How do you calculate a crosswind component?

Subtract the runway heading (runway number times ten) from the wind direction to get the wind angle. Crosswind is wind speed times the sine of that angle and headwind is wind speed times the cosine. Runway 17 with wind 140 at 25 knots gives a 30 degree angle, about 13 knots of crosswind and 22 knots of headwind.

Should I use the gust or the steady wind?

Check both. The steady wind sets your crosswind correction on final; the gust value is what the airplane may see in the flare. Compare the gust crosswind with your personal limit and the maximum demonstrated crosswind in your POH.

Is the maximum demonstrated crosswind a limit?

Usually not. It is the highest crosswind the manufacturer tested during certification, published in the POH or AFM. It is not necessarily the most the airplane can handle, but it is a sensible ceiling for a personal limit unless you have trained beyond it.

Are METAR winds magnetic or true?

METAR and TAF winds are true. Winds from the tower, ATIS and AWOS or ASOS voice broadcasts are magnetic, like runway numbers. The difference is the local magnetic variation, which can matter in parts of the country where it is large.

Pilot tool

Crosswind calculator

Crosswind equals wind speed times the sine of the angle between the wind and the runway; headwind equals wind speed times the cosine. Enter the runway number and the reported wind below to get the crosswind, the headwind or tailwind, and the same numbers at the gust.

Steady 15 kt

Crosswind

10kt

From the left

Headwind

11kt

Down the runway

Gust 25 kt, spread 10 kt

Crosswind

16kt

From the left

Headwind

19kt

Down the runway

Wind angle 40° left of the runway heading. Components rounded to the nearest knot.

How it works

A runway number is its magnetic heading rounded to the nearest ten degrees, with the last zero dropped. Runway 27 points 270°, runway 9 points 090°, runway 36 points 360°. The wind angle is the reported wind direction minus that heading, taken the short way round, so wind 350° on runway 01 is 20° from the left, not 340°.

The wind then splits into two parts. The part blowing straight down the runway is the headwind, and it shortens your ground roll. The part blowing across it is the crosswind, which you hold off with aileron into the wind and opposite rudder. Once the angle passes 90°, the headwind turns negative and becomes a tailwind, which lengthens both takeoff and landing distance.

Gusts matter twice. The gust crosswind is what you may meet in the flare, and the gust spread (gust minus steady) is a common personal minimum on its own, because a large spread means large, sudden changes in airspeed close to the ground.

Check winds and runway against the same reference. Runway numbers, tower winds and ATIS winds are magnetic; METAR and TAF winds are true.

The formula

Check the math yourself.

θ = wind direction − runway heading, where runway heading = runway number × 10

Headwind = W × cos θ. A negative result is a tailwind.

Crosswind = W × sin θ. Positive is from the right, negative from the left.

W is the wind speed in knots; repeat with the gust value. This is the trigonometry the crosswind component chart in PHAK chapter 11 draws graphically, so the two agree to within the precision of reading the chart.

Source

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 11: Aircraft Performance ↗

Worked example

PHAK chapter 11 works this problem with its crosswind component chart: runway 17, wind 140° at 25 knots.

Runway heading is 170°. θ = 140 − 170 = −30°, so the wind is 30° off the nose, from the left. Headwind = 25 × cos 30° = 21.7 knots. Crosswind = 25 × sin 30° = 12.5 knots. Rounded, 22 knots of headwind and 13 knots of crosswind, the same answer the handbook reads off the chart.

Add a gust to 35 knots and the crosswind at the gust is 35 × sin 30° = 17.5 knots, with a 10 knot gust spread. If your POH lists a 15 knot maximum demonstrated crosswind, that gust takes you past it.

Questions

How do you calculate a crosswind component?: Subtract the runway heading (runway number times ten) from the wind direction to get the wind angle. Crosswind is wind speed times the sine of that angle and headwind is wind speed times the cosine. Runway 17 with wind 140 at 25 knots gives a 30 degree angle, about 13 knots of crosswind and 22 knots of headwind.
Should I use the gust or the steady wind?: Check both. The steady wind sets your crosswind correction on final; the gust value is what the airplane may see in the flare. Compare the gust crosswind with your personal limit and the maximum demonstrated crosswind in your POH.
Is the maximum demonstrated crosswind a limit?: Usually not. It is the highest crosswind the manufacturer tested during certification, published in the POH or AFM. It is not necessarily the most the airplane can handle, but it is a sensible ceiling for a personal limit unless you have trained beyond it.
Are METAR winds magnetic or true?: METAR and TAF winds are true. Winds from the tower, ATIS and AWOS or ASOS voice broadcasts are magnetic, like runway numbers. The difference is the local magnetic variation, which can matter in parts of the country where it is large.

More pilot tools

Set your crosswind limit once.

In Ascent 1.9.6, Copilot (beta) checks a planned flight's crosswind and gust spread against the personal minimums you set. It is advisory; the decision stays with you.