3.3 Magnetic Compass Mechanics & Errors

Magnetic Compass Mechanics & Errors

The Magnetic Compass is the only self-contained, direct-reading direction-seeking instrument installed in standard aircraft panels. Because it requires neither vacuum pump suction nor electrical power, it serves as the ultimate heading backup during electrical or gyroscopic instrument failures in IMC. However, because the compass relies on the Earth's magnetic flux lines, it is subject to severe physical and magnetic dip errors whenever the aircraft accelerates, decelerates, or banks in a turn.

Mastering magnetic compass behavior and knowing how to calculate turning rollout points is mandatory for FAA Instrument Rating practical and knowledge exams.

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1. Magnetic Compass Construction & Mechanics

+-----------------------------------------------------------------------------+
|                        MAGNETIC COMPASS CONSTRUCTION                        |
|                                                                             |
|                     +-----------------------------+                         |
|                     |   Sealed Compass Bowl       |                         |
|                     |   (Damping Kerosene Fluid)  |                         |
|                     |                             |                         |
|                     |    [ Jeweled Pivot Bearing] |                         |
|                     |              |              |                         |
|                     |    +---------+---------+    |                         |
|                     |    | Float & Magnets   |    |                         |
|                     |    | (Counterweighted) |    |                         |
|                     |    +---------+---------+    |                         |
|                     |              |              |                         |
|                     |    [ Azimuth Card Scale]    |                         |
|                     +--------------+--------------+                         |
|                                    | (Glass Window & Lubber Line)           |
|                     [ Compass Correction Card ]                             |
+-----------------------------------------------------------------------------+

Internal Components

• Float Assembly: Two small magnetized bar magnets attached beneath a buoyant aluminum float, mounted on a hardened steel pivot resting inside an agate jeweled bearing cup. This design allows the float to rotate freely and tilt up to approximately $18^\circ$.
• Damping Fluid: The sealed bowl is filled with clear, acid-free mineral spirits or white kerosene. This fluid dampens high-frequency mechanical vibration, lubricates the jeweled pivot, and reduces wild swinging oscillations.
• Lubber Line: A vertical reference line etched on the front glass face against which the compass card is read.
• B-N / C-S Compensating Screws: Small adjustable magnetic rods located within the compass housing used by avionics technicians to calibrate and minimize magnetic deviation.

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2. Magnetic Dip & Terrestrial Magnetism

The Earth acts as a giant dipole magnet whose magnetic poles do not coincide with its geographic poles:

• Magnetic Flux Geometry: At the Earth's Magnetic Equator, magnetic lines of flux flow parallel to the Earth's surface ($0^\circ$ magnetic dip angle).
• Polar Flux Tilt: As one travels north or south toward the magnetic poles, flux lines curve steeply downward into the Earth ($90^\circ$ dip angle at the magnetic poles).
• Northern Hemisphere Counterweight: To prevent the north-seeking end of the magnet from dipping downward toward the floor in the Northern Hemisphere, manufacturers attach a tiny counterweight to the south-seeking end of the float assembly. This mechanical imbalance, combined with the vertical component of Earth's magnetic field, creates acceleration and turning dip errors.

+-----------------------------------------------------------------------------+
|                       EARTH'S MAGNETIC DIP FIELD                            |
|                                                                             |
|   [ NORTH MAGNETIC POLE ]  <==== Flux lines plunge 90° straight down        |
|             ^                                                               |
|             |                                                               |
|   [ MID-LATITUDES (USA) ]  <==== Flux lines tilt down at ~50° to 70° angle  |
|             ^                    (Generates severe dip errors: ANDS/NOSE)   |
|             |                                                               |
|   [ MAGNETIC EQUATOR ]     <==== Flux lines are 100% horizontal             |
|                                  (Zero magnetic dip errors)                 |
+-----------------------------------------------------------------------------+

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3. Variation & Deviation

Before analyzing dynamic dip errors, pilots must distinguish between static navigational compass errors: Variation and Deviation.

+-----------------------------------------------------------------------------+
|                   THE TVMDC HEADING CONVERSION SEQUENCE                     |
|                                                                             |
|   [ TRUE COURSE / HEADING ]                                                 |
|              |                                                              |
|              +--- (+/- VARIATION) ["East is Least (-), West is Best (+)"]   |
|              v                                                              |
|   [ MAGNETIC COURSE / HEADING ]                                             |
|              |                                                              |
|              +--- (+/- DEVIATION) [From Compass Correction Card]            |
|              v                                                              |
|   [ COMPASS HEADING ]                                                       |
+-----------------------------------------------------------------------------+

1. Variation (Magnetic Declination)

• Definition: The angular difference between True North (geographic North Pole) and Magnetic North (magnetic pole in Northern Canada/Arctic Ocean).
• Isogonic Lines: Dashed magenta lines drawn across aeronautical sectional and IFR en route charts connecting points of equal magnetic variation (e.g., $7^\circ\text{W}$ or $12^\circ\text{E}$).
• Agonic Line: The unique isogonic line where magnetic variation is $0^\circ$ (True North and Magnetic North are perfectly aligned). In North America, the agonic line runs roughly from Lake Michigan down through Georgia/Florida.
• Calculation Rule: "East is Least (subtract), West is Best (add)."
  • Example: True Heading = $090^\circ$, Variation = $6^\circ\text{W} \rightarrow \text{Magnetic Heading} = 090^\circ + 6^\circ = 096^\circ$.

