Magnetic Deviation and the Compass Swing: Hard vs. Soft Iron, Compensating Screws, and Electrical Loads
Updated: Sep 26
Modern aviation relies heavily on advanced avionics, GPS, and automation, yet one of the most fundamental navigational concepts pilots must understand is magnetic deviation. Despite its age-old roots in compass navigation, deviation remains operationally relevant in every cockpit that uses a magnetic compass — which is virtually all of them. But there's real depth to deviation beyond "the aircraft's magnetism throws off the compass": there are two distinct types of magnetism causing it (hard iron and soft iron), a specific procedure with compensating screws to minimize it (the compass swing), a regulatory acceptance standard, and even a preflight technique to check how your electrical loads affect it. Understanding deviation — how it's caused, compensated, and checked — reinforces strong foundational navigation skills.
This post covers magnetic deviation in practical depth: what it is and where it fits, the two types of magnetism causing it (hard vs. soft iron), the compass swing procedure with compensating screws, the deviation card and the 10-degree standard, checking deviation with electrical loads, glass-cockpit magnetometers, and why it still matters.
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Where Deviation Fits: The Big Picture
Before the deep dive, place deviation in the navigational framework.
The four concepts:
True North: the geographic North Pole (charts, maps)
Magnetic North: where a compass points (the magnetic pole)
Variation: the angular difference between true and magnetic north (caused by the EARTH)
Deviation: the compass error caused by the AIRCRAFT's own magnetism
The short version:
Variation is caused by the Earth (geographic, location-based)
Deviation is caused by the aircraft (onboard interference, aircraft-specific)
(For variation in depth, see our magnetic variation post; for the in-flight compass errors — ANDS, UNOS, dip — see our magnetic compass post)
This post's focus:
Deviation specifically — the aircraft-caused error
How it's created (the physics), compensated (the compass swing), and checked
The hands-on, mechanical side
The aircraft's contribution to compass error
What Magnetic Deviation Is
Magnetic deviation is the error in a magnetic compass caused by magnetic fields generated by the aircraft and its equipment.
The sources of aircraft magnetism:
Metal airframe components (ferrous metals)
Electrical wiring (current creates magnetic fields)
Radios and avionics
Alternators and generators
Speakers, motors, and even handheld electronics
The aircraft is full of magnetic influences
The effect:
These components distort the local magnetic field around the compass
The compass indicates a heading that differs from the actual magnetic heading
The difference is deviation
The aircraft "pulls" the compass off
The aircraft-specific nature:
Deviation is unique to each individual aircraft
Depends on its configuration and equipment placement
Even two identical models can differ (avionics upgrades, repairs, modifications, electrical differences)
Not charted (must be measured in that specific aircraft)
The Two Types of Magnetism: Hard Iron vs. Soft Iron
A physics distinction the basic treatment doesn't cover but that explains deviation's behavior.
Hard iron magnetism:
Permanent magnetism in the aircraft (magnetized ferrous components)
Creates a CONSTANT magnetic field (fixed in the aircraft)
Its effect on the compass varies with the aircraft's HEADING (as the aircraft rotates relative to Earth's field)
Causes "semicircular" deviation (a pattern that varies once per 360°)
Soft iron magnetism:
Magnetism INDUCED in the aircraft's ferrous components by the Earth's field
Temporary (induced, changes as the aircraft's orientation changes)
The induced field depends on the aircraft's orientation in Earth's field
Causes "quadrantal" deviation (a pattern that varies twice per 360°)
The distinction:
Hard iron: permanent aircraft magnetism (semicircular error)
Soft iron: induced magnetism from Earth's field (quadrantal error)
Both contribute to total deviation
Different patterns
Why the distinction matters:
The two types cause different deviation patterns
Compensation addresses both (the compensating screws for hard iron, and the overall pattern)
Understanding them explains why deviation varies with heading
The physics behind the card
The heading-dependence:
Because of these effects, deviation VARIES with the aircraft's heading
It's not a single constant value
Different on different headings (hence the card lists multiple headings)
Heading-specific
The Compass Swing Procedure
The hands-on procedure the basic treatment mentions but doesn't detail: how deviation is actually measured and minimized.
