PHAK 8: Flight Instruments: Compass Systems

Question 1

Compass Systems

What creates the Earth’s magnetic field?

Answer 1

The Earth acts as a giant magnet, surrounded by invisible lines of magnetic flux that extend from the magnetic North Pole to the magnetic South Pole.

Question 2

Compass Systems

What are the two key characteristics of magnetic flux lines?

Answer 2

1. A magnet free to rotate will align with them.
2. An electrical current is induced in a conductor that crosses them.

Question 3

Compass Systems

What aircraft instruments utilize magnetic flux lines?

Answer 3

Most direction indicators, such as magnetic compasses, use these flux lines to provide heading information.

Question 4

Compass Systems: Magnetic Compass

What is a magnetic compass used for in aviation?

Answer 4

It indicates direction and is a required instrument for VFR and IFR flight under 14 CFR part 91.

Question 5

Compass Systems: Magnetic Compass

What are the two poles of a magnet?

Answer 5

North and South poles. Unlike poles attract; like poles repel.

Question 6

Compass Systems: Magnetic Compass

How does the magnetic compass work?

Answer 6

Two small magnets attached to a metal float align with the Earth’s magnetic field, and a graduated card shows direction.

Question 7

Compass Systems: Magnetic Compass

What is the function of the compass fluid?

Answer 7

It provides buoyancy for the float, reduces oscillations, and allows the float to tilt up to 18°.

Question 8

Compass Systems: Magnetic Compass

How is the compass card graduated?

Answer 8

• Marked with cardinal directions (N, E, S, W)
• Numbers every 30°, omitting the final zero (e.g., 3 = 30°, 33 = 330°).

Question 9

Compass Systems: Magnetic Compass

What prevents fluid damage due to temperature changes?

Answer 9

A flexible diaphragm or metal bellows allows for expansion and contraction.

Question 10

Compass Systems: Magnetic Compass

Why can the magnetic compass be confusing to read?

Answer 10

The card remains stationary while the pilot and compass housing rotate, making the numbers appear backward.

Question 11

Compass Systems: Magnetic Compass

How many degrees do the long marks on a compass represent?

Answer 11

10°

Question 12

Compass Systems: Magnetic Compass

How many degrees do the short marks on a compass represent?

Answer 12

5°

Question 13

Compass Systems: Magnetic Compass Induced Errors, Variation

What is variation in navigation?

Answer 13

The angular difference between true directions (geographic poles) and magnetic directions (magnetic poles).

Question 14

Compass Systems: Magnetic Compass Induced Errors, Variation

What are isogonic lines?

Answer 14

Lines on a chart that show the degrees of variation in a specific area.

Question 15

Compass Systems: Magnetic Compass Induced Errors, Variation

What is the agonic line?

Answer 15

A line where the geographic North Pole and magnetic North Pole align, resulting in no variation.

Question 16

Compass Systems: Magnetic Compass Induced Errors, Variation

How do you correct for variation when flying in areas with variation?

Answer 16

• West variation: Add the variation to the true course.
• East variation: Subtract the variation from the true course.

Question 17

Compass Systems: Magnetic Compass Induced Errors, Variation

Give an example of correcting for variation.

Answer 17

• In Washington, D.C. (10° west variation), to fly a true course of 180°, fly a magnetic course of 190°.
• In Los Angeles (14° east variation), to fly a true course of 180°, fly a magnetic course of 166°.

Question 18

Compass Systems: Magnetic Compass Induced Errors, Variation

Does variation change with the heading of the aircraft?

Answer 18

No, variation remains constant anywhere along the same isogonic line.

Question 19

Compass Systems: Magnetic Compass Induced Errors, Deviation

What is deviation in a magnetic compass?

Answer 19

Deviation is an error caused by magnetic fields in the aircraft, such as from electrical current, magnetized parts, or interference with the Earth’s magnetic field.

Question 20

Compass Systems: Magnetic Compass Induced Errors, Deviation

How does deviation differ from variation?

