Advanced Pilot Training: Navigation - Section IV Compasses and Compass Errors
28. Need for compass.-Human beings do not have that sense which will allow them to move in any direction without some outside means Of orientation. Certain animals, such as pigeons, the wild duck, and the domesticated cat, have this sense highly developed and their exploits are well known. Man, however, must depend on some mechanical means or must use the celestial bodies to find his way along the surface of the earth. Even the ancient mariners depended upon the sun and stars to give them directions when at sea. When the sky was covered with fog or clouds they were forced to anchor if they were out of sight of land. It was not until the 12th century that the use of the magnetic compass became known in Europe.
29. Magnetism-a. Lodestone.-It was discovered that a certain ore called "lodestone" had the particular property of always pointing in the same direction when freely suspended in space. This discovery led to the development of the first instrument for indicating direction. It was discovered that this peculiar property of lodestone was due to magnetic influence, and this in turn brought the further discovery that the earth is a huge magnet.
b. Some laws of magnetism.- (1) The general law applying to all magnets is that like magnetic poles repel and unlike poles attract.
(2) The field is the space surrounding a magnet in which the magnetic forces act.
(3) The poles of a magnet are the places where the lines of force enter the magnet.
(4) Dip is the vertical angle between the longitudinal axis of a magnetized needle, freely suspended, and the horizontal. The dip angle is zero at the magnetic equator. As the compass is carried north or south, the angle increases until the maximum dip is reached at the magnetic poles. The magnetic equator is an imaginary, irregular line circumventing the earth. It does not coincide with the geographic Equator.
(5) A bar magnet freely suspended in a magnetic field will take a position parallel to the magnetic lines of force.
(6) A compass is simply a magnetized steel needle which is suspended to allow it to rotate freely in a horizontal plane. The compass needle will aline itself with the earth's magnetic field when it is not influenced by local magnetism.
(7) The earth being a huge magnet has a north magnetic pole and a south magnetic pole. The earth's north magnetic pole is the one situated in the Northern Hemisphere. To avoid confusion it is customary to refer to that end of the compass needle which points to the earth's north magnetic pole as the "north seeking" end of the needle. The other end is the "south seeking" end of the needle. The north seeking end of the needle or a bar magnet is said to have "red" magnetism and the south seeking end is said to have "blue" magnetism.
(8) Unfortunately the geographical poles and the magnetic poles do not coincide, thus the compass needle does not usually point toward true north. This discrepancy in direction i s* known as Civariation."
30. Magnetic compass.-a. Methods.-There are four general methods by means of which direction is established without visual reference to the surface of the earth:
(1) Some type of magnetic compass which reacts to the magnetic lines of force of the earth.
(2) A gyro compass which keeps its axis lined up with the rotational axis of the earth. This type has not yet been developed for use in aircraft.
(3) Radio reception, using a radio compass or a radio range.
(4) Celestial observations, using an instrument such as a sun compass.
b. Classes.-There are two general classes of magnetic compasses which depend upon the magnetic field of the earth for their operation:
(1) A type having a magnetic needle which tends to line up parallel to the earth's magnetic field. The Ai~ Corps types B and D operate on this principle.
(2) A type in which a generator rotates, using the earth's magnetic lines of force for its field. The earth inductor compass operates on this principle.
c. Air Corps type B compass.- (1) This type of magnetic compass is the one in general use by pilots. It may be mounted on the instrument panel with the other instruments and its card is read through a window on the rear side of the case or bowl. (See fig. 21.)
(2) The card has a mark or graduation for each 5'. (Some new compasses have 11) graduations.) There is a number or a letter at each 30° interval on the card. Each cardinal direction, north, east, south, and west, is designated by its first letter. Other directions are numbered, with the final zero of the actual value omitted. For example, the direction of 240° would be indicated by the number 24. If the entire card could be removed from the compass and be split in half near the letter N, and then could be unrolled into a straight band, the letters and numerals would appear in the following order (there would be only one N)
N 33 30 W 24 21 S15 12 E 6 3 N
Each of the 30° markings is divided into smaller graduations . Each smallest subdivision represents 5°.
d. Parts.-The principal parts of the magnetic aircraft compass are as follows:
(1) Bowl, which is usually spherical or cylindrical in shape and made of a nonmagnetic material.
