Chapter 3 - Magnetic Compass Flashcards
What is the angle of dip?
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.
What are the 3 Norths that need considering when using a magnetic compass?
True North
Grid North
Magnetic North
What is True north and how is an arrow for True North depicted?
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.
How is an arrow for Mag North depicted?
Half-diamond on the end of the pointer
How is an arrow for Compass North depicted?
Circle on the end of the pointer
Where is the magnitude of the difference between true and grid north greatest?
At higher latitudes. A result of projecting the Earth’s curved surface onto a flat chart.
What is Variation (or Declination)?
The direction of the local magnetic field with respect to True North.
How do you convert from a True hdg to a Mag hdg (or vice versa)?
Variation East, magnetic least (subtract variation from True)
Variation West, magnetic best (add variation to True)
What is a DICS?
Direct Indicating Compass System
Freely suspended magnet system that aligns with the HORIZONTAL component of the Earth’s magnetic field. Indicates Mag North.
What must a DICS have to operate efficiently in flight conditions?
Horizontality
Sensitivity
Aperiodicity
What is Horizontality?
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.
What is Sensitivity?
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.
What is Aperiodicity?
A compasses resistance to oscillations and instability following disturbance.
Aka being “Dead Beat”
How can compass sensitivity be improved?
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).
How can aperiodicity be improved?
Using multiple short magnets - keeps mass central and reduces moment of inertia.
Encasing assembly in a fluid filled bowl - has a damping effect.
What is shown on the compass card?
Four Cardinal Headings
Numbers every 30 degrees
Long indication lines every 10 degrees
Some have short indication lines every 5 degrees
What is a lubber line?
A line etched on the glass front of the compass assembly to enable an exact reading of the compass.
What is the operational accuracy of DICS?
+/- 10 degrees
can approach a target accuracy of +/-2.5 degrees
What are the advantages of DICS?
Cheap to purchase, install and maintain
Small and light
Simple to use
Requires no power supply to operate (except for lighting)
What are the limitations of DICS?
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.
Note - Look at DICS components diagram at 4-3-2 in the course manual.
Learn now!
What errors can effected a DICS?
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
What sort of gyro does a gyro-magnetic compass use?
Displacement gyro - Spin axis mounted horizontally
What are the 3 basic components of a gyro-magnetic compass?
Transmission and display system
Gyroscope - short term azimuth stability
Detector unit (Fluxvalve) - Detecting elements providing long-term azimuth reference for the gyroscope
Note - Go to the course manual and learn the gyro compass operation diagram.
Not major but look at if there’s time.
What errors affect a gyro-magnetic compass system?
Gyroscope: Drift Earth rate Transport wander Gimbal error
Detector Unit (Fluxvalve): Turn Coriolis Vehicle Movement Vibration
What is the purpose of a compass swing?
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.
When should a compass swing be carried out?
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)
Note - Check compass swing procedure if there’s time, but not that important.
Section 4-3-4
Where can compass corrections be found to account for the errors in an individual DICS?
On a deviation card - usually in the aircraft cockpit
