difficult to measure to that level of accuracy, and in practical
application, it is rarely necessary. A number of flight planning
publications publish airport coordinates and waypoints with
this precision and the unit accepts those figures. There is a
subtle but important difference in CDI operation and display
in the RNAV modes.
In the RNAV modes, course deviation is displayed in terms
of linear deviation. In the RNAV en route mode, maximum
deflection of the CDI typically represents 5 NM on either
side of the selected course without regard to distance from the
waypoint. In the RNAV approach mode, maximum deflection
of the CDI typically represents 1¼ NM on either side of the
selected course. There is no increase in CDI sensitivity as the
aircraft approaches a waypoint in RNAV mode.
The RNAV approach mode is used for instrument approaches.
Its narrow scale width (¼ of the en route mode) permits very
precise tracking to or from the selected waypoint. In VFR
cross-country navigation, tracking a course in the approach
mode is not desirable because it requires a great deal of
attention and soon becomes tedious.
A fourth, lesser-used mode on some units is the VOR
Parallel mode. This permits the CDI to display linear
(not angular) deviation as the aircraft tracks to and from
VORTACs. It derives its name from permitting the pilot
to offset (or parallel) a selected course or airway at a fixed
distance of the pilot’s choosing, if desired. The VOR parallel
mode has the same effect as placing a waypoint directly
over an existing VORTAC. Some pilots select the VOR
parallel mode when utilizing the navigation (NAV) tracking
function of their autopilot for smoother course following
near the VORTAC.
Navigating an aircraft with VOR/DME-based RNAV can be
confusing, and it is essential that the pilot become familiar
with the equipment installed. It is not unknown for pilots to
operate inadvertently in one of the RNAV modes when the
operation was not intended, by overlooking switch positions
or annunciators. The reverse has also occurred with a pilot
neglecting to place the unit into one of the RNAV modes by
overlooking switch positions or annunciators. As always, the
prudent pilot is not only familiar with the equipment used,
but never places complete reliance in just one method of
navigation when others are available for cross-check.
Automatic Direction Finder (ADF)
Many general aviation-type aircraft are equipped with ADF
radio receiving equipment. To navigate using the ADF,
the pilot tunes the receiving equipment to a ground station
known as a nondirectional radio beacon (NDB). The NDB
stations normally operate in a low or medium frequency band
of 200 to 415 kHz. The frequencies are readily available on
aeronautical charts or in the Chart Supplement U.S.
All radio beacons, except compass locators, transmit
a continuous three-letter identification in code, except
during voice transmissions. A compass locator, which is
associated with an instrument landing system, transmits a
two-letter identification.
Standard broadcast stations can also be used in conjunction
with ADF. Positive identification of all radio stations is
extremely important and this is particularly true when using
standard broadcast stations for navigation.
NDBs have one advantage over the VOR in that low or
medium frequencies are not affected by line-of-sight. The
signals follow the curvature of the Earth; therefore, if the
aircraft is within the range of the station, the signals can be
received regardless of altitude.
The following table gives the class of NDB stations, their
power, and their usable range:
NONDIRECTIONAL RADIO BEACON (NDB)
(Usable radius distances for all altitudes)
Power Distance
Class (Watts) (Miles)
Compass Locator Under 25 15
MH Under 50 2 5
H 50–1999 *50
H H 2000 or more 75
*Service range of individual facilities may be less than
50 miles.
One of the disadvantages that should be considered when
using low frequency (LF) for navigation is that LF signals are
very susceptible to electrical disturbances, such as lightning.
These disturbances create excessive static, needle deviations,
and signal fades. There may be interference from distant
stations. Pilots should know the conditions under which these
disturbances can occur so they can be more alert to possible
interference when using the ADF.
Basically, the ADF aircraft equipment consists of a tuner,
which is used to set the desired station frequency, and the
navigational display.