(2) Card assembly, which comprises the card, card magnets, spider, pivot, and float when one is used.
(3) Lubber line, which is a wire or thin piece of material fixed with reference to the compass and by which the compass card is read.
(4) Damping fluid, a water white, acid f ree kerosene which completely fills the bowl.
(5) Compensating chamber, where the compensating magnets are held.
(6) Expansion and contraction device, which allows for temperature changes of the liquid.
(7) Antivibration mount, which is the frame by which the compass is attached to the airplane.
(8) Light for illumination of the card at night.
e. Construction.- (1) A magnetic compass is actuated by one or more bar magnets held parallel in a common frame. This frame or card assembly is pivoted at a point above its center of gravity in such a manner that it will balance horizontally. The movement of the card assembly is damped by a liquid in order to minimize the effects of vibration and the relative unsteadiness of the airplane. A shockabsorbing system, consisting usually of springs and felt pads, is also provided to take up a certain amount of vibration in the interest of preserving the pivot and jewel. The liquid serves two other functions: one to prevent corrosion of the pivot and the other parts inside of the bowl; and the other to keep the jewel washed clean of insoluble particles which tend to settle to the bottom of the bowl. A cross-sectional diagram of a magnetic compass is shown in figure
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f. Operation.-If a bar magnet is suspended so as to turn in any direction about its center of gravity, it will take a position with one end pointing northerly and the other end pointing southerly. For this reason the ends of the magnets are known as the north seeking or N end, and the south seeking or S end, respectively. Since the magnetic force acting on the N end is equal and opposite to the force on the S end, the effort on the N end only is considered. The position taken by the magnets in the usual compass is parallel to the earth's magnetic field. Hence, the needles hold the card in one position (errors will be discussed later), so that the direction of the longitudinal axis of the airplane with respect to magnetic north may be determined by reading that part of the card which is designated by the lubber line. When the airplane is turned, and the card comes to rest, the magnetic needles hold the card in the same previous directional position. The window and lubber line have been moved around the card to show a different section of it, and hence a different direction is indicated by the lubber line.
31. Aperiodic or Air Corps type D compass.-This type of compass is used extensively in airplanes which carry a member of the crew designated as navigator. Its description and operation are contained in section III, chapter 2.
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33. Variation.-a. Definition.-The magnetic compass needle if operating perfectly and undisturbed by outside forces will point to the magnetic north pole. The earth's magnetic poles, with the magnetic field which controls the compass, are not located at the geographic poles of the earth. The magnetic pole in the Northern Hemisphere is at approximately lat. 711 N. and long. 961 W., while the southern pole is at lat. 731 S. and long. 1561 E. Variation (Var.) is the angle between the plane of the true meridian and a line passing thru a freely suspended compass needle which is influenced solely by the earth's magnetism. It is named east or west according to the direction of the compass needle from true north. Variation changes with time and place.
b. Isogonic lines.-If the earth's crust were composed of a homogeneous material, the magnetic lines of force would be great circles joining the magnetic poles. The composition of the earth's crust is such, however, that in most localities the direction of the magnetic lines of force deviates considerably from the great circle. Fortunately, science not only has accurately located the magnetic poles but has also determined, with sufficient accuracy for the navigator, the direction of the magnetic lines of force at all places on the earth's surface; furthermore, the small changes in the direction which are gradually taking place have also been computed. An imaginary line connecting points of equal variation is called an "isogonic line." At all points along any given isogonic line the magnetic variation is the same. Figure 24 shows the lines of equal magnetic variation in the United States in 1935 at 5' intervals. Referring to the figure, it may be seen that in the eastern part of the United States the magnetic compass points west of true north (that is, the variation is westerly) ; in the western part of the country the magnetic compass points east of true north (easterly variation). The dividing line between these two areas of opposite variation, the line of 00 variation, is known as the "agonic line." At all points along this line the direction of magnetic north and true north are the same. Minor bends and turns in the isogonic lines are chiefly the result of local attraction. The isogonic lines shown in the figure are not magnetic meridians and should not be so called.
