Flight Training - July 2020 - 15

HOW IT WORKS

BY IAN J. TWOMBLY

COURSE DEVIATION INDICATOR
THE NEEDLE KNOWS

THERE'S A REASON VORS ARE GOING OUT OF VOGUE. THEY ARE CHALLENGING TO USE,

not as accurate as more modern methods of navigation, and the ground stations are
expensive to maintain.
But as much as we'd like to rip the theory pages out of our books and the equipment out of our airplanes, VORs have their place. Thousands of aircraft still don't have
GPS, and the VOR network serves as a critical backup.
The VOR system has many components, including a ground-based radio transmission station, a panel-mounted navigation radio to receive signals, and a navigator
head. The panel's navigator head includes the OBS (omni-bearing selector) and CDI
(course deviation indicator). Learning to use a CDI with a VOR also will apply to GPS
navigation in an airplane with a panel-mounted GPS.
The CDI acts somewhat like a course line, showing the pilot the relationship of
the airplane to the selected course. If you're flying westbound, for example, and the
OBS is properly tuned and the CDI is to the left (or south), you know the airplane is
north of the desired course.
To show a course line the navigation radio receives two signals from the VOR
station, one fixed and one variable. The fixed signal is the same regardless of the radial
selected, while each radial produces a slightly different variable signal. The cockpit
portion of the system determines which radial the airplane is on based on the phase
difference between the fixed and variable signals. By spinning the OBS the pilot can
center the CDI or select a desired course.

TRUE OR MAGNETIC?
Because a VOR is an electronic navigation aid, the
easiest and most direct
way to orient the course
is to magnetic north.
This means on a day with
no wind, your compass
would match the OBS,
which would line up with
the runway, and so on.
But not every VOR is
oriented to magnetic
north. In northern Canada where compasses
aren't reliable, VORs are
oriented to true north,
or the geographic north
pole. Curiously, some
VORs in the continental
United States are also
oriented to true north,
most notably a few in
central Florida.

0-degree radial

N

12
90-degree radial

270-degree radial

21

STEVE KARP

S

15

24

FR
FR

180-degree radial

12

OBS

Course
deviation
indicator

E

W

TO

6

30

33

AOPA.ORG/FT  15


http://www.AOPA.ORG/FT

Flight Training - July 2020

Table of Contents for the Digital Edition of Flight Training - July 2020

Contents
Flight Training - July 2020 - Intro
Flight Training - July 2020 - Cover1
Flight Training - July 2020 - Cover2
Flight Training - July 2020 - Contents
Flight Training - July 2020 - 2
Flight Training - July 2020 - 3
Flight Training - July 2020 - 4
Flight Training - July 2020 - 5
Flight Training - July 2020 - 6
Flight Training - July 2020 - 7
Flight Training - July 2020 - 8
Flight Training - July 2020 - 9
Flight Training - July 2020 - 10
Flight Training - July 2020 - 11
Flight Training - July 2020 - 12
Flight Training - July 2020 - 13
Flight Training - July 2020 - 14
Flight Training - July 2020 - 15
Flight Training - July 2020 - 16
Flight Training - July 2020 - 17
Flight Training - July 2020 - 18
Flight Training - July 2020 - 19
Flight Training - July 2020 - 20
Flight Training - July 2020 - 21
Flight Training - July 2020 - 22
Flight Training - July 2020 - 23
Flight Training - July 2020 - 24
Flight Training - July 2020 - 25
Flight Training - July 2020 - 26
Flight Training - July 2020 - 27
Flight Training - July 2020 - 28
Flight Training - July 2020 - 29
Flight Training - July 2020 - 30
Flight Training - July 2020 - 31
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Flight Training - July 2020 - 33
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Flight Training - July 2020 - 35
Flight Training - July 2020 - 36
Flight Training - July 2020 - 37
Flight Training - July 2020 - 38
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Flight Training - July 2020 - 40
Flight Training - July 2020 - 41
Flight Training - July 2020 - 42
Flight Training - July 2020 - 43
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Flight Training - July 2020 - 45
Flight Training - July 2020 - 46
Flight Training - July 2020 - 47
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Flight Training - July 2020 - 55
Flight Training - July 2020 - 56
Flight Training - July 2020 - Cover3
Flight Training - July 2020 - Cover4
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