Chapter 3 Basic Instruments Introduction Instruments mechanically measure physical quantities or properties with varying degrees of accuracy. Much of a navigator’s work consists of applying corrections to the indications of various instruments and interpreting the results. Therefore, navigators must be familiar with the capabilities and limitations of the instruments available to them. A navigator obtains the following flight information from basic instruments: direction, altitude, temperature, airspeed, drift, and groundspeed (GS). Some of the basic instruments are discussed in this chapter. The more complex instruments that make accurate and long distance navigation possible are discussed in later chapters. 3-1 Direction The force of the magnetic field of the earth can be divided into two components: the vertical and the horizontal. The Basic Instruments relative intensity of these two components varies over the The navigator must have a fundamental background in earth so that, at the magnetic poles, the vertical component navigation to ensure accurate positioning of the aircraft. Dead is at maximum strength and the horizontal component is reckoning (DR) procedures aided by basic instruments give minimum strength. At approximately the midpoint between the navigator the tools to solve the three basic problems of the poles, the horizontal component is at maximum strength navigation: position of the aircraft, direction to destination, and the vertical component is at minimum strength. Only and time of arrival. Using only a basic instrument, such as the the horizontal component is used as a directive force for a compass and drift information, you can navigate directly to magnetic compass. Therefore, a magnetic compass loses its any place in the world. Various fixing aids, such as celestial usefulness in an area of weak horizontal force, such as the and radar, can greatly improve the accuracy of basic DR area around the magnetic poles. The vertical component procedures. This chapter discusses the basic instruments used causes the end of the needle nearer to the magnetic pole to for DR and then reviews the mechanics of DR, plotting, wind tip as the pole is approached. [Figure 3-1] This departure effect, and computer solutions. from the horizontal is called magnetic dip. Directional information needed to navigate is obtained by Compasses use of the earth’s magnetic lines of force. A compass system uses a device that detects and converts the energy from these A compass may be defined as an instrument that indicates lines of force to an indicator reading. The magnetic compass direction over the earth’s surface with reference to a known operates independently of the aircraft electrical systems. datum. Various types of compasses have been developed, Later developed compass systems require electrical power each of which is distinguished by the particular datum used to convert these lines of force to an aircraft heading. as the reference from which direction is measured. Two basic types of compasses are in current use: the magnetic Earth’s Magnetic Field and gyrocompass. The earth has some of the properties of a bar magnet; however, its magnetic poles are not located at the geographic The magnetic compass uses the lines of force of the earth’s poles, nor are the two magnetic poles located exactly opposite magnetic field as a primary reference. Even though the each other as on a straight bar. The north magnetic pole is earth’s field is usually distorted by the pressure of other located approximately at 73° N latitude and 100° W longitude local magnetic fields, it is the most widely used directional on Prince of Wales Island. The south magnetic pole is located reference. The gyrocompass uses as its datum an arbitrary at 68° S latitude and 144° E longitude on Antarctica. fixed point in space determined by the initial alignment of the gyroscope axis. Compasses of this type are widely used today The earth’s magnetic poles, like those of any magnet, can be and may eventually replace the magnetic compass entirely. considered to be connected by a number of lines of force. These lines result from the magnetic field that envelops the earth. Magnetic Compass They are considered Earth’sto be emanating magnetic from field the south magnetic The magnetic compass