What's true?
A course is a course, of course
By Bruce Williams.
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 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 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.” 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 VOR-based airways. Your CDI and moving map in the panel and the controller’s display on the ground show you on centerline. In fact, as the AIM reveals, “PBN systems, (with the exception of VOR/DME RNAV equipment) navigate by reference to true north and display magnetic course only for pilot reference.”
However, if you’re obsessive about such discrepancies, not understanding the background can lead you astray. For example, if you use OBS mode to set a course to join a charted track between two fixes, the difference between the GPS and charted numbers may tempt you to “correct” the displayed GPS course to match the chart. Instead, enter or select the charted course. For a scenario where you might make that mistake, see “Resolving a Database Dilemma,” August 2024 AOPA Pilot.
In the future, such details may be irrelevant. The International Civil Aviation Organization (ICAO) and other authorities are considering adopting true north as the standard for all air navigation. (In the far north, courses and runways are already referenced to true north.) According to a briefing from ICAO, at conferences in 2012 and 2018 Canada proposed moving from a magnetic to a true north reference. ICAO says adopting a true north reference worldwide “would simplify charting and aircraft operations and may result in considerable cost savings for air operators, air navigation service providers, airports, avionics original equipment manufacturers, and flight procedures designers going forward.” Indeed, a presentation at webinar in 2022 estimated that 5,656 runways worldwide are currently out of magnetic alignment and 25,732 must be renumbered as the variation changes. That study estimated that updating markings and signs at just a single runway hold-short point costs $10,000.
But I’m not counting on a switch to true anytime soon. And I have other issues to contend with in the meantime. When ATC asks, “What will be your on-course heading?” I still must suppress the snarky reply, “Which do you want to know? My intended course or my heading?” 
Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com
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