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Compasses (Part One)

in Basic Instruments

A compass may be defined as an instrument that indicates direction over the earth’s surface with reference to a known datum. Various types of compasses have been developed, each of which is distinguished by the particular datum used as the reference from which direction is measured. Two basic types of compasses are in current use: the magnetic and gyrocompass.

The magnetic compass uses the lines of force of the earth’s magnetic field as a primary reference. Even though the earth’s field is usually distorted by the pressure of other local magnetic fields, it is the most widely used directional reference. The gyrocompass uses as its datum an arbitrary fixed point in space determined by the initial alignment of the gyroscope axis. Compasses of this type are widely used today and may eventually replace the magnetic compass entirely.

Magnetic Compass

The magnetic compass indicates direction in the horizontal plane with reference to the horizontal component of the earth’s magnetic field. This field is made up of the earth’s field in combination with other magnetic fields in the vicinity of the compass. These secondary fields are caused by the presence of ferromagnetic objects.

Magnetic compasses may be divided into two classes:

  1. The direct-indicating magnetic compass in which the measurement of direction is made by a direct observation of the position of a pivoted magnetic needle; and
  2. The remote-indicating gyro-stabilized magnetic compass.

Magnetic direction is sensed by an element located at positions where local magnetic fields are at a minimum, such as the vertical stabilizer and wing tips. The direction is then transmitted electrically to repeater indicators on the instrument panels.

Direct-Indicating Magnetic Compass

Basically, the magnetic compass is a magnetized rod pivoted at its middle, with several features incorporated to improve its performance. One type of direct-indicating magnetic compass, the B-16 compass (often called the whiskey compass), is illustrated in Figure 3-2. It is used as a standby compass in case of failure of the electrical system that operates the remote compasses. It is a reliable compass and gives good navigational results if used carefully.

Figure 3-2. Magnetic compass.

Figure 3-2. Magnetic compass. [click image to enlarge]

Magnetic Variation and Compass Errors

The earth’s magnetic poles are joined by irregular curves called magnetic meridians. The angle between the magnetic meridian and the geographic meridian is called the magnetic variation. Variation is listed on charts as east or west. When variation is east, magnetic north (MN) is east of true north (TN). Similarly, when variation is west, MN is west of TN. [Figure 3-3] Lines connecting points having the same magnetic variation are called isogonic lines. [Figure 3-4] Compensate for magnetic variation to convert a compass direction to true direction.

Figure 3-3. Effects of variation.

Figure 3-3. Effects of variation. [click image to enlarge]

Compass error is caused by nearby magnetic influences, such as magnetic material in the structure of the aircraft and its electrical systems. These magnetic forces deflect a compass needle from its normal alignment. The amount of such deflection is called deviation which, like variation, is labeled “east” or “west” as the north-seeking end of the compass is deflected east or west of MN, respectively.

Figure 3-4. Isogonic lines show same magnetic variation.

Figure 3-4. Isogonic lines show same magnetic variation. [click image to enlarge]

The correction for variation and deviation is usually expressed as a plus or minus value and is computed as a correction to true heading (TH). If variation or deviation is east, the sign of the correction is minus; if west, the sign is plus. A rule of thumb for this correction is easily remembered as east is least and west is best.

Aircraft headings are expressed as TH or magnetic headings (MH). If the heading is measured in relation to geographical north, it is a TH. If the heading is in reference to MN, it is a MH; if it is in reference to the compass lubber line, it is a compass heading (CH). CH corrected for variation and deviation is TH. MH corrected for variation is TH.

Figure 3-5. Find true heading by working backwards.

Figure 3-5. Find true heading by working backwards.

This relationship is best expressed by reference to the navigator’s log, where the various headings and corrections are listed as TH, variation (var), MH, deviation (dev), and CH. [Figure 3-5] Thus, if an aircraft is flying in an area where the variation is 10° E and the compass has a deviation of 3° E, the relationship would be expressed as follows, assuming a CH of 125°:

TH var MH dev CH
138 – 10 = 128 – 3 = 125


Variation has been measured throughout the world and the values found have been plotted on charts. Isogonic lines are printed on most charts used in aerial navigation so that, if the aircraft’s approximate position is known, the amount of variation can be determined by visual interpolation between the printed lines. At high altitudes, these values can be considered quite realistic. Conversely, at low altitudes, these magnetic values are less reliable because of local anomalies.

Variation changes slowly over a period of years and the yearly amount of such change is printed on most charts. Variation is also subject to small diurnal (daily) changes that may generally be neglected in air navigation.




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