Magnetic Compass

Magnetic Compass


Magnetism Overview:

  • NASA, The Earth's Magnetosphere
    NASA, The Earth's Magnetosphere
  • The Earth is a huge magnet with lines of flux which make its magnetic field
  • These lines extend from the poles around the Earth [Figure 1]
  • Characteristics:
    • Any magnet that is free to rotate (such as an aircraft's magnetic compass) will align with the poles
  • A magnet, typically made of iron, attracts and holds lines of flux
    • Magnets have a north and a South Pole
    • Opposite poles attract, while similar poles repel
  • NASA, The Earth's Magnetosphere
    NASA, The Earth's Magnetosphere

Magnetic Compass Construction:

  • A compass is a simple magnetic bar suspended in fluid
  • The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup
  • The magnets align with the Earth's magnetic field, and the pilot reads the direction on the scale opposite the lubber line
  • When the pilot is flying North as the compass shows, East is to the pilot's right, but on the card "33", which represents 330° (West of North), is to the right of North
  • The reason for this apparent backward graduation is that the card remains stationary, and the compass housing and the pilot turn around it, always viewing the card from its backside
  • A compensator assembly mounted on the top or bottom of the compass allows an aviation maintenance technician (AMT) to create a magnetic field inside the compass housing that cancels the influence of local outside magnetic fields
    • The compensator corrects for deviation error
  • The compensator assembly has two shafts whose ends have screwdriver slots accessible from the front of the compass
  • Each shaft rotates one or two small compensating magnets
  • The end of one shaft is marked E-W, and its magnets affect the compass when the aircraft points East or West
  • The other shaft is marked N-S, and its magnets affect the compass when the aircraft points North or South
  • An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid
    • The compass housing is full of compass fluid, similar to kerosene, to avoid freezing at lower temperatures/higher altitudes
  • The buoyancy of the float takes weight off the pivot, and the fluid damps the oscillation of the float and card
  • This jewel-and-pivot type mounting allows the float the freedom to rotate and tilt up to approximately 18° angle of bank 45° pitch up/down
  • At steeper bank angles, the compass indications are erratic and unpredictable
  • The rear of the compass case is sealed with a flexible diaphragm or with a metal bellow in some compasses to prevent damage or leakage when the fluid expands and contracts with temperature changes
  • The magnetic compass is a reliable, self-contained unit requiring no external power source
    • It is extremely useful as a standby or emergency instrument
  • A graduated scale, called a card, is wrapped around the float and viewed through a glass window with a lubber line across it
    • The card is marked with letters representing the cardinal directions, North, East, South, and West, and a number for each 30° between these letters
    • The final "0" is omitted from these directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°
    • There are long and short graduation marks between the letters and numbers, with each long mark representing 10° and each short mark representing 5°

Magnetic Compass Errors:

  • The magnetic compass is the simplest instrument in the panel, but it is subject to several errors to consider
  • The acronym "VD-MONA" helps pilots remember compass errors:
  • Variation:

    • Latitude and longitude are based on "true" directions meaning they provide a constant horizontal and vertical plane with which to reference on maps and charts [Figure 2]
    • The magnetic pole to which the magnetic compass points is not collocated with the geographic "true" North pole but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions
    • In aerial navigation, the difference between true and magnetic directions is called variation
      • This same angular difference in surveying and land navigation is called declination
    • An "isogonic" line connects points of equal variation on a map
    • The amount of variation depends on your location relative to the poles
      • Variation in equatorial regions will be less dramatic, as isogonic lines are farther from each other
      • Conversely, at extreme northern and southern regions, isogonic lines are more pronounced as lines are closer together
    • The Agonic Line:

