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2.31.1. Most major commands who do ARAs plan to reach to minimum descent altitude (MDA) at 2
NM and continue at that altitude to the missed approach point (MAP) at 1 NM.
2.31.2. The ARA may begin with or without ground based-vectors. In either case, once on the final
approach, the navigator is directing the aircraft. During the ARA, the navigator must keep the pilot
informed of the altitudes and headings to the runway.
2.32. Summary. In-flight duties for the navigator are many and diverse. As it is true that pilots earn their
money on takeoffs and landings, navigators earn their money during the cruise portion, but they are also
responsible for monitoring the departure and approach. Since crew safety and mission accomplishment
are affected by each crewmember's performance, the success of any mission depends in part on the
navigator's competence. Conscientious performance of in-flight duties can avert embarrassing and
dangerous situations for the entire crew.
AFPAM11-216 1 MARCH 2001 83
Figure 2.8. Typical Airborne Radar Approach Construction Graph.
84 AFPAM11-216 1 MARCH 2001
Chapter 3
BASIC INSTRUMENTS
Section 3A— Introduction
3.1. Basics. 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.
3.1.1. A navigator obtains the following flight information from basic instruments: direction, altitude,
temperature, airspeed, drift, groundspeed (GS).
3.1.2. Some of the basic instruments are discussed in this chapter. The more complex instruments which
make accurate and long distance navigation possible are discussed in later chapters or in specific aircraft
technical orders.
Section 3B— Direction
3.2. Basic Instruments. The navigator must have a fundamental background in navigation to ensure
accurate positioning of the aircraft. Dead reckoning (DR) procedures aided by basic instruments give the
navigator the tools to solve the three basic problems of navigation: position of the aircraft, direction to
destination, and time of arrival. Using only a basic instrument such as the compass and drift information,
you can navigate directly to any place in the world. Various fixing aids, such as celestial and radar, can
greatly improve the accuracy of basic DR procedures. This chapter will discuss the basic instruments
used for DR and then review the mechanics of DR, plotting, wind effect, and computer solutions.
Directional information needed to navigate is obtained by use of the earth's magnetic lines of force. A
compass system uses a device that detects and converts the energy from these lines of force to an
indicator reading. The magnetic compass operates independently of the aircraft electrical systems. Later
developed compass systems require electrical power to convert these lines of force to an aircraft heading.
3.3. Earth's Magnetic Field. The earth has some of the properties of a bar magnet; however, its
magnetic poles are not located at the geographic poles, nor are the two magnetic poles located exactly
opposite each other as on a straight bar. The north magnetic pole is located approximately at 73o N
latitude and 100o W longitude on Prince of Wales Island. The south magnetic pole is located at 68o S
latitude and 144o E longitude on Antarctica.
3.3.1. The earth's magnetic poles, like those of any magnet, can be considered to be connected by a
number of lines of force. These lines result from the magnetic field which envelops the earth. They are
considered to be emanating from the south magnetic pole and terminating at the north magnetic pole as
illustrated in Figure 3.1.
3.3.2. The force of the magnetic field of the earth can be divided into two components: the vertical and
the horizontal. The relative intensity of these two components varies over the earth so that, at the
magnetic poles, the vertical component is at maximum strength and the horizontal component is
minimum. At approximately the midpoint between the poles, the horizontal component is at maximum
AFPAM11-216 1 MARCH 2001 85
strength and the vertical component is minimum. Only the horizontal component is used as a directive
force for a magnetic compass. Therefore, a magnetic compass loses its usefulness in an area of weak
horizontal force such as the area around the magnetic poles.
Figure 3.1. Earth's Magnetic Field Compared to a Bar Magnet.
3.3.3. The vertical component causes the end of the needle nearer to the magnetic pole to tip as the pole
is approached (Figure 3.1). This departure from the horizontal is called magnetic dip.
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