Aircraft MagnetismNavigation — Instrumentation — DGCA CPL practice questions

All 4 questions — Aircraft Magnetism

Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.

1. Q1. European regulations (EU OPS-1) state that the maximum permissible deviations after compensation are:

   • A.one degree for a remote indicating compass, and ten degrees for a direct reading magnetic compass✓
   • B.three degrees for a direct reading magnetic compass, and one degree for a remote indicating compass
   • C.ten degrees for a remote indicating compass, and one degree for a direct reading magnetic compass
   • D.one degree for a direct reading magnetic compass, and eleven degrees for a slaved compass

   Why: CS OPS-1 / EU OPS-1 specifies: DRMC = ±10° maximum residual deviation; RIMC = ±1° maximum residual deviation. See Section 7 . — RIMC = 1° (very tight, because it is a precision instrument feeding autopilots and FMS). DRMC = 10° (larger tolerance for a simpler instrument). Easy mnemonic: "Remote = 1; Direct = 10."

2. Q2. Compass swings should be carried out:

   • A.on the apron
   • B.only on the compass swinging base or site✓
   • C.at the holding point
   • D.on the active runway

   Why: The compass swing must be conducted in an area specifically selected for this purpose — the compass swinging base or site — which is chosen to be free from magnetic interference (underground cables, reinforcing, aircraft equipment). See Section 2 . — The compass swinging base is a specially prepared area on the airfield, away from hangars, pipelines, and other magnetic interference. The landing/datum compass used must also be calibrated and away from interference.

3. Q3. Aircraft magnetism caused by vertical soft iron:

   • A.varies with magnetic heading but not with magnetic latitude
   • B.varies with magnetic latitude but not with heading✓
   • C.is not affected by magnetic latitude or heading
   • D.varies as the cosine of the compass heading

   Why: Vertical Soft Iron (VSI) magnetism is induced by the vertical component Z of the earth's field. Z increases with latitude (zero at equator, maximum at poles). So VSI magnetism varies with magnetic latitude. However, for a given latitude, the component Z is the same regardless of which way the aircraft is pointing — so VSI magnetism does not vary with heading. See Section 4 . — Compare with hard iron (constant regardless of heading OR latitude) and soft iron (varies with latitude). This distinction is a frequent exam topic.

4. Q4. Aircraft magnetism caused by hard iron:

   • A.is not usually influenced by the earth's magnetic field✓
   • B.varies directly with magnetic latitude
   • C.varies indirectly with magnetic latitude
   • D.is maximum on east and west

   Why: Hard iron magnetism is due to permanent magnets — parts of the aircraft that retain their magnetism independently. They are not induced by the earth's field and therefore are not influenced by changes in heading or latitude. Their magnetic force is constant and fixed with the aircraft. See Section 3 . — Hard iron = permanent, constant, not affected by external fields. Soft iron = induced, varies with the inducing field (earth's field). Clear contrast.