Variation, deviation & course conversion
| Variation | Deviation | |
|---|---|---|
| Cause | Earth's field: angle between true and magnetic north (they sit in different places) | Aircraft's own magnetism: metal and electrics tug the needle (AC 100-001) |
| Depends on | Location: fixed along an isogonic line | Heading: changes with heading, not one fixed number (PHAK ch.16) |
| Read from | Chart | Deviation card by the compass, per heading (PHAK ch.16) |
| Fix | None: swinging cannot change it; "no one can reduce or change variation error" (PHAK ch.8) | Swing the compass |
Chart lines (PHAK glossary):
| Line | Joins |
|---|---|
| Isogonic | Places of equal variation |
| Agonic | Zero variation: true and magnetic north line up |
- East of the agonic line, magnetic North Pole lies west of geographic North Pole: correction must be applied to compass indication for true direction (PHAK ch.8).
Signs (PHAK ch.8); deviation is applied like variation:
| Convert | West | East |
|---|---|---|
| True to magnetic to compass | + (add) | − (subtract) |
| Compass to magnetic to true | − (subtract) | + (add) |
- True to magnetic: 10°W added to 180° true = 190° magnetic. 14°E subtracted from 180° true = 166° magnetic.
- Magnetic to true: 190° magnetic with 10°W = 180° true.
Sequence (PHAK ch.8):
- From desired true course, apply variation and deviation in correct sequence to get compass course to steer.
- Compass course to true course: deviation first, then variation.
Compass Course ± Deviation = Magnetic Course ± Variation = True Course
Trap: mixing deviation (aircraft's magnetism, deviation card, swung out) with variation (earth's field, from chart).
References
- AC 100-001 glossary (deviation)
- PHAK ch.8 (FAA-H-8083-25)
- PHAK ch.16
- PHAK glossary: isogonic lines, agonic line