      • The line that passes near Des Moines, Iowa & Little Rock, Arkansas has a variation of 0°, making it the agonic line
        • Right of this line, the magnetic pole is to the West of the geographic pole, and a correction must be applied to a compass indication to get a true direction
          • Flying in the Washington, D.C. area, for example, the variation is 10° West
          • If the pilot wants to fly a true course of South (180°), the variation must be added to this resulting in a magnetic course to fly of 190°
        • Left of this line, the magnetic pole is to the East of the geographic pole, and a correction must be applied to a compass indication to get a true direction
          • Flying in the Los Angeles, California area, the variation is 14° East
          • To fly a true course of 180° there, the pilot would have to subtract the variation and fly a magnetic course of 166°
      • East/West can be tricky to remember given they would otherwise be considered opposite; however, if you think of looking at the world from the pole (vice your location toward the pole), then it makes sense
    • The variation error does not change with the heading of the aircraft; it is the same anywhere along the isogonic line
    • Isogonic lines are depicted on sectional charts with a dashed magenta line and the number associated
    • Used to convert true course to magnetic course
    • Correcting For Variation:

      • True course (170°) ± variation (+10°) = magnetic course (180°)
      • The magnetic course (180°) is flown if there is no deviation error to be applied
    • Variation Memory Aids:

      • West is best (+), East is least (-), or;
      • Variation East, magnetic track least (-) while variation west, magnetic track best (+)
    • Isogonic Lines of Variation
      Pilot Handbook of Aeronautical Knowledge,
      Isogonic Lines of Variation
  • Deviation:

    • Magnetic Compass Deviation Card
      Pilot Handbook of Aeronautical Knowledge,
      Magnetic Compass Deviation Card
    • Local magnetic fields in an aircraft caused by electrical current flowing in the structure, in nearby wiring, or any magnetized part of the structure cause a compass error called deviation
    • Deviation manifests itself differently between aircraft and depending on heading, however, but it is not affected by the geographic location
    • Deviation error is minimized when a pilot or Aviation Maintenance Technition (AMT) performs the maintenance task known as "swinging the compass"
    • Most airports have a compass rose, which is a series of lines marked out on a taxiway or ramp at some location where there is no magnetic interference
    • Lines, oriented to magnetic North, are painted every 30°
    • The pilot or AMT aligns the aircraft on each magnetic heading and adjusts the compensating magnets to minimize the difference between the compass indication and the actual magnetic heading of the aircraft
    • Any error that remains is recorded on a compass correction card and placed in a card-holder near the compass [Figure 3]
    • If the pilot wants to fly a magnetic heading of 120° and the aircraft is operating with radios on, the pilot should fly a compass heading of 123°
    • The corrections for variation and deviation must be applied in the correct sequence and is shown below, starting from the true course desired
    • Error due to magnetic interference with metal components in the aircraft as well as magnetic fields from the aircraft's electrical equipment
    • Compensating magnets inside the compass casing can help reduce this error but not eliminate it
    • Correcting For Deviation:

      • Magnetic Course ± Deviation = Compass Course
      • Assume a magnetic course of 180° as above, ± Deviation (-4° (180-176 = -4, assuming RDO ON), from correction card) = Compass Course (176°)
    • Magnetic Compass Deviation Card
      Pilot Handbook of Aeronautical Knowledge,
      Magnetic Compass Deviation Card
    • Note that intermediate magnetic courses between those listed on the compass card need to be interpolated. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°
    • To find the true course when the compass course is known:
      • Compass course ± deviation = magnetic course ± variation = true course
  • Magnetic Dip:

    • Compass tends to dip toward the magnetic pole, most dominant as latitude increases
      • The lines of magnetic flux leave the Earth at the magnetic North Pole and enter at the magnetic South Pole, pronouncing magnetic dip near the poles
      • At both locations, the lines are perpendicular to the Earth's surface
      • At the magnetic equator, which is roughly halfway between the poles, the lines are parallel with the surface
      • Within 300 miles of the poles, the instrument is unreliable due to extreme errors
    • The south end of the compass is therefore weighted to minimize this error
    • The magnets in a compass align with this field; near the poles, they dip or tilt the float and card
    • A small dip-compensating weight balances the float, so it stays relatively level when operating in the middle latitudes of the northern hemisphere
    • To counter this, the pivot point on which the bar magnet swings is deliberately placed at a position other than the magnet's