2. Deviation

• Definition: Magnetic interference caused by localized electromagnetic fields, avionics circuits, radar, engine ignition, and ferrous metal airframe components inside the aircraft.
• Compass Swing & Correction Card: A technician aligns the aircraft on a surveyed airport compass rose on the ground with engine and radios running, adjusting the internal compensating magnets to minimize error. Remaining errors are recorded on the Compass Correction Card mounted below the compass.

| FOR (Heading) | $000^\circ$ | $030^\circ$ | $060^\circ$ | $090^\circ$ | $120^\circ$ | $150^\circ$ | $180^\circ$ | $210^\circ$ | $240^\circ$ | $270^\circ$ | $300^\circ$ | $330^\circ$ | | :--- | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | | STEER | $001^\circ$ | $028^\circ$ | $059^\circ$ | $092^\circ$ | $121^\circ$ | $152^\circ$ | $179^\circ$ | $208^\circ$ | $239^\circ$ | $272^\circ$ | $301^\circ$ | $329^\circ$ |

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4. Magnetic Dip Errors: Acceleration Errors (ANDS)

Acceleration errors occur exclusively when flying on East ($090^\circ$) or West ($270^\circ$) headings (or headings with an east/west component) in the Northern Hemisphere.

+-----------------------------------------------------------------------------+
|                     ACCELERATION DIP ERROR: ANDS                            |
|                                                                             |
|   ON EAST OR WEST HEADINGS (Northern Hemisphere):                           |
|                                                                             |
|   [ ACCELERATE ] ---> Inertia holds south counterweight back                |
|                  ---> Float tilts forward                                   |
|                  ---> Vertical magnetic dip pulls north needle DOWN        |
|                  ===> COMPASS ROTATES INDICATING FALSE TURN TO NORTH        |
|                                                                             |
|   [ DECELERATE ] ---> Inertia throws south counterweight forward             |
|                  ---> Float tilts aft                                       |
|                  ---> Vertical magnetic dip pulls north needle              |
|                  ===> COMPASS ROTATES INDICATING FALSE TURN TO SOUTH        |
|                                                                             |
|   MEMORY RULE: ANDS = Accelerate North, Decelerate South                    |
+-----------------------------------------------------------------------------+

Why ANDS Occurs

• When the aircraft accelerates on an east or west heading, inertia causes the heavier, counterweighted south end of the float to lag behind the pivot point.
• This tilts the float assembly. As the float tilts, the vertical component of Earth's magnetic flux pulls the north-seeking end of the magnet downward, rotating the card.
• Result: The compass card rotates to indicate a false turn toward the North.
• When the aircraft decelerates, inertia throws the counterweighted south end forward, tilting the card aft $ ightarrow$ vertical flux pulls the magnet downward $ ightarrow$ compass indicates a false turn toward the South.
• On Direct North or South Headings: Acceleration/deceleration causes the float to tilt along its magnetic axis, resulting in pitch oscillation but no rotational dip error.

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5. Magnetic Dip Errors: Turning Errors (NOSE / UNOS)

Turning errors occur when turning from or through North or South headings in the Northern Hemisphere.

+-----------------------------------------------------------------------------+
|                       TURNING DIP ERROR: NOSE / UNOS                        |
|                                                                             |
|   TURNING FROM A NORTH HEADING:                                             |
|   - Centrifugal force swings the float's center of gravity outward.         |
|   - The float tilts, allowing vertical magnetic flux to pull North needle.  |
|   - Compass initially turns in the OPPOSITE direction or LAGS behind.       |
|   ===> NORTH OPPOSITE (Undershoot North)                                    |
|                                                                             |
|   TURNING FROM A SOUTH HEADING:                                             |
|   - Centrifugal force tilts float.                                          |
|   - Vertical flux pulls North needle in the direction of the turn.          |
|   - Compass immediately RACES AHEAD / LEADS the actual turn rate.           |
|   ===> SOUTH EXCEEDS (Overshoot South)                                      |
+-----------------------------------------------------------------------------+

Calculating Compass Turning Rollout Points (Lead/Lag Rule)

When flying partial-panel (without a heading indicator or attitude indicator), pilots must execute compass turns using the timed standard rate turn or the Compass Lead/Lag Rollout Formula: $\text{Rollout Lead / Lag Degrees} = \text{Aircraft Latitude} + \frac{1}{2} \times \text{Bank Angle}$

+-----------------------------------------------------------------------------+
|                     COMPASS ROLLOUT CALCULATION WORKFLOW                    |
|                                                                             |
|   EXAMPLE: Aircraft at 30°N Latitude executing a turn with 15° Bank Angle:  |
|   Correction = 30° + (15° / 2) = 30° + 7.5° ≈ 38°                          |
|                                                                             |
|   [ 1. TURNING TO NORTH (e.g., 360°) ]                                      |
|        Compass LAGS -> Pilot must UNDERSHOOT (Roll out early).              |
|        - Left turn to 360°: Roll out when compass reads 038° (360° + 38°)   |
|        - Right turn to 360°: Roll out when compass reads 322° (360° - 38°)  |
|                                                                             |
|   [ 2. TURNING TO SOUTH (e.g., 180°) ]                                      |
|        Compass LEADS -> Pilot must OVERSHOOT (Roll out late).               |
|        - Left turn to 180°: Roll out when compass reads 142° (180° - 38°)   |
|        - Right turn to 180°: Roll out when compass reads 218° (180° + 38°)  |
|                                                                             |
|   [ 3. TURNING TO EAST (090°) OR WEST (270°) ]                              |
|        Zero turning dip error -> Roll out directly at 090° or 270°.         |
+-----------------------------------------------------------------------------+

Summary of Compass Memory Rules

• ANDS: Accelerate North, Decelerate South (on East/West headings).
• NOSE: North Opposite (initial reverse/lag), South Exceeds (leads ahead).
• UNOS: Undershoot North, Overshoot South.
• Oscillation Error: Fluid turbulence and rough mechanical motion cause the compass card to oscillate erratically. Always average the swing of the compass card during smooth, unaccelerated straight-and-level flight.