR station is passed, the course deviation needle
fluctuates, then settles down, and the “TO” indication
changes to “FROM.” If the aircraft passes to one side of the
station, the needle deflects in the direction of the station as
the indicator changes to “FROM.”
Generally, the same techniques apply when tracking
outbound as those used for tracking inbound. If the intent is
to fly over the station and track outbound on the reciprocal of
the inbound radial, the course selector should not be changed.
Corrections are made in the same manner to keep the needle
centered. The only difference is that the omnidirectional
range indicator indicates “FROM.”
If tracking outbound on a course other than the reciprocal of
the inbound radial, this new course or radial must be set in
the course selector and a turn made to intercept this course.
After this course is reached, tracking procedures are the same
as previously discussed.
Tips on Using the VOR
• Positively identify the station by its code or voice
identification.
• Remember that VOR signals are “line-of-sight.” A
weak signal or no signal at all is received if the aircraft
is too low or too far from the station.
• When navigating to a station, determine the inbound
radial and use this radial. Fly a heading that will
maintain the course. If the aircraft drifts, fly a heading
to re-intercept the course then apply a correction to
compensate for wind drift.
• If minor needle fluctuations occur, avoid changing
headings immediately. Wait a moment to see if the
needle recenters; if it does not, then you must correctly
recenter the course to the needle.
• When flying “TO” a station, always fly the selected
course with a “TO” indication. When flying “FROM” a
station, always fly the selected course with a “FROM”
indication. If this is not done, the action of the course
deviation needle is reversed. To further explain this
reverse action, if the aircraft is flown toward a station
with a “FROM” indication or away from a station
with a “TO” indication, the course deviation needle
indicates in a direction opposite to that which it should
indicate. For example, if the aircraft drifts to the right
of a radial being flown, the needle moves to the right
or points away from the radial. If the aircraft drifts to
the left of the radial being flown, the needle moves
left or in the direction opposite of the radial.
• When navigating using the VOR, it is important to
fly headings that maintain or re-intercept the course.
Just turning toward the needle will cause overshooting
the radial and flying an S turn to the left and right of
course.
Time and Distance Check From a Station Using a
RMI
To compute time and distance from a station, first turn the
aircraft to place the RMI bearing pointer on the nearest 90°
index. Note the time and maintain the heading. When the
RMI bearing pointer has moved 10°, note the elapsed time
in seconds and apply the formulas in the following example
to determine the approximate time and distance from a given
station. [Figure 16-33]
The time from station may also be calculated by using a short
method based on the above formula, if a 10° bearing change
is flown. If the elapsed time for the bearing change is noted
Figure 16-34. Time-distance check formula using a CDI.
A
B
= 60 x minutes flown between bearing change
degrees of bearing change
Time to
station
= TAS x minutes flown
degrees of bearing change
Distance to station
Time-Distance Check Formula
in seconds and a 10° bearing change is made, the time from
the station, in minutes, is determined by counting off one
decimal point. Thus, if 75 seconds are required to fly a 10°
bearing change, the aircraft is 7.5 minutes from the station.
When the RMI bearing pointer is moving rapidly or when
several corrections are required to place the pointer on the
wingtip position, the aircraft is at station passage.
The distance from the station is computed by multiplying TAS
or GS (in miles per minute) by the previously determined time
in minutes. For example, if the aircraft is 7.5 minutes from
station, flying at a TAS of 120 knots or 2 NM per minute,
the distance from station is 15 NM (7.5 × 2 = 15).
The accuracy of time and distance checks is governed by
existing wind, degree of bearing change, and accuracy of
timing. The number of variables involved causes the result
to be only an approximation. However, by flying an accurate
heading and checking the time and bearing closely, the pilot
can make a reasonable estimate of time and distance from
the station.
Time and Distance Check From a Station Using a
CDI
To compute time and distance from a station using a CDI,
first tune and identify the VOR station and determine the
radial on which you are located. Then turn inbound and
re-center the needle if necessary. Turn 90° left or right, of
the inbound course, rotating the OBS to the nearest 10°
increment opposite the direction of turn. Maintain heading
and when the CDI centers, note the time. Maintaining the
same heading, rotate the OBS 10° in the same direction as
was done previously and note the elapsed time when the
CDI again centers. Time and distance from the station is
determined from the formula shown in Figure 16-34.
Course Intercept
Course interceptions are performed in most phases of
instrument navigation. The equipment used varies, but an
intercept heading must be flown that results in an angle or
rate of intercept sufficient for solving a particular problem.
Rate of Intercept
Rate of intercept, seen by the aviator as bearing pointer or
HSI movement, is a result of the following factors:
• The angle at which the aircraft is flown toward a
desired course (angle of intercept)
• True airspeed and wind (GS)
• Distance from the station
Angle of Intercept
The angle of intercept is the angle between the heading
of the aircraft (intercept heading) and the desired course.
Controlling this angle by selection/adjustment of the intercept
heading is the easiest and most effective way to control
course interceptions. Angle of intercept must be greater than
the degrees from course, but should not exceed 90°. Within
this limit, make adjustments as needed, to achieve the most
desirable rate of intercept.