      Turning Error:

      • Turning Error
        Turning Error
      • The pull of the vertical component of the Earth's magnetic field causes northerly turning error, which is apparent on a heading of north or south
      • When an aircraft flying on a heading of North makes a turn toward East, the aircraft banks to the right and the compass card tilts to the right
      • The vertical component of the Earth's magnetic field pulls the north-seeking end of the magnet to the right, and the float rotates, causing the card to rotate toward West, the direction opposite the direction the turn is made
      • When turning from a northerly heading, the compass will initially turn the opposite direction and catch up by due East/West [Figure 4]
      • When turning from a southerly heading, the compass will indicate a turn in the proper direction but will lead the actual heading, slowing down by due East/West [Figure 4]
      • The rule for this error is: when starting a turn from a northerly heading, the compass indication lags behind the turn
      • The rule for this error is: When starting a turn from a southerly heading, the compass indication leads the turn
      • If the turn is made from north to West, the aircraft banks to the left, and the compass card will tilt down on the left side
      • The magnetic field pulls on the end of the magnet that causes the card to rotate toward East
      • This indication is again opposite to the direction the turn is made
      • When an aircraft is flying on a heading of south and begins a turn toward East, the Earth's magnetic field pulls on the end of the magnet that rotates the card toward East, the same direction the turn is made
      • If the turn is made from south toward West, the magnetic pull starts the card rotating toward West-again, in the same direction the turn is being made
      • Turning Error Memory Aids:
        • UNOS (northern hemisphere):
          • Undershoot North
          • Overshoot South
      • New heading: roll-out heading ± latitude - half the bank angle
  • Acceleration/Deceleration Error:

    • The dip-correction weight causes the end of the float and card marked N (the south-seeking end) to be heavier than the opposite end
    • When the aircraft is flying at a constant speed on a heading of East or West, the float and card is level
    • When the aircraft accelerates on a heading of East/West, inertia causes the weight to lag, and the card rotates toward North [Figure 5]
    • When the aircraft decelerates on a heading of East/West, inertia causes the weight to move ahead, and the card rotates toward South [Figure 5]
    • The airspeed changes that are needed to make this noticeable are infrequent
    • As soon as the speed of the aircraft stabilizes, the card swings back to its East indication
    • Acceleration/Deceleration Memory Aids:
      Acceleration/Deceleration Error
      Acceleration/Deceleration Error
      • ANDS:
        • Accelerate North
        • Decelerate South
      • SAND:
        • South Accelerate
        • North Decelerate
  • Oscillation Error:

    • Fluid fills the compass body, which provides damping, thereby decreasing unwanted oscillations due to turbulence of the magnet and float
    • Fluid should therefore fill the compass, and no air bubbles or compass fluid discoloration should be present
    • The clear compass face (window) has on it a vertical line called a "LUBBER LINE" so that the pilot can use it as a datum to set the required heading
    • Oscillation is a combination of all of the other errors, and it results in the compass card swinging back and forth around the heading flown
    • Turbulence causes the compass to "bounce" or move in the container
    • When setting the gyroscopic heading indicator to agree with the magnetic compass, use the average indication between the swings
  • The Vertical Card Magnetic Compass:

    • The floating magnet type of compass not only has all the errors just described but also lends itself to confused reading
    • It is easy to begin a turn in the wrong direction because its card appears backward
    • East is on what the pilot would expect to be the West side
    • The vertical card magnetic compass eliminates some of the errors and confusion
    • The dial of this compass is graduated with letters representing the cardinal directions, numbers every 30° and marks every 5°
    • A set of gears rotates the dial from the shaft-mounted magnet, and the nose of the symbolic airplane on the instrument glass represents the lubber line for reading the heading of the aircraft from the dial
    • Eddy currents induced into an aluminum-damping cup damp oscillation of the magnet

    Flux Gate Compass System:

