Chapter 3 - Magnetic Compass

Question 1

What is the angle of dip?

Answer 1

The angle between the horizontal and the actual force line at a point in Earth’s magnetic field.

Varies from 0 degrees at the equator to 90 degrees at the magnetic poles.

Question 2

What are the 3 Norths that need considering when using a magnetic compass?

Answer 2

True North

Grid North

Magnetic North

Question 3

What is True north and how is an arrow for True North depicted?

Answer 3

True North is the direction of the meridians of longitude, which converge at the geographic North and South poles - the Earth’s axis of rotation.

Depicted by a diamond on the end of the pointer.

Question 4

How is an arrow for Mag North depicted?

Answer 4

Half-diamond on the end of the pointer

Question 5

How is an arrow for Compass North depicted?

Answer 5

Circle on the end of the pointer

Question 6

Where is the magnitude of the difference between true and grid north greatest?

Answer 6

At higher latitudes. A result of projecting the Earth’s curved surface onto a flat chart.

Question 7

What is Variation (or Declination)?

Answer 7

The direction of the local magnetic field with respect to True North.

Question 8

How do you convert from a True hdg to a Mag hdg (or vice versa)?

Answer 8

Variation East, magnetic least (subtract variation from True)

Variation West, magnetic best (add variation to True)

Question 9

What is a DICS?

Answer 9

Direct Indicating Compass System

Freely suspended magnet system that aligns with the HORIZONTAL component of the Earth’s magnetic field. Indicates Mag North.

Question 10

What must a DICS have to operate efficiently in flight conditions?

Answer 10

Horizontality

Sensitivity

Aperiodicity

Question 11

What is Horizontality?

Answer 11

A compass must lie horizontally in flight as much as possible. Increasing dip angle at higher latitudes will make this difficult.

Mount the magnet assembly on a pendulous pivot.

Question 12

What is Sensitivity?

Answer 12

How well a freely suspended magnet senses the horizontal (H) component of the Earth’s magnetic field at all points over the Earth’s surface.

Needs to do so quickly and accurately.

Depends on the strength of H (weak near the poles) and the magnetic moment of the magnet assembly.

Question 13

What is Aperiodicity?

Answer 13

A compasses resistance to oscillations and instability following disturbance.

Aka being “Dead Beat”

Question 14

How can compass sensitivity be improved?

Answer 14

Increasing the magnetic moment of the magnet assembly by:

Having a longer magnet

Increasing the pole strength

Reducing friction (jewelled bearing points at the pivot + lubricate with liquid that fills the case, also reduces the weight of the suspended magnet on the pivot).

Question 15

How can aperiodicity be improved?

Answer 15

Using multiple short magnets - keeps mass central and reduces moment of inertia.

Encasing assembly in a fluid filled bowl - has a damping effect.

Question 16

What is shown on the compass card?

Answer 16

Four Cardinal Headings

Numbers every 30 degrees

Long indication lines every 10 degrees

Some have short indication lines every 5 degrees

Question 17

What is a lubber line?

Answer 17

A line etched on the glass front of the compass assembly to enable an exact reading of the compass.

Question 18

What is the operational accuracy of DICS?

Answer 18

+/- 10 degrees

can approach a target accuracy of +/-2.5 degrees

Question 19

What are the advantages of DICS?

Answer 19

Cheap to purchase, install and maintain

Small and light

Simple to use

Requires no power supply to operate (except for lighting)

Question 20

What are the limitations of DICS?

Answer 20

Unsuitable as a primary heading system as it gives significant error indications when turning, accelerating/decelerating.

Unusable near the poles + unreliable at high latitudes due to weak H field.

Subject to electromagnetic interference in the cockpit (particularly windscreen heating) and the aircraft’s own magnetism. Causes Deviation.

Need to consider/apply variation and deviation when using.

Question 21

Note - Look at DICS components diagram at 4-3-2 in the course manual.

Answer 21

Learn now!

Question 22

What errors can effected a DICS?

Answer 22

Acceleration/Deceleration - CoG of DICS displaced, causes couples which rotate the compass.

(CHECK MANUAL FOR DETAILS ON DIRECTION IF reqd.)

Turning - Compass needle swings to temporarily show inaccurate results

Question 23

What sort of gyro does a gyro-magnetic compass use?

Answer 23

Displacement gyro - Spin axis mounted horizontally

Question 24

What are the 3 basic components of a gyro-magnetic compass?

Answer 24

Transmission and display system

Gyroscope - short term azimuth stability

Detector unit (Fluxvalve) - Detecting elements providing long-term azimuth reference for the gyroscope

Question 25

Note - Go to the course manual and learn the gyro compass operation diagram.

Answer 25

Not major but look at if there’s time.

Question 26

What errors affect a gyro-magnetic compass system?

Answer 26

Gyroscope:
Drift
Earth rate
Transport wander
Gimbal error

Detector Unit (Fluxvalve):
Turn
Coriolis
Vehicle Movement
Vibration

Question 27

What is the purpose of a compass swing?

Answer 27

To ensure the compass is accurate and to record any DEVIATION due to electromagnetic interference within the aircraft of the aircraft’s own magnetism in order to create a deviation card.

Question 28

When should a compass swing be carried out?

Answer 28

Accuracy in doubt
After major service 
Compass components replaced or installed
Repair/modification
After a lightning strike
Aircraft in long term storage on one heading.
Etc
(SEE BOOK FOR WHOLE LIST)

Question 29

Note - Check compass swing procedure if there’s time, but not that important.

Answer 29

Section 4-3-4

Question 30

Where can compass corrections be found to account for the errors in an individual DICS?

Answer 30

On a deviation card - usually in the aircraft cockpit