The navigational display consists of a dial upon which the
azimuth is printed and a needle which rotates around the
dial and points to the station to which the receiver is tuned.
Figure 16-38. ADF terms.
Radio station
N-S
E-W
3330
24
21
15 12
6
3W
S
E
N
Magnetic bearing to station
Relative bearing
Magnetic heading
Magnetic North
Figure 16-37. ADF with fixed azimuth and magnetic compass.
N-S
E-W
33
30
24 21
15
12
6 3
W S
E N
Some of the ADF dials can be rotated to align the
azimuth with the aircraft heading; others are fixed with 0°
representing the nose of the aircraft and 180° representing
the tail. Only the fixed azimuth dial is discussed in this
handbook. [Figure 16-37]
Figure 16-38 illustrates terms that are used with the ADF
and should be understood by the pilot.
To determine the magnetic bearing “FROM” the station,
180° is added to or subtracted from the magnetic bearing to
the station. This is the reciprocal bearing and is used when
plotting position fixes.
Keep in mind that the needle of fixed azimuth points to the
station in relation to the nose of the aircraft. If the needle
is deflected 30° to the left for a relative bearing of 330°,
this means that the station is located 30° left. If the aircraft
is turned left 30°, the needle moves to the right 30° and
indicates a relative bearing of 0° meaning that the aircraft
is pointing toward the station. If the pilot continues flight
toward the station keeping the needle on 0°, the procedure
is called homing to the station. If a crosswind exists, the
ADF needle continues to drift away from zero. To keep the
needle on zero, the aircraft must be turned slightly resulting
in a curved flight path to the station. Homing to the station
is a common procedure but may result in drifting downwind,
thus lengthening the distance to the station.
Tracking to the station requires correcting for wind drift and
results in maintaining flight along a straight track or bearing
to the station. When the wind drift correction is established,
the ADF needle indicates the amount of correction to the
right or left. For instance, if the magnetic bearing to the
station is 340°, a correction for a left crosswind would
result in a magnetic heading of 330°, and the ADF needle
would indicate 10° to the right or a relative bearing of 010°.
[Figure 16-39]
When tracking away from the station, wind corrections are
made similar to tracking to the station, but the ADF needle
points toward the tail of the aircraft or the 180° position on
the azimuth dial. Attempting to keep the ADF needle on
the 180° position during winds results in the aircraft flying
a curved flight leading further and further from the desired
track. When tracking outbound, corrections for wind should
be made in the direction opposite of that in which the needle
is pointing.
Although the ADF is not as popular as the VOR for radio
navigation, with proper precautions and intelligent use, the
ADF can be a valuable aid to navigation.
Global Positioning System
The GPS is a satellite-based radio navigation system.
Its RNAV guidance is worldwide in scope. There are no
symbols for GPS on aeronautical charts as it is a space-based
system with global coverage. Development of the system is
underway so that GPS is capable of providing the primary
means of electronic navigation. Portable and yoke-mounted
units are proving to be very popular in addition to those
permanently installed in the aircraft. Extensive navigation
databases are common features in aircraft GPS receivers.
Figure 16-39. ADF tracking.
330
°
33
30
24 21
15
12
63
W
S
E
N
330
°
33
30
24 21
15
12
63
W
S
E
N
340° bearing to station
The GPS is a satellite radio navigation and time dissemination
system developed and operated by the U.S. Department of
Defense (DOD). Civilian interface and GPS system status
is available from the U.S. Coast Guard.
It is not necessary to understand the technical aspects of
GPS operation to use it in VFR/IFR navigation. It does differ
significantly from conventional, ground-based electronic
navigation and awareness of those differences is important.
Awareness of equipment approvals and limitations is critical
to the safety of flight.
The GPS navigation system broadcasts a signal that is used
by receivers to determine precise position anywhere in the
world. The receiver tracks multiple satellites and determines
a pseudorange measurement to determine the user location.
