Explain the term 'isogonic variation' and its effect on magnetic navigation.

• A. Variation is constant globally and negligible.
• B. Magnetic variation varies with location; it affects compass readings used for navigation; you must apply variation to true headings.
• C. Variation affects airspeed only.
• D. Variation is a weather parameter.

Answer: (B)

Isogonic variation refers to the difference between true north and magnetic north at a location, which is not the same everywhere and can change over time. The lines on charts that connect places with the same amount of variation are called isogonic lines. Because the magnetic north is offset from true north by this varying amount, a compass reading (magnetic heading) does not match the true heading you intend to fly unless you correct for that offset.

In practice, you convert a true heading to a magnetic heading by applying the local variation shown on your chart. If the variation is to the east, you subtract it from the true heading to get the magnetic heading; if it’s to the west, you add it. For example, a true heading of 045° with an eastern variation of 10° becomes a magnetic heading of 035°. Conversely, a western variation would yield a magnetic heading of 055°. This adjustment is essential for accurate navigation because ignoring variation would cause you to drift off your intended track.

The other statements don’t fit because variation isn’t constant worldwide or negligible, it isn’t about airspeed, and it isn’t a weather parameter.