Chapter 4 - Compass Systems Flashcards
When is a material considered to be magnetically saturated?
All the molecules (or domains) are aligned
What are the three classifications of magnetic materials?
- Hard iron
- Semi-hard iron
- Soft iron
Which alloying elements have a tendency to be magnetic?
- Tungsten
- Vanadium nickel
- Chromium
- Cobalt
How is an object degaussed?
Using a demagnetising coil energised by single phase AC of 250 volts, 50 Hz
The part is passed through the insulated coil slowly in an East/West direction
What is an isogonic or isogonal line?
The Earth’s magnetic lines of force don’t run parallel to the geographic lines of longitude
Charts are available which show variation angles over a certain area, and isogonal or isogonic lines are those that join points of equal variation
What is the line which joins points of zero variation?
Agonic line
What are the forces called which act on a freely suspended magnet?
Horizontal component - H
Vertical component - Z
Resultant - T
What are isoclinals?
Lines of equal dip showing magnetic latitude
How can the effect of dip be reduced on a direct reading compass?
The magnet is made pendulous, therefore the centre of gravity is below the pivot point
What is deviation as it pertains to aircraft compasses?
The difference between the direction of magnetic North and the compass North
Describe the magnetic components of the aircraft’s magnetic fields?
P - acting fore and aft (longitudinal)
Q - acting across the aircraft (lateral)
R - acting up and down (vertical)
When are the magnetic components of an aircraft’s magnetic fields considered positive?
When the component attracts the compass needle forward, starboard or downwards
What is the deviation produced by the P component known as?
B error
At what aircraft heading does the P component produce the largest deviation?
East or West
At what aircraft heading does the Q component produce the largest deviation?
North or South
What is the deviation produced by the Q component?
C error
What is the difference between variation and deviation?
Variation - true North to magnetic North
Deviation - magnetic North to compass North
When is the angle of deviation considered positive or easterly?
When measured clockwise from the direction of magnetic North
What is the correct order of performing corrections to compass heading to find true heading?
Compass heading - Deviation - Magnetic heading - Variation - True heading
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What are the three types of errors an aircraft magnetic compass is subject to?
- Aircraft’s magnetic structure (coefficients of deviation)
- Mechanical sources
- Aircraft movement (dynamic errors)
What factors cause coefficient A errors?
- Effect of the aircraft’s ‘soft iron’
- Not correctly aligned with the aircraft’s longitudinal axis
How is coefficient A error removed?
During a compass swing by turning the compass bowl or detector unit
At what heading is coefficient A error maximum?
Same on all compass headings
How can mechanical errors be categorised in an aircraft compass?
- Pivot errors
- Liquid swirl
- Parallax error
What is pivot friction?
When a compass is deflected away from its natural North seeking position and then allowed to swing back and settle, it may not settle in the original position due to pivot friction
How many coefficients of deviation are there?
5, A-E
Which coefficients of deviation are not normally corrected for?
D and E
How are the errors in the B and C coefficients corrected for in a direct reading compass and a detector unit?
Direct reading compass - Small permanent magnets are moved
Detector unit - Value of a DC electromagnet is changed
What effect does liquid swirl have on a compass and why?
Tendency of the liquid to move around inside the compass body in an attempt to follow the aircraft’s movement will provide an additive error on the reading