A minimum of four satellites is necessary to establish an
accurate three-dimensional position. The Department of
Defense (DOD) is responsible for operating the GPS satellite
constellation and monitors the GPS satellites to ensure proper
operation.
The status of a GPS satellite is broadcast as part of the data
message transmitted by the satellite. GPS status information
is also available from the U.S. Coast Guard navigation
information service at (703) 313-5907 or online at www.
navcen.uscg.gov. Additionally, satellite status is available
through the NOTAM system.
The GPS receiver verifies the integrity (usability) of the
signals received from the GPS constellation through receiver
autonomous integrity monitoring (RAIM) to determine if
a satellite is providing corrupted information. At least one
satellite, in addition to those required for navigation, must
be in view for the receiver to perform the RAIM function;
thus, RAIM needs a minimum of five satellites in view or
four satellites and a barometric altimeter (baro-aiding) to
detect an integrity anomaly. For receivers capable of doing
so, RAIM needs six satellites in view (or five satellites with
baro-aiding) to isolate the corrupt satellite signal and remove
it from the navigation solution. Baro-aiding is a method of
augmenting the GPS integrity solution by using a nonsatellite
input source. GPS derived altitude should not be relied upon
to determine aircraft altitude since the vertical error can
be quite large and no integrity is provided. To ensure that
baro-aiding is available, the current altimeter setting must be
entered into the receiver as described in the operating manual.
RAIM messages vary somewhat between receivers; however,
generally there are two types. One type indicates that there
are not enough satellites available to provide RAIM integrity
monitoring and another type indicates that the RAIM integrity
monitor has detected a potential error that exceeds the limit
for the current phase of flight. Without RAIM capability, the
pilot has no assurance of the accuracy of the GPS position.
Selective Availability
Selective Availability (SA) is a method by which the accuracy
of GPS is intentionally degraded. This feature is designed
to deny hostile use of precise GPS positioning data. SA was
discontinued on May 1, 2000, but many GPS receivers are
designed to assume that SA is still active.
The baseline GPS satellite constellation consists of 24
satellites positioned in six earth-centered orbital planes
with four operation satellites and a spare satellite slot in
each orbital plane. The system can support a constellation
of up to thirty satellites in orbit. The orbital period of a GPS
satellite is one-half of a sidereal day or 11 hours 58 minutes.
The orbits are nearly circular and equally spaced about the
equator at a 60-degree separation with an inclination of
Figure 16-40. Satellite constellation.
55 degrees relative to the equator. The orbital radius (i.e.
distance from the center of mass of the earth to the satellite)
is approximately 26,600 km.
With the baseline satellite constellation, users with a clear
view of the sky have a minimum of four satellites in view.
It is more likely that a user would see six to eight satellites.
The satellites broadcast ranging signals and navigation data
allowing users to measure their pseudoranges in order to
estimate their position, velocity and time, in a passive, listen-
only mode. The receiver uses data from a minimum of four
satellites above the mask angle (the lowest angle above the
horizon at which a receiver can use a satellite). The exact
number of satellites operating at any one particular time
varies depending on the number of satellite outages and
operational spares in orbit. For current status of the GPS
constellation, please visit http://tycho.usno.navy.mil/gpscurr.
html. [Figure 16-40]
VFR Use of GPS
GPS navigation has become a great asset to VFR pilots
providing increased navigation capability and enhanced
situational awareness while reducing operating costs due
to greater ease in flying direct routes. While GPS has many
benefits to the VFR pilot, care must be exercised to ensure
that system capabilities are not exceeded.
Types of receivers used for GPS navigation under VFR are
varied from a full IFR installation being used to support a
VFR flight to a VFR only installation (in either a VFR or IFR
capable aircraft) to a hand-held receiver. The limitations of
each type of receiver installation or use must be understood
by the pilot to avoid misusing navigation information. In all
cases, VFR pilots should never rely solely on one system
of navigation. GPS navigation must be integrated with
other forms of electronic navigation, as well as pilotage
and dead reckoning. Only through the integration of these
techniques can the VFR pilot ensure accuracy in navigation.