    • As mentioned earlier, the lines of flux in the Earth's magnetic field have two basic characteristics: a magnet aligns with these lines of flux, and an electrical current is induced or generated in any wire crossed by them
    • The flux-gate compass that drives slaved gyros uses the characteristic of current induction
    • The flux valve is a small, segmented ring made of soft iron that readily accepts lines of magnetic flux
    • An electrical coil is wound around each of the three legs to accept the current induced in this ring by the Earth's magnetic field
    • A coil wound around the iron spacer in the center of the frame has 400-Hz alternating current (A.C.) flowing through it
    • During the times when this current reaches its peak, twice during each cycle, so much magnetism is produced by this coil that the frame cannot accept the lines of flux from the Earth's field
    • But as the current reverses between the peaks, it demagnetizes the frame so it can accept the flux from the Earth's field
    • As this flux cuts across the windings in the three coils, it causes current to flow in them
    • These three coils connect in such a way that the current flowing in them changes as the aircraft heading changes
    • The three coils connect to three similar but smaller coils in a synchro inside the instrument case
    • The synchro rotates the dial of a Radio Magnetic Indicator (RMI) or a horizontal situation indicator (HSI)
    • Made of soft iron that readily accepts a line of flux
    • A coil wound around the iron spacer has 400-Hz AC flowing through it
    • As current peaks, twice during each cycle, there is so much magnetism that the frame cannot accept the lines of flux, but as the current reverses, it demagnetizes to accept more
    • The three coils connect to three similar but smaller coils in a synchro inside case
    • The synchro rotated the dial of a Radio Magnetic Indicator (RMI or an HSI)


    • Magnetic direction indicators (of which a magnetic compass satisfies) are required by Federal Aviation Regulation 91.205 to be installed and operational on an aircraft for it to be considered airworthy

    Magnetic Compass Preflight Actions:

    • Full of fluid; card in place and indicating a correct heading
    • Compass card installed (does not need to be filled out)
    • Check the alignment of the direction indicator to comparing to the magnetic compass (indicator) once electrical power is applied and the gyros have had a chance to spin up

    Compass Swing:

    • A compass swing must be performed whenever any ferrous component of the system (i.e. flux valve compensator, or Standby Compass) is installed, removed, repaired, or a new compass is installed
    • The magnetic compass can be checked for accuracy by using a compass rose located on an airport
    • Finding a Compass Rose:

      • Willmar Municipal Airport Compass Rose
        Willmar Municipal Airport Compass Rose
      • While there is no official listings of airports with compass roses, they can be found on many airports [Figure 6]
      • Painted surfaces are of course static, and may be invalidated with the gradual shift in magnetic poles
    • The compass swing is normally effected by placing the aircraft on various magnetic headings and comparing the deviations with those on the deviation cards
    • Refer to CFR 14 23.1327, 23.1547, and the equipment or aircraft manufacturer's manual
    • A compass swing must be performed when:
      • The accuracy of the compass is suspected
      • After any cockpit modification or major replacement involving ferrous metal
      • Whenever a compass has been subjected to a shock; for example, after a hard landing or turbulence
      • After aircraft has passed through a severe electrical storm
      • After lighting strike
      • Whenever a change is made to the electrical system
      • Whenever a change of cargo is likely to affect the compass
      • When an aircraft operation is changed to a different geographic location with a major change in magnetic deviation (e.g., from Miami, Florida to Fairbanks, Alaska.)
      • After aircraft has been parked on one heading for over a year
      • When flux valves are replaced
    • Procedures and precautions are found in Advisory Circular (43.13-1B CHG 1) Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair


    • The magnetic compass remains a time tested reliable instrument, although as avionics evolve, the magnetic compass is being seen increasingly as a standby or back-up instrument
    • Although NOT approved for use by the FAA, products such as the SUUNTO Core All Black Military Men's Outdoor Sports Watch [Amazon] provide a back-up method of direction-finding if the magnetic compass is ever inoperable or in question
    • Deviation errors are unavoidable but is a moving target as any modification may require a compass swing
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