When a compass swing is performed:
During initial aircraft certification
After major maintenance (that could affect magnetism)
After avionics installation (new equipment, new magnetic fields)
When compass accuracy is in doubt
Periodically (some operations require it)
Where it's done:
On a COMPASS ROSE (a painted circle at an airport with accurate magnetic headings marked)
Away from buildings, other aircraft, and magnetic interference
Aligned to known magnetic headings
A dedicated location
The compass rose:
A large circle painted on a ramp/taxiway with magnetic headings marked (N, 30, 60, E, etc.)
Surveyed to accurate magnetic directions
The aircraft is aligned to each heading
The reference for the swing
The procedure step by step:
Position the aircraft on the compass rose, aligned to magnetic NORTH (using the rose's markings)
With the engine running and typical electrical loads on (radios, etc.), read the compass
Adjust the NORTH-SOUTH compensating screw to minimize the error on north (and check south)
Align to magnetic EAST, adjust the EAST-WEST compensating screw to minimize the error on east (and check west)
Continue around the rose (typically every 30°), reading the compass on each heading
Record the REMAINING error on each heading on the deviation card
The remaining errors (after compensation) go on the card
The compensating screws:
The compass has two adjustment screws: N-S and E-W
Non-magnetic screwdriver (a brass tool) used to adjust them
They move small compensating magnets inside the compass
Adjusting them cancels some of the hard-iron deviation
The N-S and E-W screws:
The N-S screw is adjusted while aligned north/south (minimizes deviation there)
The E-W screw while aligned east/west
These cancel the semicircular (hard iron) deviation on the cardinal headings
Then the residual is recorded
Why residual deviation remains:
The screws can't cancel ALL deviation (especially the quadrantal/soft-iron component)
Some deviation always remains after compensation
The remaining error goes on the card
Hence the card exists
The Coefficients (A, B, C): How Compensation Works
An advanced concept that explains the swing, for the curious.
The deviation coefficients:
Deviation is analyzed as mathematical coefficients (A, B, C, and higher)
Coefficient A: a constant error on all headings (often a misalignment)
Coefficient B: the east-west (semicircular) component (corrected by the E-W screw)
Coefficient C: the north-south (semicircular) component (corrected by the N-S screw)
Higher coefficients (D, E): quadrantal (soft iron) components
How the swing addresses them:
The E-W screw corrects coefficient B (the east-west hard-iron error)
The N-S screw corrects coefficient C (the north-south hard-iron error)
Coefficient A (constant) may be addressed by realigning the compass
The quadrantal (soft iron) errors generally can't be adjusted out (recorded on the card)
Why this matters:
The compensating screws target the semicircular (hard iron) errors (B and C)
The quadrantal (soft iron) errors remain and go on the card
This is why the card is still needed after compensation
The math behind the procedure
The practical takeaway:
You don't need to calculate coefficients as a pilot
But understanding that the screws fix the cardinal-heading (hard iron) errors, and the rest goes on the card, explains the swing
The compensation is partial by nature
The card captures the residual
The Compass Deviation Card
The card, expanded.
What it is:
A card mounted near the magnetic compass
Lists the compass errors (deviation) on selected headings
Commonly every 30° (or 45°)
The correction reference
The card format:
"For (magnetic heading) steer (compass heading)"
Example: "For 090° steer 088°" (to fly a magnetic heading of 090°, steer 088° on the compass)
Or lists the deviation (+/-) per heading
The corrections
Reading the card:
If the compass reads 090° and the card shows -1° deviation, the actual magnetic heading is 089°
Or: to fly 090° magnetic, steer the compass heading the card specifies
Apply the correction
Small adjustments
The 10-degree standard:
After a compass swing, the acceptance standard is that deviation should not exceed 10° on any heading
If deviation exceeds 10° after compensation, there's a problem (excessive interference)
The card should show errors within acceptable limits (ideally much less than 10°)
The maximum acceptable
Why the card must be current:
After any change affecting magnetism (avionics, etc.), the card should be updated (re-swing)
An outdated card gives wrong corrections
Keep it current
Accuracy
Checking Deviation with Electrical Loads
A practical technique the basic treatment doesn't mention: deviation changes with electrical loads.