Answer 20

• Deviation: Depends on the aircraft’s heading and is caused by onboard magnetic fields.
• Variation: Depends on geographic location and the difference between true and magnetic north.

Question 21

Compass Systems: Magnetic Compass Induced Errors, Deviation

What is “swinging the compass”?

Answer 21

A maintenance procedure performed by an AMT to minimize compass deviation by aligning the aircraft with known headings on a compass rose and adjusting compensator magnets.

Question 22

Compass Systems: Magnetic Compass Induced Errors, Deviation

What is a compass rose?

Answer 22

A series of lines marked every 30° on an airport ramp, oriented to magnetic north, used for minimizing magnetic deviation.

Question 23

Compass Systems: Magnetic Compass Induced Errors, Deviation

How is deviation corrected in the compass?

Answer 23

Using a compensator assembly with adjustable magnets for east-west and north-south headings. The remaining error is recorded on a compass correction card.

Question 24

Compass Systems: Magnetic Compass Induced Errors, Deviation

What is the sequence for correcting variation and deviation errors?

Answer 24

• Magnetic Course = True Course ± Variation.
• Compass Course = Magnetic Course ± Deviation.

Question 25

# Compass Systems: Magnetic Compass Induced Errors, Deviation Example of correcting for deviation and variation:

Answer 25

* True Course: 180° * Variation: +10° (West) → Magnetic Course = 190° * Deviation: –2° (from correction card) → Compass Course = 188°

Question 26

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What is the Earth's magnetic dip angle?

Answer 26

The angle created by the vertical pull of the Earth's magnetic field in relation to the Earth's surface, increasing as you move toward the magnetic poles.

Question 27

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Where is the Earth's magnetic field parallel to the surface?

Answer 27

At the Magnetic Equator, halfway between the Magnetic North and South Poles.

Question 28

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Why doesn't the compass needle pivot freely in three dimensions?

Answer 28

To prevent the needle from pointing up or down with the dip angle, the compass is designed to rotate only in the horizontal plane by lowering the center of gravity below the pivot point.

Question 29

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What happens to a compass near the magnetic poles?

Answer 29

The horizontal component of the Earth's magnetic field becomes too small to align the compass, making it unusable for navigation.

Question 30

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Why is dip angle not of navigational interest?

Answer 30

The compass is designed to work in the horizontal plane, eliminating the effects of the vertical component of the Earth's magnetic field.

Question 31

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What errors are caused by the Earth's magnetic dip?

Answer 31

Northerly and southerly turning errors.

Question 32

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What causes northerly turning errors in a magnetic compass?

Answer 32

The center of gravity of the float assembly is below the pivot point, and magnetic dip causes the float to swing in the same direction as the turn, leading to a false northerly turn indication.

Question 33

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How does magnetic dip affect compass readings during a northerly turn?

Answer 33

Magnetic dip causes the compass card to lead, indicating the turn prematurely.

Question 34

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How can pilots correct for northerly turning errors?

Answer 34

Stop the turn 15 degrees plus half the latitude before reaching the desired heading.

Question 35

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Provide an example of correcting for northerly turning error at 40° latitude.

Answer 35

Stop the turn 15 + (40 ÷ 2) = 35 degrees before the desired heading.

Question 36

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Where is the northerly turning error most amplified?

Answer 36

Near the magnetic poles due to the increased effects of magnetic dip.

Question 37

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What causes southerly turning errors in a magnetic compass?

Answer 37

The float assembly lags due to magnetic dip, leading to a false southerly turn indication.

Question 38

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How does magnetic dip affect compass readings during a southerly turn?

Answer 38

Magnetic dip causes the compass card to lag, indicating the turn later than it actually occurs.

Question 39

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How can pilots correct for southerly turning errors?

Answer 39

Allow the compass to pass the desired heading by 15 degrees plus half the latitude before stopping the turn.

Question 40

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Provide an example of correcting for southerly turning error at 30° latitude.

