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Magnetic Compass

Introduction:

  • The magnetic compass was one of the first flight instruments developed
  • A compass is a simple magnetic bar suspended in fluid
  • It floats in a hardened steel pivot in its center that rides inside a special, spring-loaded, hard glass jewel cup
  • The magnetic compass is a reliable, self-contained unit requiring no external power source
    • It is extremely useful as a standby or emergency instrument
  • "Accurate" up to 18° pitch up
  • 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
Figure 1: Magnetic Compass

Magnetism:

  • The Earth is a huge magnet with lines of flux which make its magnetic field
  • These lines extend from the poles around the Earth
  • Characteristics:
    • Any magnet that is free to rotate (such as an aircraft's magnetic compass) will align with them
    • An electrical current is induced into any conductor that cuts across them
  • 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

Magnetic Compass Construction:

  • The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup
  • 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 entirely full of compass fluid, similar to kerosene, to avoid freezing at lower temperatures/higher altitudes
  • The buoyancy of the float takes most of the weight off the pivot, and the fluid damps the oscillation of the float and card
  • This jewel-and-pivot type mounting allows the float freedom to rotate and tilt up to approximately 18° angle of bank
  • At steeper bank angles, the compass indications are erratic and unpredictable
  • To prevent damage or leakage when the fluid expands and contracts with temperature changes, the rear of the compass case is sealed with a flexible diaphragm or with a metal bellows in some compasses

Magnetic Compass Theory of Operations:

  • 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
  • This is done to correct 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 is pointed east or west
  • The other shaft is marked N-S and its magnets affect the compass when the aircraft is pointed north or south

Magnetic Compass Induced Errors:

  • The magnetic compass is the simplest instrument in the panel, but it is subject to a number of errors that must be considered
  • Acronym: VD-MONA
    • Variation
    • Deviation
    • Magnetic Dip
    • Oscillation
    • Northern Turning Errors (part of magnetic dip)
    • Acceleration Errors (part of magnetic dip)

Variation:

  • Maps and charts are drawn using meridians of longitude that pass through the geographic poles
  • Directions measured from the geographic poles are called true directions
  • The north magnetic pole to which the magnetic compass points is not collocated with the geographic north pole, but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions
  • The amount of variation depends on your location in relation to the poles
  • 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
  • Points of equal variation can be connected by an isogonic line on a map
  • The line that passes near Chicago (0°) is called the agonic line
    • East 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°
    • West 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, CA 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°
  • The variation error does not change with the heading of the aircraft; it is the same anywhere along the isogonic line
  • Isogonic lines can be found on sectional charts with a dashed magenta line and the number associated
  • Used to convert true course to magnetic course
  • Memory Aid: West is best (+), East is least (-)

Deviation:

  • 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
  • Different on each heading, but it is not affected by the geographic location
  • Deviation error can be minimized when a pilot or 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 cannot be removed is recorded on a compass correction card and placed in a card-holder near the compass
  • If the pilot wants to fly a magnetic heading of 120° and the aircraft is operating with the 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 completely eliminate it

  • Step 1: Determine the Magnetic Course:
    • True Course (180°) ± Variation (+10°) = Magnetic Course (190°)
    • The Magnetic Course (190°) is steered if there is no deviation error to be applied
    • The compass card must now be considered for the compass course of 190°
  • Step 2: Determine the Compass Course:
    • Magnetic Course (190°, from step 1) ± Deviation (-2°, from correction card) = Compass Course (188°)

NOTE:
Intermediate magnetic courses between those listed on the compass card need to be interpreted. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°

  • To find the true course that is being flown when the compass course is known:
    • Compass Course ± Deviation = Magnetic Course ± Variation = True Course

Magnetic Dip Errors:

  • Compass tends to dip toward the magnetic pole, most dominate as latitude increases
    • The lines of magnetic flux are considered to leave the Earth at the magnetic north pole and enter at the magnetic South Pole
    • At both locations, the lines are perpendicular to the Earth's surface
    • At the magnetic equator, which is 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 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
  • The float is balanced with a small dip-compensating weight, so it stays relatively level when operating in the middle latitudes of the northern hemisphere
  • When the aircraft is flying at a constant speed on a heading of east or west, the float and card is level and the effects of magnetic dip and the weight are approximately equal
  • This dip along with this weight causes two very noticeable errors:
    • Northerly 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 being made
      • When turning from a northerly heading the compass will initially turn the opposite direction and catch up by due east/west
      • 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
      • 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 tilts 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 being 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 being 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
      • UNOS:
        • 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 behind and the card rotates toward north
      • When the aircraft decelerates on a heading of east/west, inertia causes the weight to move ahead and the card rotates toward south
      • 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
      • ANDS:
        • Accelerate North
        • Decelerate South

Oscillation Error:

  • Oscillation is a combination of all of the other errors, and it results in the compass card swinging back and forth around the heading being flown
  • Turbulence causes 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°
  • The dial is rotated by a set of gears 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

The 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 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, there is so much magnetism 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 are connected in such a way that the current flowing in them changes as the heading of the aircraft changes
  • The three coils are connected 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)
  • A magnet aligns with lines of flux and an electrical current is induced, or generated, in any wire crossed by them
  • Made of soft iron that readily accepts 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 are connected 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)

Regulation:


Preflight Check:

  • Full of fluid; card in place and indicating a correct heading
  • Compass card installed (does not need to be filled out)

References: