Terrestrial MagnetismNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 7
The red pole of a freely suspended magnet will point towards ....... and at latitude 60°N will point ....... at an angle known as the angle of ......
All 7 questions — Terrestrial Magnetism
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. The red pole of a freely suspended magnet will point towards ....... and at latitude 60°N will point ....... at an angle known as the angle of ......
- A.the nose of the aircraft, downwards, deviation
- B.the North magnetic pole, downwards, variation
- C.the nearest pole, downwards, declination
- D.the North magnetic pole, downwards, dip✓
Why: The red (north-seeking) pole of a freely suspended magnet points towards the north magnetic pole. In the northern hemisphere (60°N), it also dips downwards below the horizontal, and the angle of this dip below horizontal is called the angle of dip. See Section 2 and Section 9 . — Three important angles: Variation (true vs. magnetic meridian), Deviation (magnetic meridian vs. compass needle due to aircraft magnetism), Dip (magnet axis vs. horizontal). Know which is which.
Q2. If the total force of the earth's field at a point is T and the horizontal and vertical components H and Z, the value of H is found by the formula:
- A.H = T sin dip
- B.H = Z tan dip
- C.H = T cos dip✓
- D.H = T tan dip
Why: H is the horizontal component of T. In the right triangle formed by T, H, and Z, H is adjacent to the dip angle → H = T × cos(dip). At dip 0° (equator): H = T. At dip 90° (pole): H = 0. See Section 10 . — H = T cos dip (H is the horizontal = adjacent = cos). Z = T sin dip (Z is vertical = opposite = sin). Draw the right triangle and label it.
Q3. The directive force of the earth's magnetic field:
- A.varies with the heading of the aircraft
- B.increases as the magnetic variation increases
- C.increases as magnetic latitude increases
- D.is greatest at the magnetic equator✓
Why: At the magnetic equator, dip = 0° → H = T cos 0° = T (maximum). As latitude increases towards the poles, dip increases → H decreases. Directive force H is maximum at the equator and zero at the poles. See Section 10 . — Directive force (H) and dip are inversely related. More dip = less H = worse compass. Equator = best compass. Poles = useless compass.
Q4. The slow change in the earth's magnetic variation is known as the ....... change and is caused by ......
- A.annual, westerly movement of the magnetic pole
- B.diurnal, easterly movement of the magnetic pole
- C.secular, westerly movement of the magnetic pole✓
- D.annual, sunspot activity
Why: The most significant regular change in earth magnetism is the secular change, caused by the slow (approximately 960-year cycle) westerly movement of the north magnetic pole about the geographic pole. This is what causes the annual decrease in westerly variation in the UK. See Section 11.1 . — Secular = slow, ~960 years, westerly pole movement. Sunspots = magnetic storms = unpredictable. These are the two main categories of change tested in DGCA.
Q5. Soft iron is comparatively ....... to magnetize whilst hard iron is ....... to demagnetize.
- A.easy; difficult✓
- B.easy; easy
- C.difficult; easy
- D.difficult; difficult
Why: Soft iron is easy to magnetize (weak field sufficient) but also easy to demagnetize (loses magnetism when field removed — temporary). Hard iron is difficult to magnetize (requires strong field) and difficult to demagnetize (retains magnetism permanently). See Section 6.2 . — Remember: Soft = Easy in, Easy out (temporary). Hard = Hard in, Hard out (permanent). The words describe magnetic characteristics, not physical hardness.
Q6. Which of the following materials are classed as ferromagnetic:
- A.iron, steel, carbon-fibre
- B.nickel, iron, steel
- C.copper, iron, carbon steel
- D.iron, cobalt steel, chromium steel✓
Why: Ferromagnetic materials are iron and steel alloys (iron alloyed with cobalt, nickel, chromium, tungsten, carbon). Cobalt steel and chromium steel are both iron-based alloys and are ferromagnetic. See Section 6.1 . — Non-magnetic materials (don't affect compass): aluminium, duralumin, brass, copper, plastic, paint. Ferromagnetic: iron, steel and its alloys with Co, Ni, Cr, W, C.
Q7. The magnetic moment of a magnet:
- A.is the product of pole strength and effective length✓
- B.varies inversely as the square of the distance between the poles
- C.varies directly as the square of the distance between the poles
- D.decreases as the magnet length increases
Why: Magnetic moment (M) = pole strength (m) × effective length (l). This is the fundamental definition. A magnet with greater pole strength and/or greater effective length has a higher magnetic moment and therefore greater ability to align itself with an external field (greater sensitivity). See Section 1 and associated magnetism theory. — Magnetic Moment = Pole Strength × Effective Length. In compass design, increasing the magnetic moment (stronger magnets or longer effective length) increases sensitivity.