How do true and magnetic headings differ in flight simulation?
Learn how true and magnetic headings differ in flight simulation, how to convert them, what reference to follow and why displays disagree.
In aviation and real-world flying, true heading is the direction the aircraft’s nose points relative to geographic north; magnetic heading measures it from magnetic north. Their difference is local magnetic variation. Sim cockpits usually show magnetic heading, while geographic maps and METAR wind directions use true north; navigation computers may convert between both.
What causes true and magnetic headings to differ?
Magnetic variation, also called declination, is the angle between true north and magnetic north at a particular location. It changes across the world and slowly changes over time, so there is no single correction that works for an entire flight.
| Reference | Measured from | Typical use |
|---|---|---|
| True heading | Geographic north | Geographic mapping, some planning calculations and true-reference operations |
| Magnetic heading | Magnetic north | Heading indicators, HSIs, most ATC instructions and runway references |
| Compass heading | The aircraft’s magnetic compass indication | Magnetic heading affected by compass deviation and, temporarily, turning or acceleration errors |
True values are commonly marked with °T and magnetic values with °M. A conventional directional gyro is not itself a magnetic sensor, but the pilot aligns it with the magnetic compass; slaved instruments and most glass displays perform that alignment automatically. Our overview of how onboard navigation systems handle position and direction explains the wider relationship between these references.
How do you convert true heading to magnetic heading?
Use the magnetic variation at the aircraft’s location and keep east and west corrections separate.
- Find the variation. Read it from the applicable chart, navigation display or simulator data. Our guide to interpreting aeronautical chart information covers how directional values are presented.
- Convert true to magnetic. Subtract easterly variation and add westerly variation: “east is least, west is best”.
- Normalise the result. If the answer falls below 000°, add 360°; if it reaches 360° or more, subtract 360°.
For example, a heading of 090°T with 10°E variation becomes 080°M. The same 090°T heading with 10°W variation becomes 100°M. Using east as a positive value, the formula is Magnetic = True − Variation; reverse it with True = Magnetic + Variation.
Magnetic variation is not a wind correction. Wind changes the heading required to hold a desired course or track; variation only changes the north reference used to express that direction.
Which heading should you use in a flight simulator?
Use the reference specified by the instrument, chart or instruction rather than assuming every directional number uses the same north.
- Cockpit and ATC: Heading indicators, heading bugs and ordinary ATC heading assignments normally use magnetic degrees. True-reference procedures exist in some high-latitude operations and should be clearly identified.
- Charts and flight plans: Most published runway directions and route bearings are magnetic unless marked otherwise, even though the underlying map is geographically aligned.
- Weather: METAR wind directions are reported relative to true north, while operational broadcasts and controller wind reports commonly use magnetic references. The exact convention is covered in our explanation of true and magnetic wind directions in ATIS and weather reports.
- FMS and GPS displays: Check for a
MAGorTRUEannunciation. Do not manually apply variation if the system has already converted the value.
Are runway numbers true or magnetic?
Runway numbers normally represent the runway’s approximate magnetic direction rounded to the nearest ten degrees, with the final zero omitted. Runway 27 therefore points roughly towards 270°M, not necessarily exactly 270°.
Magnetic variation changes can eventually cause a real runway to be renumbered. In a simulator, painted numbers, airport data and navigation data may represent different dates, so a runway label can disagree with the displayed magnetic course by several degrees. Some high-latitude regions also use special true-reference conventions.
Why do simulator headings sometimes disagree?
Most apparent heading errors are reference or label mismatches rather than a broken compass.
- Identify the quantity. Heading is where the nose points, course is the intended path, track is the path over the ground and bearing points towards an object. In a crosswind, heading and GPS track should differ even when both use magnetic north. See our worked FSX examples separating heading, course, track and bearing for the practical distinctions.
- Check for MAG or TRUE. A difference close to the local magnetic variation usually means one source is true and the other magnetic.
- Avoid correcting twice. Flight-management systems, GPS units and many planning tools apply variation automatically. Adding it again creates an error roughly equal to the local variation.
- Synchronise the heading indicator. An unslaved directional gyro drifts and must periodically be aligned with the magnetic compass. If the discrepancy grows during the flight, gyro drift is more likely than a true-versus-magnetic problem.
- Allow for different data epochs. The simulator, an add-on aircraft, navigation data and airport scenery may use different magnetic models or dates. Updating navigation data alone does not necessarily change painted runway numbers or the simulator’s magnetic-variation model.
- Consider compass behaviour. If only the standby magnetic compass disagrees, aircraft deviation and turning or acceleration errors may be responsible. How fully these effects are modelled depends on the simulator and aircraft.
A useful diagnosis is simple: a fixed difference matching local variation points to true versus magnetic references; a discrepancy that grows with time suggests gyro drift; and a heading-versus-track difference that changes with wind is normal aerodynamic behaviour.