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Pilot's Handbook of Aeronautical Knowledge

Radio Navigation

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




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, and this affects 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
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

If an aircraft has two VOR receivers installed, a dual VOR
receiver check can be made. To accomplish the dual receiver
check, a pilot tunes both VOR receivers to the same VOR
ground facility. The maximum permissible variation between
the two indicated bearings is 4 degrees. A list of the airborne
and ground checkpoints is published in the A/FD.

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 which states the name of the station followed
by "VOR." Many FSS transmit voice messages on the same
frequency that the VOR operates. Voice transmissions should
not be relied upon to identify stations, because many FSS
remotely transmit over several omniranges, which 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
(1) omnibearing selector (OBS) sometimes referred to as
the course selector, (2) a CDI needle (Left-Right Needle),
and (3) 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
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 15-29] If
FROM" is displayed and the course shown is followed, the
aircraft is flown away from the station.

VOR indicator.
Figure 15-29. VOR indicator.