Some critical concerns in VFR use of GPS include RAIM
capability, database currency, and antenna location.
RAIM Capability
Many VFR GPS receivers and all hand-held units are not
equipped with RAIM alerting capability. Loss of the required
number of satellites in view, or the detection of a position
error, cannot be displayed to the pilot by such receivers.
In receivers with no RAIM capability, no alert would
be provided to the pilot that the navigation solution had
deteriorated and an undetected navigation error could occur.
A systematic cross-check with other navigation techniques
would identify this failure and prevent a serious deviation.
In many receivers, an updatable database is used for
navigation fixes, airports, and instrument procedures.
These databases must be maintained to the current update
for IFR operation, but no such requirement exists for VFR
use. However, in many cases, the database drives a moving
map display that indicates Special Use Airspace and the
various classes of airspace in addition to other operational
information. Without a current database, the moving map
display may be outdated and offer erroneous information
to VFR pilots wishing to fly around critical airspace areas,
such as a Restricted Area or a Class B airspace segment.
Numerous pilots have ventured into airspace they were trying
to avoid by using an outdated database. If there is not a current
database in the receiver, disregard the moving map display
when making critical navigation decisions.
In addition, waypoints are added, removed, relocated, or re-
named as required to meet operational needs. When using
GPS to navigate relative to a named fix, a current database
must be used to properly locate a named waypoint. Without
the update, it is the pilot’s responsibility to verify the
waypoint location referencing to an official current source,
such as the Chart Supplement U.S., sectional chart, or en
route chart.
In many VFR installations of GPS receivers, antenna location
is more a matter of convenience than performance. In IFR
installations, care is exercised to ensure that an adequate
clear view is provided for the antenna to communicate with
satellites. If an alternate location is used, some portion of
the aircraft may block the view of the antenna increasing the
possibility of losing navigation signal.
This is especially true in the case of hand-held receivers. The
use of hand-held receivers for VFR operations is a growing
trend, especially among rental pilots. Typically, suction cups
are used to place the GPS antennas on the inside of aircraft
windows. While this method has great utility, the antenna
location is limited by aircraft structure for optimal reception
of available satellites. Consequently, signal loss may occur
in certain situations where aircraft-satellite geometry causes
a loss of navigation signal. These losses, coupled with a lack
of RAIM capability, could present erroneous position and
navigation information with no warning to the pilot.
While the use of hand-held GPS receivers for VFR operations
is not limited by regulation, modification of the aircraft, such
as installing a panel- or yoke-mounted holder, is governed by
14 CFR part 43. Pilots should consult a mechanic to ensure
compliance with the regulation and a safe installation.
Tips for Using GPS for VFR Operations
Always check to see if the unit has RAIM capability. If no
RAIM capability exists, be suspicious of a GPS displayed
position when any disagreement exists with the position
derived from other radio navigation systems, pilotage, or
dead reckoning.
Check the currency of the database, if any. If expired, update
the database using the current revision. If an update of an
expired database is not possible, disregard any moving map
display of airspace for critical navigation decisions. Be aware
that named waypoints may no longer exist or may have been
relocated since the database expired. At a minimum, the
waypoints to be used should be verified against a current
official source, such as the Chart Supplement U.S. or a
Sectional Aeronautical Chart.
While a hand-held GPS receiver can provide excellent
navigation capability to VFR pilots, be prepared for
intermittent loss of navigation signal, possibly with no RAIM
warning to the pilot. If mounting the receiver in the aircraft,
be sure to comply with 14 CFR part 43.
Plan flights carefully before taking off. If navigating to user-
defined waypoints, enter them prior to flight, not on the fly.