When selecting an intercept heading, the key factor is the
relationship between distance from the station and degrees
from the course. Each degree, or radial, is 1 NM wide at
a distance of 60 NM from the station. Width increases or
decreases in proportion to the 60 NM distance. For example,
1 degree is 2 NM wide at 120 NM—and ½ NM wide at 30
NM. For a given GS and angle of intercept, the resultant rate
of intercept varies according to the distance from the station.
When selecting an intercept heading to form an angle of
intercept, consider the following factors:
• Degrees from course
• Distance from the station
• True airspeed and wind (GS)
Distance Measuring Equipment (DME)
Distance measuring equipment (DME) consists of an ultra
high frequency (UHF) navigational aid with VOR/DMEs and
VORTACs. It measures, in NM, the slant range distance of
an aircraft from a VOR/DME or VORTAC (both hereafter
referred to as a VORTAC). Although DME equipment is
very popular, not all aircraft are DME equipped.
To utilize DME, the pilot should select, tune, and identify
a VORTAC, as previously described. The DME receiver,
utilizing what is called a “paired frequency” concept,
automatically selects and tunes the UHF DME frequency
associated with the VHF VORTAC frequency selected by
the pilot. This process is entirely transparent to the pilot.
After a brief pause, the DME display shows the slant range
distance to or from the VORTAC. Slant range distance is the
direct distance between the aircraft and the VORTAC and
is therefore affected by aircraft altitude. (Station passage
directly over a VORTAC from an altitude of 6,076 feet AGL
Figure 16-36. RNAV controls.
4
3
3
9
7
2
BEARING LOAD DISTANCE
OFFSET EN ROUTE
R
N
A
V
3 3
4 4
8 8
12 12
Figure 16-35. Flying an RNAV course.
Area Navigation Direct Route
would show approximately 1.0 NM on the DME.) DME is a
very useful adjunct to VOR navigation. A VOR radial alone
merely gives line of position information. With DME, a pilot
may precisely locate the aircraft on a given line (radial).
Most DME receivers also provide GS and time-to-station
modes of operation. The GS is displayed in knots (NMPH).
The time-to-station mode displays the minutes remaining to
VORTAC station passage, predicated upon the present GS.
GS and time-to-station information is only accurate when
tracking directly to or from a VORTAC. DME receivers
typically need a minute or two of stabilized flight directly
to or from a VORTAC before displaying accurate GS or
time-to-station information.
Some DME installations have a hold feature that permits a
DME signal to be retained from one VORTAC while the
course indicator displays course deviation information from
an ILS or another VORTAC.
VOR/DME RNAV
Area navigation (RNAV) permits electronic course guidance
on any direct route between points established by the pilot.
While RNAV is a generic term that applies to a variety
of NAVAIDS, such as GPS and others, this section deals
with VOR/DME-based RNAV. VOR/DME RNAV is not a
separate ground-based NAVAID, but a method of navigation
using VOR/DME and VORTAC signals specially processed
by the aircraft’s RNAV computer. [Figure 16-35]
NOTE: In this section, the term “VORTAC” also includes
VOR/DME NAVAIDs.
In its simplest form, VOR/DME RNAV allows the pilot to
electronically move VORTACs around to more convenient
locations. Once electronically relocated, they are referred
to as waypoints. These waypoints are described as a
combination of a selected radial and distance within the
service volume of the VORTAC to be used. These waypoints
allow a straight course to be flown between almost any
origin and destination, without regard to the orientation of
VORTACs or the existence of airways.
While the capabilities and methods of operation of VOR/
DME RNAV units differ, there are basic principles of
operation that are common to all. Pilots are urged to study
the manufacturer’s operating guide and receive instruction
prior to the use of VOR/DME RNAV or any unfamiliar
navigational system. Operational information and limitations
should also be sought from placards and the supplement
section of the AFM/POH.
VOR/DME-based RNAV units operate in at least three
modes: VOR, en route, and approach. A fourth mode, VOR
Parallel, may also be found on some models. The units need
both VOR and DME signals to operate in any RNAV mode.
If the NAVAID selected is a VOR without DME, RNAV
mode will not function.
In the VOR (or non-RNAV) mode, the unit simply functions
as a VOR receiver with DME capability. [Figure 16-36] The
unit’s display on the VOR indicator is conventional in all
respects. For operation on established airways or any other
ordinary VOR navigation, the VOR mode is used.
To utilize the unit’s RNAV capability, the pilot selects and
establishes a waypoint or a series of waypoints to define a
course. A VORTAC (or VOR/DME) needs to be selected
as a NAVAID, since both radial and distance signals are
available from these stations. To establish a waypoint, a
point somewhere within the service range of a VORTAC is
defined on the basis of radial and distance. Once the waypoint
is entered into the unit and the RNAV en route mode is
selected, the CDI displays course guidance to the waypoint,
not the original VORTAC. DME also displays distance to
the waypoint. Many units have the capability to store several
waypoints, allowing them to be programmed prior to flight,
if desired, and called up in flight.
RNAV waypoints are entered into the unit in magnetic
bearings (radials) of degrees and tenths (i.e., 275.5°) and
distances in NM and tenths (i.e., 25.2 NM). When plotting
RNAV waypoints on an aeronautical chart, pilots find it