The electrical-load effect:
Current flowing through wires creates magnetic fields
Turning on electrical equipment (radios, lights, pitot heat) changes the magnetic environment
This changes the deviation
Deviation isn't just static — it shifts with electrical loads
Why it matters:
The compass swing is done with TYPICAL electrical loads on (to represent normal operation)
But turning on high-draw equipment (landing light, pitot heat) can shift the compass
The deviation with everything on differs from everything off
A dynamic factor
The preflight check:
Some pilots check the compass with various electrical loads
Turn on the landing light, pitot heat, etc., and watch for compass movement
Significant movement indicates electrical interference affecting the compass
A diagnostic
The practical implication:
On a heading, turning on a high-draw item might shift the compass a degree or two
Be aware the compass can be affected by electrical loads
The card assumes typical loads
Account for it if using the compass precisely
The "compass check" in the run-up:
Part of the instrument check: verify the compass is full of fluid (no bubbles), reads correctly, and moves freely
Also note if electrical loads affect it
A pre-takeoff verification
Good practice
Deviation vs. the In-Flight Compass Errors
The distinction, brief (the compass post covers the in-flight errors in depth).
Deviation is one of several compass limitations:
Deviation: aircraft magnetism (constant for a heading, correctable via the card)
Turning errors (UNOS): dip-induced, on north/south headings
Acceleration errors (ANDS): dip-induced, on east/west headings
Dip error: the vertical component of Earth's field
Oscillation/lag: in turbulence
How deviation differs:
Deviation is CONSTANT for a given heading (predictable, on the card)
The in-flight errors (ANDS, UNOS) are DYNAMIC (occur during acceleration/turns)
Deviation is correctable (the card); the dynamic errors are managed by technique
Deviation is the "static" aircraft error
The manageable nature:
Deviation is one of the more manageable compass errors (constant, correctable)
Provided the pilot applies the card
(The dynamic errors — ANDS, UNOS — are covered in our magnetic compass post)
Apply the card, know the dynamic errors
Applying Deviation: The TVMDC Sequence
Where deviation fits in the conversion, expanded.
The sequence:
True → Variation → Magnetic → Deviation → Compass
Deviation is the final step (magnetic to compass)
Converts the magnetic heading to what you steer on the compass
The last conversion
The full example:
True course from the chart: 095°
Apply variation (10°W → add): 105° magnetic
Apply deviation (+2° from the card): 107° compass
Fly 107° on the compass to maintain the 095° true course
The complete flow
The deviation step (M to C):
Take the magnetic heading (after variation)
Apply the deviation from the card (for that heading)
Get the compass heading to steer
The card provides the correction
Why it's last:
Deviation is a compass-specific error
It's applied after variation (which handles Earth's field)
The compass heading is what you actually steer
The end of the sequence
Deviation in Glass Cockpits and Magnetometers
A modern topic the basic treatment doesn't cover: even glass cockpits deal with deviation.
The magnetometer:
Glass-cockpit aircraft use a MAGNETOMETER (an electronic magnetic sensor) for heading
Usually mounted in a wingtip or tail (away from interference)
Provides the magnetic heading to the electronic displays
The modern "compass"
Magnetometer calibration:
The magnetometer must be CALIBRATED (compensated for the aircraft's magnetism)
Similar concept to a compass swing (an electronic version)
Done during installation and after certain maintenance
The glass-cockpit equivalent of the swing
Why placement matters:
The magnetometer is mounted away from interference (wingtip, tail)
Reduces deviation at the source
Better than a panel-mounted compass (near all the avionics)
Design reduces the error
The backup magnetic compass:
Glass-cockpit aircraft STILL have a traditional magnetic compass (backup)
Required (91.205) and independent of electrical power
With its own deviation card
The last-resort reference
The modern reality:
The magnetometer provides heading to the glass displays (calibrated for deviation)
The backup compass has a deviation card
Both deal with deviation (electronically and traditionally)
Deviation persists in modern aircraft
Why Deviation Still Matters in the GPS Age
The enduring relevance, expanded.
Deviation still matters because:
The magnetic compass is REQUIRED equipment (91.205)
It's independent of electrical power (works when everything else fails)
It's a last-resort heading reference
Checkrides and training test deviation knowledge
Many aircraft use magnetic heading sources for backups
The emergency scenario:
In an electrical failure (glass cockpit goes dark)
The magnetic compass (with its deviation card) is your heading reference
Accurate deviation knowledge is critical
The backup
The partial-panel connection:
In a partial-panel situation (gyro/heading indicator failure)
The magnetic compass becomes the heading reference
Applying deviation matters
A raw-navigation skill
The professional standard:
Understanding deviation is part of foundational navigation competence
Not just following automation
Knowing why the compass behaves as it does
The mark of a knowledgeable pilot
Common Deviation Mistakes
The errors, expanded.