Verify the planned flight against a current source, such as a
current sectional chart. There have been cases in which one
pilot used waypoints created by another pilot that were not
where the pilot flying was expecting. This generally resulted
in a navigation error. Minimize head-down time in the aircraft
and maintain a sharp lookout for traffic, terrain, and obstacles.
Just a few minutes of preparation and planning on the ground
makes a great difference in the air.
Another way to minimize head-down time is to become very
familiar with the receiver’s operation. Most receivers are not
intuitive. The pilot must take the time to learn the various
keystrokes, knob functions, and displays that are used in
the operation of the receiver. Some manufacturers provide
computer-based tutorials or simulations of their receivers.
Take the time to learn about the particular unit before using
it in flight.
In summary, be careful not to rely on GPS to solve all VFR
navigational problems. Unless an IFR receiver is installed in
accordance with IFR requirements, no standard of accuracy
or integrity can be assured. While the practicality of GPS is
compelling, the fact remains that only the pilot can navigate the
aircraft, and GPS is just one of the pilot’s tools to do the job.
VFR Waypoints
VFR waypoints provide VFR pilots with a supplementary
tool to assist with position awareness while navigating
visually in aircraft equipped with area navigation receivers.
VFR waypoints should be used as a tool to supplement current
navigation procedures. The use of VFR waypoints include
providing navigational aids for pilots unfamiliar with an area,
waypoint definition of existing reporting points, enhanced
navigation in and around Class B and Class C airspace, and
enhanced navigation around Special Use Airspace. VFR
pilots should rely on appropriate and current aeronautical
charts published specifically for visual navigation. If
operating in a terminal area, pilots should take advantage of
the Terminal Area Chart available for the area, if published.
The use of VFR waypoints does not relieve the pilot of any
responsibility to comply with the operational requirements
of 14 CFR part 91.
VFR waypoint names (for computer entry and flight plans)
consist of five letters beginning with the letters “VP” and are
retrievable from navigation databases. The VFR waypoint
names are not intended to be pronounceable, and they are not
for use in ATC communications. On VFR charts, a stand-
alone VFR waypoint is portrayed using the same four-point
star symbol used for IFR waypoints. VFR waypoint collocated
with a visual checkpoint on the chart is identified by a small
magenta flag symbol. A VFR waypoint collocated with a
visual checkpoint is pronounceable based on the name of the
visual checkpoint and may be used for ATC communications.
Each VFR waypoint name appears in parentheses adjacent
to the geographic location on the chart. Latitude/longitude
data for all established VFR waypoints may be found in the
appropriate regional Chart Supplement U.S.
When filing VFR flight plans, use the five-letter identifier as
a waypoint in the route of flight section if there is an intended
course change at that point or if used to describe the planned
route of flight. This VFR filing would be similar to VOR use
in a route of flight. Pilots must use the VFR waypoints only
when operating under VFR conditions.
Any VFR waypoints intended for use during a flight should
be loaded into the receiver while on the ground and prior to
departure. Once airborne, pilots should avoid programming
routes or VFR waypoint chains into their receivers.
Pilots should be especially vigilant for other traffic while
operating near VFR waypoints. The same effort to see and
avoid other aircraft near VFR waypoints is necessary, as
is the case when operating near VORs and NDBs. In fact,
the increased accuracy of navigation through the use of
GPS demands even greater vigilance as there are fewer
off-course deviations among different pilots and receivers.
When operating near a VFR waypoint, use all available
ATC services, even if outside a class of airspace where
communications are required. Regardless of the class of
airspace, monitor the available ATC frequency closely for
information on other aircraft operating in the vicinity. It is
also a good idea to turn on landing light(s) when operating
near a VFR waypoint to make the aircraft more conspicuous
to other pilots, especially when visibility is reduced.
Lost Procedures
Getting lost in flight is a potentially dangerous situation,
especially when low on fuel. If a pilot becomes lost, there
are some good common sense procedures to follow. If a town
or city cannot be seen, the first thing to do is climb, being
mindful of traffic and weather conditions. An increase in
altitude increases radio and navigation reception range and
also increases radar coverage. If flying near a town or city, it
may be possible to read the name of the town on a water tower.
If the aircraft has a navigational radio, such as a VOR or ADF
receiver, it can be possible to determine position by plotting
an azimuth from two or more navigational facilities. If GPS
is installed, or a pilot has a portable aviation GPS on board,
it can be used to determine the position and the location of
the nearest airport.
Communicate with any available facility using frequencies
shown on the sectional chart. If contact is made with a
controller, radar vectors may be offered. Other facilities may
offer direction finding (DF) assistance. To use this procedure,
the controller requests the pilot to hold down the transmit
button for a few seconds and then release it. The controller
may ask the pilot to change directions a few times and repeat
the transmit procedure. This gives the controller enough
information to plot the aircraft position and then give vectors
to a suitable landing site. If the situation becomes threatening,
transmit the situation on the emergency frequency 121.5 MHz
and set the transponder to 7700. Most facilities, and even
airliners, monitor the emergency frequency.
Flight Diversion
There may come a time when a pilot is not able to make it to
the planned destination. This can be the result of unpredicted
weather conditions, a system malfunction, or poor preflight
planning. In any case, the pilot needs to be able to safely
and efficiently divert to an alternate destination. Risk
management procedures become a priority during any type
of flight diversion and should be used the pilot. For example,
the hazards of inadvertent VFR into IMC involve a risk that
the pilot can identify and assess and then mitigate through a
pre-planned or in-flight diversion around hazardous weather.
Before any cross-country flight, check the charts for airports
or suitable landing areas along or near the route of flight.
Also, check for navigational aids that can be used during a
diversion. Risk management is explained in greater detail in
Chapter 2, Aeronautical Decision-making.
Computing course, time, speed, and distance information in
flight requires the same computations used during preflight
planning. However, because of the limited flight deck space
and because attention must be divided between flying the
aircraft, making calculations, and scanning for other aircraft,
take advantage of all possible shortcuts and rule-of-thumb
computations.
When in flight, it is rarely practical to actually plot a course
on a sectional chart and mark checkpoints and distances.
Furthermore, because an alternate airport is usually not very far
from your original course, actual plotting is seldom necessary.
The course to an alternate destination can be measured
accurately with a protractor or plotter but can also be
measured with reasonable accuracy using a straightedge
and the compass rose depicted around VOR stations. This
approximation can be made on the basis of a radial from a
nearby VOR or an airway that closely parallels the course
to your alternate destination. However, remember that the
magnetic heading associated with a VOR radial or printed
airway is outbound from the station. To find the course to
the station, it may be necessary to determine the reciprocal of
that heading. It is typically easier to navigate to an alternate
airport that has a VOR or NDB facility on the field.
After selecting the most appropriate alternate destination,
approximate the magnetic course to the alternate using a
compass rose or airway on the sectional chart. If time permits,
try to start the diversion over a prominent ground feature.
However, in an emergency, divert promptly toward your
alternate destination. Attempting to complete all plotting,
measuring, and computations involved before diverting to the
alternate destination may only aggravate an actual emergency.
Once established on course, note the time, and then use the
winds aloft nearest to your diversion point to calculate a
heading and GS. Once a GS has been calculated, determine a
new arrival time and fuel consumption. Give priority to flying
the aircraft while dividing attention between navigation and
planning. When determining an altitude to use while diverting,
consider cloud heights, winds, terrain, and radio reception.
Chapter Summary
This chapter has discussed the fundamentals of VFR
navigation. Beginning with an introduction to the charts that
can be used for navigation to the more technically advanced
concept of GPS, there is one aspect of navigation that remains
the same—the pilot is responsible for proper planning and
the execution of that planning to ensure a safe flight.