FAA Flight Instructor Airplane (FIA)Navigation and AirspaceMedium
While planning a flight from Ohio to Colorado, a pilot notices the compass rose on the sectional chart shows a line labeled '4°W' passing through the route. A flight instructor explains this line represents:
- AAn isogonic line connecting points of equal magnetic variation
- BAn agonic line where magnetic variation is zero
- CA line of constant magnetic dip angle used for compass calibration
- DThe boundary between two magnetic time zones used for navigation
Show answer & explanationAnswer & explanation
Correct answer: A. An isogonic line connecting points of equal magnetic variation
Isogonic lines on aeronautical charts connect points having the same amount of magnetic variation; the specific line where variation equals zero is called the agonic line. A '4°W' line is an isogonic line indicating 4° westerly variation.
Why the other options are wrong
- B. An agonic line specifically represents zero variation, not 4°.
- C. Magnetic dip refers to vertical component of magnetism, unrelated to these chart lines.
- D. There is no concept of 'magnetic time zones' in aviation.
Isogonic and Agonic Lines
Isogonic lines connect points of equal magnetic variation; the agonic line specifically connects points where variation is zero.
- Magnetic variation is the angle between true north and magnetic north
- Isogonic lines are labeled with variation values (e.g., 4°W)
- The agonic line in the U.S. runs roughly through the Great Lakes/Midwest
Memory trick: 'Iso = same, Agonic = agony-free (zero variation)'