c. Importance.-When a course is referred to magnetic north rather than true north it is known-as a magnetic course. The magnetic course is the true course with variation applied, or the true course plus or minus variation. A magnetic course has no importance of its own to a pilot; it is simply a necessary step in converting a true course to a compass heading, and it must have some name for reference. The correct application of variation is, however, one of the most important steps in navigation. Ships have been piled on the rocks arid airplanes have become completely lost because of misapplication of magnetic variation.
d. Rule.-In
applying variation it is necessary to learn the following rule so
thoroughly that a wrong application is impossible: To convert a
true course into a magnetic course, add westerly variation. Many
pilots have found it helpful to remember the rhymes "East is
least, west is best" or "East is minus, west is plus."
It is important to note that the rule applies only when changing
from true to magnetic course. It must be reversed to change from
magnetic to true course. If the Pilot can fix in his mind the relation
Pictured in figure 25, there will be no question as to the correct
application of magnetic variation. In the figure, N represents the
true geographic meridian, and angle 1 is the true course for the
route shown. M represents the direction of magnetic north in the
vicinity of point 0 and is west of true north as indicated. Angle
NOM is the magnetic variation which is westerly. When magnetic north
lies to the west of true north, the angle NOM must be added to the
true course (angle 1) to obtain the magnetic course (angle 2), or
the magnetic direction of the route. If westerly variation is to
be added, easterly variation must be subtracted; but by always remembering
the rule, add westerly variation, there will never be any danger
of an erroneous treatment. For instance, near Portland, Maine, the
variation is about 17° west, and the compass reading is 17° greater
than the corresponding true course; near Portland, Oregon, the variation
is about 22° east, and the compass reading is 22° less than the
true course for any chosen course.
e. How variation is applied- (1) In order to use the magnetic compass in air navigation, it is necessary to measure the course angle to get the true course, and then to make the correction for variation in order to know the magnetic course which will correspond to the true course. This magnetic course is later converted to compass course as will be described in the following paragraphs. For long flights, after dividing the route into sections of practical length and determining the series of true courses, the average magnetic variation for each section is applied in order to find the series of magnetic courses. (This is unnecessary on short flights in United States latitudes.)
(2) If the above procedure is disregarded and a long route is flown on one mean magnetic course, considerable departure from the intended track may result. For example, figure 26 shows the conditions actually existing in 1935 along the Canadian border between longitudes 90° and 96°, a distance of 273 miles. The true course for the route from point 0 to point C is 270°, the magnetic course at the point 0 is 268°. While the mean magnetic course is flown for the entire distance beginning at 0, the course is in error by about 4°, and the plane will track the broken line south of the parallel. At the center of the route the track will be 4.1 miles south of the parallel, gradually returning to meet it at C. The departure from this course is greatest where the greatest differences in magnetic variation occur. The maximum departure does not always occur at the midpoint of the route. It will only occur at the midpoint when the isogons are equally spaced.
(3) Variation cannot be reduced or eliminated, but it is constant for all headings of the aircraft in any one locality.