Answer 40

Stop the turn 15 + (30 ÷ 2) = 30 degrees after passing the desired heading.

Question 41

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors Where is the southerly turning error most amplified?

Answer 41

Near the magnetic poles due to the stronger effects of magnetic dip.

Question 42

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What is the mnemonic to remember Northern/Southerly turning errors?

Answer 42

**UNOS** Undershoot North Overshoot South

Question 43

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What is the mnemonic to remember East/West acceleration errors?

Answer 43

**ANDS** Acceleration North Deceleration South

Question 44

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What causes acceleration errors in a magnetic compass?

Answer 44

Magnetic dip and inertia from changes in airspeed, combined with the pendulous mounting of the compass.

Question 45

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How does the compass behave when accelerating on an easterly or westerly heading?

Answer 45

The compass shows a false turn toward north.

Question 46

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors How does the compass behave when decelerating on an easterly or westerly heading?

Answer 46

The compass shows a false turn toward south.

Question 47

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors What mnemonic helps remember magnetic compass acceleration error?

Answer 47

"ANDS": Acceleration-North, Deceleration-South.

Question 48

# Compass Systems: Magnetic Compass Induced Errors, Dip Errors On which headings is acceleration error most noticeable?

Answer 48

Easterly and westerly headings.

Question 49

# Compass Systems: Magnetic Compass Induced Errors, Oscillation Error What is oscillation error in a magnetic compass?

Answer 49

A combination of all compass errors, causing fluctuations of the compass card relative to the actual heading.

Question 50

# Compass Systems: Magnetic Compass Induced Errors, Oscillation Error How can a pilot manage oscillation error when setting the gyroscopic heading indicator?

Answer 50

Use the average indication between the swings.

Question 51

# Compass Systems: The Vertical Card Magnetic Compass What is the primary advantage of the vertical card magnetic compass?

Answer 51

It eliminates some of the errors and confusion associated with standard magnetic compasses.

Question 52

# Compass Systems: The Vertical Card Magnetic Compass How is the dial of the vertical card magnetic compass graduated?

Answer 52

It uses letters for cardinal directions, numbers every 30°, and tick marks every 5°.

Question 53

# Compass Systems: The Vertical Card Magnetic Compass What does the nose of the symbolic aircraft on the vertical card magnetic compass represent?

Answer 53

The lubber line, used to read the aircraft's heading.

Question 54

# Compass Systems: The Vertical Card Magnetic Compass What happens to the compass during a turn from a northerly heading?

Answer 54

The compass lags behind the turn.

Question 55

# Compass Systems: The Vertical Card Magnetic Compass What happens to the compass during a turn from a northerly heading?

Answer 55

The compass leads the turn.

Question 56

# Compass Systems: The Vertical Card Magnetic Compass What is eddy current damping in a vertical card magnetic compass?

Answer 56

Eddy currents are created by the oscillating magnet, producing a magnetic flux that opposes the oscillations, reducing their amplitude.

Question 57

# Compass Systems: The Vertical Card Magnetic Compass What is the purpose of the Outside Air Temperature (OAT) gauge?

Answer 57

It measures the outside air temperature, providing useful information such as the temperature lapse rate with altitude changes.

Question 58

# Compass Systems: The Vertical Card Magnetic Compass How does the OAT gauge measure temperature?

Answer 58

Using a bimetallic-type thermometer, where two dissimilar materials are welded together and twisted into a helix.

Question 59

# Compass Systems: The Vertical Card Magnetic Compass Where is the sensing element of the OAT gauge located?

Answer 59

It is mounted to ensure it is exposed to the outside air.

Question 60

# Compass Systems: The Vertical Card Magnetic Compass How is the OAT gauge calibrated?

Answer 60

It is calibrated in degrees Celsius (°C), Fahrenheit (°F), or both.

Question 61

# Compass Systems: The Vertical Card Magnetic Compass What information can the OAT gauge provide to a pilot?

Answer 61

It helps determine the temperature lapse rate as altitude changes.