Heading Systems

8.4.1. Heading Information. Heading information is usually obtained by using the Earth’s magnetic lines of force. The magnetic compass is a self-contained instrument that operates independently of the electrical system and converts these magnetic lines of force to aircraft heading information. Other heading systems require electrical power to convert the magnetic lines of force to aircraft heading.

8.4.2. Basic Operation of Magnetic Compass. The magnetic field that surrounds the earth consists of invisible lines of flux. These lines leave the surface of the earth at the magnetic north

pole and reenter at the magnetic South Pole. The Earth’s magnetic north pole does not actually coincide with the geographic North Pole, but is located at a position that approximates 200 nm north of Resolute Bay, Canada. Also, the location tends to move within the confines of a magnetic polar area several hundred miles in diameter. Thus, there is actually no exact position of the magnetic North Pole. These lines of magnetic flux have two important characteristics:

any magnet that is free to rotate will align with them, and an electrical current is induced into any conductor that cuts across them. Most magnetic direction indicators installed in an aircraft make use of one or both of these two characteristics. One of the oldest and simplest instruments for indicating direction is the magnetic, or “whiskey” compass. An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated compass card marked with the four cardinal directions of the compass (N, S, E, and W) as well as smaller increments, is wrapped around the float and viewed through a glass window with a lubber line across it. The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup. 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 to freely rotate and tilt up to approximately 18° angle of bank. At steeper bank angles, the compass indications may become erratic and unpredictable.

8.4.2.1. Variation. Variation is the difference between true and magnetic direction. 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. Directions measured from the magnetic poles are called “magnetic” directions. Isogonic lines (see Figure 8.10) are lines of equal variation traced on the surface of the globe. They connect places at which the deviation of the magnetic needle from true north is the same. The line that passes near Chicago is called the agonic line, and anywhere along this line the two poles are aligned, and there is no variation.

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. West of this line, the magnetic pole is to the east of the geographic pole and a correction must be applied as well. Add west variation to the true course to obtain the magnetic course. Subtract east variation to the true course to obtain the magnetic course. Variation does not change with the heading of the aircraft; it is the same anywhere along the isogonic line.

Figure 8.10. Isogonic Lines.

8.4.2.2. Deviation. Deviation is an error in the magnetic heading introduced by production of local magnetic fields. Electrical currents flowing in the structure of the aircraft induce localized magnetic fields that can cause deviation in the magnetic compass. Localized magnetic fields outside the aircraft can induce deviation errors as well. Whereas variation error cannot be reduced or changed, deviation error can be minimized by calibrating the magnetic compass to compensate for known localized magnetic fields within the aircraft. This is a maintenance task known as a “compass swing”. Most airports have a compass rose, which is a series of precisely aligned lines drawn in an area of the airport where there is no magnetic interference. By aligning the aircraft with the known magnetic headings, maintenance personnel can calibrate the magnetic compass. The compass correction card shows the amount of deviation on various compass headings. Corrections for variation and deviation must be applied in the correct sequence. To find the magnetic course when the true course is known: True Course ± Variation = Magnetic Course ± Deviation = Compass Course. To find the true course that is being flown when the magnetic course is known: Compass Course ± Deviation = Magnetic Course ± Variation = True Course.

8.4.2.3. Dip Errors. 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. Since the magnets in the compass are designed to align with the horizontal component of the magnetic field, the compass card tends to progressively dip or tilt (error induced become more pronounced) near the poles. 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. This dip along with this weight causes two very noticeable errors:

northerly turning error and acceleration error.

8.4.2.3.1. 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 is flying on a heading of north, if a turn toward east is made, as the aircraft banks to the

right it will cause the compass card to tilt 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.

If the turn is made from north to west, the aircraft banks to the left and the card tilts to the left.

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. The rule for this error is:

when starting a turn from a northerly heading, the compass indication lags behind the turn (see figure 8.11). 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 will start the card rotating toward west—again, in the same direction the turn is being made. The rule for this error is: When starting a turn from a southerly heading, the compass indication leads the turn (see figure 8.11).

Figure 8.11. Northerly and Southerly Turning Error.

8.4.2.3.2. Acceleration/Deceleration Error. In the acceleration error, the dip-correction weight causes the end of the float and card marked N (this is the south-seeking end) to be heavier than

the opposite end. When the aircraft is flying at a constant speed on a heading of either east or west, the float and card are level. The effects of magnetic dip and the weight are approximately equal. If the aircraft accelerates on a heading of east, the inertia of the weight holds its end of the float back, and the card rotates toward north. As soon as the speed of the aircraft stabilizes, the card will swing back to its east indication. If, while flying on this easterly heading, the aircraft decelerates, the inertia causes the weight to move ahead and the card rotates toward south until the speed again stabilizes. When flying on a heading of west, the acceleration errors or induced.

Inertia from acceleration causes the weight to lag, and the card rotates toward north. When the aircraft decelerates on a heading of west, inertia causes the weight to move ahead and the card rotates toward south. An acronym that is used to describe this error is “ANDS” which means that when an aircraft is on an easterly or westerly heading, “Accelerate = North, Decelerate = South.” Figure 8.12 provides a graphical presentation of the acceleration/deceleration error.

Figure 8.12. Acceleration/Deceleration Error.

8.4.2.4. 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. When setting the gyroscopic heading indicator to agree with the magnetic compass, use the average indication between the swings.

8.4.3. Slaved and Non-Slaved Heading Systems. There are many types of heading systems, but each may be classified as either slaved or non-slaved. The non-slaved system uses a gyro to supply the directional reference, while the slaved system uses the signals from a remote compass transmitter to orient the system to magnetic north. In both systems the gyro acts as a stabilizing component to reduce the inherent errors.

8.4.3.1. Heading System Errors. All heading systems are subject to errors produced by real and apparent precession. In many modern aircraft heading systems, provisions to correct for these errors are usually incorporated making them negligible and transparent to the crew.

8.4.3.1.1. Real Precession. During turns and periods of acceleration and deceleration, forces are produced which combine with the force of gravity causing the erection mechanism and the remote compass transmitter to induce errors in the heading system. However, once wings-level, unaccelerated flight is resumed, the remote compass transmitter and the erection mechanism sense true gravity and correct any errors.

8.4.3.1.2. Apparent Precession. As discussed earlier, apparent precession is caused by two factors: the rotation of the Earth; and transportation of the gyro from one location on the Earth to another. The gyro is kept horizontal by the erection mechanism.

8.5. Angle of Attack System. Angle of attack information is obtained by comparing the