What is a great circle route?

A great circle route is the shortest path between two points on Earth's surface. It follows the curvature of the Earth and appears as a curved line on most flat maps. For long-distance flights, great circle routes can save significant fuel and time compared to flying a constant heading.

What is a rhumb line?

A rhumb line (or loxodrome) crosses all meridians at the same angle, resulting in a constant heading. It appears as a straight line on a Mercator chart but is actually longer than a great circle route. Rhumb lines are simpler to fly but less efficient for long distances.

When should I use great circle vs. rhumb line navigation?

Use great circle for flights over 500nm where fuel savings justify the complexity of changing headings. Use rhumb line for shorter flights where the distance difference is negligible and constant heading is more practical. Most GPS units automatically compute great circle routes.

What is magnetic variation?

Magnetic variation is the angular difference between true north and magnetic north at a specific location. It varies by location and changes slowly over time. To convert true heading to magnetic heading, add westerly variation or subtract easterly variation (remember: 'East is least, West is best').

How do I calculate wind correction angle?

Wind correction angle (WCA) compensates for wind drift. The formula involves the crosswind component and true airspeed: WCA = arcsin(crosswind / TAS). For a 20-knot crosswind at 100 knots TAS, the WCA is about 12 degrees. Point the nose into the wind by this amount to maintain your desired ground track.

What is the difference between heading and course?

Course is the intended path over the ground from origin to destination. Heading is the direction the aircraft's nose is pointing. In wind, these differ - you must point the nose (heading) into the wind to track the desired course. The difference is the wind correction angle.

Aviation Navigation Basics - Great Circle & Wind Correction

Great Circle vs. Rhumb Line

Great Circle

✓ Shortest distance between points
✓ Saves fuel on long flights
✗ Heading changes continuously
✗ Appears curved on flat maps

Best for: Flights over 500nm

Rhumb Line

✓ Constant heading throughout
✓ Simple to fly manually
✗ Longer than great circle
✗ Less fuel efficient

Best for: Short flights, VFR navigation

Example: New York to London

Great circle: 3,459 nm | Rhumb line: 3,540 nm

Difference: 81 nm (2.3%) - saves ~800 lbs of fuel on a widebody jet

Magnetic Variation

True north (geographic) and magnetic north differ. This difference is called magnetic variation or declination.

The Memory Aid

"East is least, West is best"

Subtract easterly variation, add westerly variation when converting true to magnetic.

Location	Variation	Example
Los Angeles	12°E	True 090° → Mag 078°
New York	13°W	True 090° → Mag 103°
London	0° (agonic)	True = Magnetic

Wind Correction Angle

To maintain your course in wind, you must "crab" into the wind by the wind correction angle (WCA).

WCA = arcsin(Crosswind Component ÷ True Airspeed)

Point the nose into the wind by this angle

10 kt XW

@ 100 kt TAS

≈ 6° WCA

20 kt XW

@ 100 kt TAS

≈ 12° WCA

30 kt XW

@ 120 kt TAS

≈ 14° WCA

Course vs. Heading vs. Track

Course: The intended path over the ground (what you want to fly)

Heading: Direction the nose is pointing (corrected for wind)

Track: Actual path over the ground (what you're actually flying)

Goal: Adjust heading so that track equals course, despite wind.

Aviation Navigation Basics