Wind correction angle and the wind triangle
The airplane flies through the air. The air moves over the ground. The wind triangle is how you reconcile those two facts, and the wind correction angle is the answer it gives you.
The three vectors
The triangle has three sides. The first is where the airplane is pointed and how fast it moves through the air: true heading and true airspeed. The second is the wind: the direction it is coming from and its speed. The third is the result, the path over the ground and the speed along it: true course and ground speed.
Two of those you know. You know the course you want to fly because you drew it on the chart, and you know the wind from the forecast. The triangle gives you the other two: the heading to fly, and how fast you will actually get there.
Wind correction angle
The wind correction angle is the difference between the course you want and the heading you fly. Crab into the wind and the drift cancels out. A wind from the right needs a correction to the right, which is added to the course; a wind from the left is subtracted.
A worked example
True course 342, true airspeed 105 knots, wind 020 at 8 knots. The wind is from the right of the course by 38 degrees, so the crosswind component is small: 8 times the sine of 38, about 5 knots. The correction angle is the angle whose sine is 5 divided by 105, roughly 3 degrees, so you crab 3 degrees right and fly a true heading of 345. The headwind component is 8 times the cosine of 38, about 6 knots, so ground speed is about 99 knots.
If the local variation is 7 west, add 7 to get a magnetic heading of 352, then apply the deviation card to get the compass heading you actually fly.
The rule of thumb
For quick mental math, the correction angle in degrees is roughly the crosswind component divided by the true airspeed, times 60. At 100 knots that is about 0.6 degrees per knot of crosswind, so 10 knots of crosswind is about a 6 degree correction. It is close enough for a mid-flight sanity check, not close enough for the flight plan.
Where the wind number comes from
The winds aloft forecast is published at 3,000, 6,000, 9,000 and 12,000 feet and up, so unless you are flying at exactly one of those, you interpolate between the two nearest levels. That is covered in reading a winds aloft forecast. Interpolating direction and speed separately is a common mistake; the right way is to break each level into components and interpolate those.
Why it changes every leg
The wind does not change between checkpoints, but your course usually does. A route with a dogleg has a different correction angle and a different ground speed on each leg, from the same forecast wind. That is why the navlog has a row per leg rather than one number for the flight.
Checking your own work
EasyNavLog solves the triangle for every leg and shows the working for one of them, so you can compare its answer against your E6B before you trust it.
See the triangle solved for your own route
Enter a route and your airplane, and every leg gets its correction angle and ground speed with the math shown.
Open EasyNavLogEasyNavLog is a planning and training aid, not a substitute for an official briefing. Verify every number against your POH or AFM, current charts and an approved weather source.