Ignoring the deviation card entirely:
Not applying the card corrections
Assuming the compass is exact
Apply the card
A basic error
Assuming deviation is "close enough" to zero:
Deviation can be several degrees (up to 10° acceptable)
Small errors compound over distance
Apply it
Don't dismiss it
Forgetting deviation changes with heading:
Deviation is different on different headings (the card lists them)
Not a single value
Use the correct heading's deviation
Heading-specific
Confusing deviation with variation:
Deviation: aircraft's (onboard, card)
Variation: Earth's (geographic, charts)
Different concepts
Don't mix them
Using outdated or missing deviation cards:
After avionics changes, the card should be updated (re-swing)
A missing/outdated card is a problem (and non-compliant)
Keep it current and present
Final Thoughts
Magnetic deviation may seem like a small correction — often just a few degrees — but aviation is a discipline where small errors compound over time and distance. Understanding deviation deeply — the two types of magnetism causing it (hard and soft iron), the compass swing that compensates it (the N-S and E-W screws targeting the semicircular errors), the deviation card that captures the residual, and even how electrical loads shift it — reinforces strong foundational navigation skills and ensures pilots remain competent even when technology fails.
In aviation, knowing why instruments behave the way they do is just as important as knowing how to use them — and magnetic deviation is a perfect example. Whether you're flying steam gauges with a panel compass and its deviation card, or a glass cockpit with a calibrated magnetometer and a backup compass, deviation is the quiet aircraft-specific error that foundational airmanship accounts for.
On the Written Test and Checkride
Magnetic deviation appears on tests and checkride orals. The most commonly tested topics:
What deviation is (aircraft magnetism) vs. variation (Earth)
The compass deviation card and how to apply it
Compass swinging (how deviation is determined)
The TVMDC sequence (deviation's place: magnetic to compass)
That deviation varies with heading
Why the magnetic compass still matters (backup, required equipment)
Quick Reference
Deviation vs. Variation:
Deviation: aircraft's own magnetism (onboard, aircraft-specific, card)
Variation: Earth's field (geographic, location, charts)
What Causes Deviation:
Metal, wiring, radios, avionics, alternators, motors, electronics
Distort the field around the compass
Aircraft-specific (even identical models differ)
Two Types of Magnetism:
Hard iron: permanent aircraft magnetism → semicircular deviation (varies once per 360°)
Soft iron: induced by Earth's field → quadrantal deviation (varies twice per 360°)
Both cause deviation to vary with heading
Compass Swing:
Done on a compass rose (accurate magnetic headings)
Engine running, typical electrical loads on
Align to headings (every 30°), read the compass
Adjust N-S and E-W compensating screws (cardinal headings)
Record residual errors on the card
When: certification, major maintenance, avionics install, accuracy in doubt
Compensating Screws:
N-S screw: adjusts north/south deviation (coefficient C)
E-W screw: adjusts east/west deviation (coefficient B)
Non-magnetic (brass) screwdriver
Cancel hard-iron (semicircular) errors; soft-iron residual goes on the card
Deviation Card:
"For (magnetic) steer (compass)" every 30°
10° maximum acceptable deviation after swinging
Keep current (re-swing after changes)
Electrical Loads:
Deviation changes with electrical loads (current creates fields)
Swing done with typical loads on
Check: turn on landing light/pitot heat, watch for compass movement
Be aware loads shift the compass
TVMDC Sequence:
True → Variation → Magnetic → Deviation → Compass
Deviation is the LAST step (magnetic → compass)
Example: 095° true → +10°W var → 105° mag → +2° dev → 107° compass
Glass Cockpits:
Use a magnetometer (electronic, wingtip/tail) for heading — calibrated for deviation
Still have a backup magnetic compass (required, with a card)
Deviation persists (electronically and traditionally)
Why It Still Matters:
Magnetic compass is required (91.205), independent of power
Last-resort heading reference (electrical failure, partial panel)
Tested on checkrides
Key Principle:
Magnetic deviation is the aircraft-caused compass error (vs. variation, the Earth-caused one), from hard-iron (permanent) and soft-iron (induced) magnetism that makes it vary with heading. A compass swing on a compass rose minimizes it using the N-S and E-W compensating screws (targeting the cardinal-heading errors), with the residual recorded on the deviation card (10° max). Deviation is the final step in the TVMDC sequence (magnetic to compass), and it persists even in glass cockpits (via the calibrated magnetometer and backup compass).
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.