indicates direction in the horizontal pole and terminating at the north magnetic pole. [Figure 3-1] 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 Magnetic north pole of the compass. These secondary fields are caused by the presence of ferromagnetic objects. Magnetic compasses may be divided into two classes: N 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 S Magnetic lines of force needle; and 2. The remote-indicating gyro-stabilized magnetic compass. South magnetic pole Magnetic direction is sensed by an element located at positions where local magnetic fields are at a minimum, Figure 3-1. Earth’s magnetic field. The Earth’s magnetic field compared to a bar magnet 3-2 such as the vertical stabilizer and wing tips. The direction meridian and the geographic meridian is called the magnetic is then transmitted electrically to repeater indicators on the variation. Variation is listed on charts as east or west. instrument panels. When variation is east, magnetic north (MN) is east of true north (TN). Similarly, when variation is west, MN is Direct-Indicating Magnetic Compass west of TN. [Figure 3-3] Lines connecting points having Basically, the magnetic compass is a magnetized rod pivoted the same magnetic variation are called isogonic lines. at its middle, with several features incorporated to improve [Figure 3-4] Compensate for magnetic variation to convert its performance. One type of direct-indicating magnetic a compass direction to true direction. compass, the B-16 compass (often called the whiskey compass), is illustrated in Figure 3-2. It is used as a standby Compass error is caused by nearby magnetic influences, such compass in case of failure of the electrical system that as magnetic material in the structure of the aircraft and its operates the remote compasses. It is a reliable compass and electrical systems. These magnetic forces deflect a compass gives good navigational results if used carefully. needle from its normal alignment. The amount of such deflection is called deviation which, like variation, is labeled Magnetic Variation and Compass Errors “east” or “west” as the north-seeking end of the compass is The earth’s magnetic poles are joined by irregular curves deflected east or west of MN, respectively. called magnetic meridians. The angle between the magnetic Instrument lamp Float Filler hole and plug Lubber line Compass card N 33 30 27 Lens Fluid chamber Sensing magnet N-S E-W Compensating screws Expansion unit Pivot assembly Outer case Compensating magnet Figure 3-2. Magnetic compass. NP NP NP East West variation MP MP variation MP Zero variation N N N N 33 33 33 33 3 3 3 3 30 30 30 30 30 6 6 6 6 6 W W W W E E E E 24 24 24 12 12 12 21 21 21 15 15 15 S S S SP SP SP Figure 3-3. Effects of variation. 3-3 105˚E 120˚E 135˚E 150˚E 165˚E 180˚ 165˚W 150˚W 135˚W 120˚W 105˚W 90˚W 75˚W 60˚W 45˚W 30˚W 15˚W 0˚ 15˚E 30˚E 45˚E 60˚E 75˚E 80˚ 100˚ 120˚ 140˚ 160˚ 180˚ 160˚ 140˚ 120˚ 100˚ 80˚ 60˚ 40˚ 20˚ 0˚ 20˚ 40˚ 60˚ 80˚ 100˚ 20˚E 30˚E 30˚E 10˚E 75˚N 30˚E 75˚N 50˚E 70˚W 20˚W 10˚W 20˚E 40˚E 60˚W 50˚W 20˚E 0˚ 78˚ 0˚ 78˚ 40˚W 30˚E 10˚E 60˚N 60˚N 30˚W 10˚E 10˚W NO VARIATION 20˚E 45˚N 45˚N NO VARIATION 30˚N 10˚E 30˚N NO VARIATION 15˚N 15˚N 10˚W 10˚E NO VARIATION 10˚W 0˚ 20˚W 0˚ 20˚W 30˚W 20˚W 15˚S 40˚W 10˚W 30˚W 15˚S 20˚E 50˚W 30˚S 30˚S 40˚W 30˚E ˚ 40˚ 20˚ 0˚ 6 ˚ 40˚ 20˚ 0˚ 6 50˚W 60 45˚S 60 40˚E 45˚S 60˚W 70˚W 50˚E ˚ ˚ 70˚W 60˚E 80˚W 69 69 60˚ 80˚ 100˚ 120˚ 140˚ 160˚ 180˚ 160˚ 140˚ 120˚ 100˚ 80˚ 60˚ 40˚ 20˚ 0˚ 20˚ 40˚ 60˚ 80˚ 100˚ 105˚E 120˚E 135˚E 150˚E 165˚E 180˚ 165˚W 150˚W 135˚W 120˚W 105˚W 90˚W 75˚W 60˚W 45˚W 30˚W 15˚W 0˚ 15˚E 30˚E 45˚E 60˚E 75˚E Figure 3-4. Isogonic lines show same magnetic variation. The correction for variation and deviation is usually expressed as a plus or minus value and is computed as a correction to True north Magnetic north true heading (TH). If variation or deviation is east, the sign Compass north of the correction is minus; if west, the sign is plus. A rule of thumb for this correction is easily remembered as east is TO FIND COMPASS HEADING least and west is best. TH VARMH DEV CH 138 −10 ? −3 ? Aircraft headings are expressed as TH or magnetic headings Deviation 3°E C o 138 −10 128 −3 125 m (MH). If the heading is measured in relation to geographical Variation 10°E p M a a s g s north, it is a TH.