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37. Errors of compass in flight.-a. General.-Whenever the compass card is tilted to the eastward or westward it is acted upon by the vertical component of the earth's magnetic force and is rotated so as to give a false reading. In the magnetic compass the vertical component exerts its greatest downward pull at the northseeking end of the card in north latitudes and at the south-seeking end in south latitudes. In either case the force causing rotation will be the greatest when the card is tilted about the N-S line; that is, when either the east or west side of the card is depressed.
b. Acceleration errors.-Compass cards are held level under normal conditions, due to the fact that the card is designed with its center of gravity below the pivot, and it therefore acts like a pendulum. When acceleration forces act upon the pendulum coinpass card assembly, it will swing away from the vertical like any other pendulum, and the plane of the compass needles will be tilted with respect to the horizontal. As stated before, the resulting rotation of the card will be a maximum when the acceleration forces cause the card to tilt about its N-S line; in other words, when the acceleration is in an east or west direction. Such accelerations occur when a plane headed east or west speeds up or slows down (speed error), and when a plane headed north or south turns toward the east or west (northerly turning error). In every case in northern latitudes the north side of the cord will rotate from north toward the low side of card. In south latitudes the south side of the card will rotate from south toward the low side of the card. At the magnetic equator, where there is no vertical component, there will be no errors due to acceleration.
(1) Northerly turning error.-When making a turn, the forces of acceleration act at right angles to the heading of the plane. Thus, in northern latitudes, if the aircraft is headed in a northerly direction but is making a turn to the east, the pendulous assembly will be pulled to the west by the forces of acceleration, and the east side of the card will be depressed below the horizontal. The vertical component of the earth's magnetism will then cause the north point of the card to rotate toward the east or low side. Thus, the compass reading will be less than it should be and may, in certain cases, even falsely indicate that a turn is being made to the westward. When flying blind this is a dangerous thing as the pilot applying sufficient rudder to make the compass indicate a turn to the eastward may place the aircraft in a spinning attitude. This same action occurs when a turn is made to the westward from a northerly heading, in that the compass will read less than it should and may even indicate a turn in the opposite direction. On southerly headings in north latitudes a turn is not so dangerous, because the card indicates the turn in the correct direction but of greater magnitude than actually made. In southern latitudes the situation is reversed, and a turn from southerly headings is the most dangerous, since here the compass will indicate a turn of less magnitude than is being made, or may even indicate a turn in the opposite direction.
(2) Speed errors.-Speed errors due to acceleration or deceleration of speed are maximum on east and west headings. In normal flight, speed accelerations are small. The accelerations become large when entering and pulling out of a dive, however, and if the dives are made on east or west headings large deviations of the compass will be noted. The compasses will also be slightly deflected during takeoff s and landings on east and west headings.
(3) Swirl errors.-Swirl errors occur whenever turns are made. They are caused by the fact that the liquid in the compass is rotated a certain amount due to the friction with the compass case. The movement of this liquid rotates the card in the direction the turn is made, the amount of rotation being dependent upon the design of the compass, the liquid used, and the magnitude of the turn.
c. Vibration error.-A certain engine speed may cause vibrations which affect the compass. Instances have been recorded when compasses have begun spinning due to vibration and continued to spin until the "period" of the vibration was changed. A change in engine speed will usually break the period of the vibration.
d. Minimizing errors.-It will be apparent f rom the above that the compass should be read only when the airplane is flying straight and level at a constant speed. By reading the compass only under these conditions, acceleration errors can be held to a minimum. The compass is not a satisfactory instrument for determining the rate of a turn or even the direction when acceleration is taking place.
38. Summary-a. The compass is the most important navigational instrument in the aircraft and its use is essential except for short flights in excellent weather. The magnetic compass is subject to the laws of magnetism.
b. Compass readings should be taken while the airplane is flying straight and level at a constant speed. The compass will not record correctly when the airplane is accelerating or decelerating as occurs when the airplane is turning, diving, or climbing. Nor will the compass indicate correct magnetic direction when the needle is vibrating.
c. Variation cannot be reduced or eliminated but it is constant for all headings in any one locality.
d. Deviation error may be either eliminated or reduced and is different for different headings in the same locality. The deviations of the compass are determined by a process called calibration. Since these deviations are determined with the airplane in flying position, the deviations will only hold good when the airplane is flying straight and level.
e. When changing a true course to a magnetic course use the rule "East is least, west is best." Subtract easterly and add westerly variation.
f. Use the same rule for applying deviations when changing from a magnetic course to a compass course.
