AOPA Pilot Magazine - December 2024 - 92

RUDDER & WRENCH
INSTRUMENT TIP /
What's true?
A course is a course, of course
BY BRUCE WILLIAMS
Your GPS alerts you
to upcoming course
changes.
But the number on the
chart usually won't match
the value shown in your
GPS. Here, along V165
south of OLM, the chart
shows a magnetic course
of 176 degrees, versus 180
degrees in the DTK (desired
track) column in the GPS
flight plan.
for any loaded instrument procedure,
despite differences in displayed magnetic
course that may be attributed to
magnetic variation application. " In other
words, your GPS (today more properly
called a " performance-based navigation
(PBN) system " ) works as intended and
keeps you on track.
The details of what's going on can get
complicated, but the fundamental issue
is a difference between the references
that human designers use when laying
out airways and instrument procedures
and the algorithms that your GPS applies
when it computes and displays courses
for you to fly.
As the AIM explains, the folks who
use the terminal instrument procedures
handbook to create procedures first
establish courses referenced to true
north. Later in the process, they apply
a magnetic variation to set the values
that appear on charts. According to the
AIM, the type of procedure or route
determines which magnetic variation
value is used: " A ground−based NAVAID
[instrument flight procedure] applies the
facility magnetic variation of record to
the true course to get the charted magnetic
course. Magnetic courses on PBN
[that is, GPS-based] procedures are calculated
two different ways. SID/STAR
procedures use the airport magnetic
variation of record, while IFR enroute
charts use magnetic reference bearing. "
YOU ' VE SEEN THE message blinking on
your GPS or primary flight display as you
approach a waypoint along your route: " Left
to 180 degrees in 10 sec. " If the autopilot
is flying, you watch as it smoothly intercepts
the next segment of an airway, feeder
route, or other course, usually without a
second thought about the flashing number.
You probably don't even have to rotate
the course knob as the CDI automatically
slews to the new value and the GPS keeps
you precisely on the centerline. But if you
cross-check the course the navigator displays
against the number on the chart, you'll
usually notice a difference between the values
and, like many IFR pilots I fly with, you
92
AOPA PILOT / December 2024
may wonder aloud what's going on and
which number is " true. "
The answer to these digital discrepancies
lurks deep in Aeronautical
Information Manual 1−1−17 Global
Positioning System (GPS), specifically 5.
GPS Instrument Approach Procedures,
paragraph j: Impact of Magnetic Variation
on PBN Systems. (Save yourself the trouble
of scrolling through that long section
and just search for the key phrase.)
Jargon in the title notwithstanding, the
entry offers reassurance that " a properly
functioning PBN system, containing a
current and accurate navigational database,
should fly the correct ground track
The GPS in your panel, however, makes
" a correction to true north by adding a
magnetic variation calculated with an
algorithm based on aircraft position, or
by adding the magnetic variation coded
in [its] navigational database. This may
result in the PBN system and the procedure
designer using a different magnetic
variation, which causes the magnetic
course displayed by the PBN system and
the magnetic course...on the [chart] to
be different. "
You've probably never fretted much
about a discrepancy that's typically only
3 to 4 degrees, and, as noted earlier, your
navigator works as intended. ATC isn't
nagging pilots to get back on course as
they follow the magenta line to fly VORbased
airways. Your CDI and moving map
AOPA Pilot Magazine - December 2024

Table of Contents for the Digital Edition of AOPA Pilot Magazine - December 2024
