Headwind vs Crosswind
Wind rarely blows straight down the runway. Whatever direction it comes from, you can split it into two parts: the headwind component along the runway, which shortens your ground roll, and the crosswind component across it, which pushes you sideways and is the part that demands rudder, aileron and proficiency.
The split depends entirely on the angle between the wind and the runway. A wind straight down the runway is all headwind. A wind 90° to the runway is all crosswind. Everything in between is a mix, and the formula below tells you exactly how much of each. FlightKit’s crosswind calculator runs it for every runway end at your airport from the live METAR.
The Crosswind Formula
First find the wind angle, the difference between the wind direction and the runway heading, then split the wind with sine and cosine.
Step 1: The Wind Angle
Runway heading is the runway number × 10 (Runway 09 = 090°). Take the difference between it and the wind direction. Anything over 180° means you went the long way round: take 360 minus your answer.
Step 2: Split the Wind
Crosswind = Wind Speed × sin(Angle)
Headwind = Wind Speed × cos(Angle)
A negative headwind is a tailwind, and it is worth naming out loud rather than treating as a small number: a 5-knot tailwind lengthens the ground roll far more than a 5-knot headwind shortens it.
Worked Example: Runway 09, Wind 120° @ 20 KT
Runway heading 090°, wind from 120° at 20 knots, so the angle is 120 − 90 = 30°.
Crosswind = 20 × sin 30° = 10 KT
Headwind = 20 × cos 30° ≈ 17 KT
You are landing with a 17-knot headwind and a 10-knot crosswind from the right. The clock rule gets the same crosswind in your head: 30° is half the wind, and half of 20 is 10.
The Clock Rule
Read the wind angle like minutes on a clock face and take that fraction of the wind speed. It is the check you run in the run-up area when the tower gives you a different runway than the one you planned.
15°
¼
A Quarter of the Wind
30°
½
Half the Wind
45°
¾
About 70% of the Wind
60°+
All
The Full Wind Speed
The rule reads slightly high at 45° (the true figure is 71%, not 75%) and at 60° (87%, not 100%). Both errors are on the conservative side, which is the right direction for a mental estimate, and also why the exact number is worth computing before a marginal landing.
Gusts, and Which Number You Check
A METAR reporting 18015G27KT describes two different airplanes to fly. Compute the crosswind at 15 knots and again at 27, and treat the gust figure as the one that matters: controllability has to hold at the worst moment of the flare, not the average one.
Adding part of the gust factor to final approach speed is a common convention, but the authority is your POH and your own procedure. Whatever you add lengthens the landing roll, so cross-check it against takeoff and landing distances for the runway you picked.
True vs Magnetic Wind
Runway numbers are magnetic. So are the winds a tower controller or an ATIS reads to you. Winds in a METAR, a TAF or a winds-aloft forecast are referenced to true north.
Mix the two and your angle is off by the local magnetic variation, which runs past 15° in parts of the country: enough to turn a comfortable crosswind into an uncomfortable one. Convert first, or compare like with like.
Max Demonstrated Crosswind
Your POH lists a maximum demonstrated crosswind component, the strongest crosswind a test pilot handled during certification, usually in Section 4. It is not a regulatory limit. But exceeding it puts you beyond what the manufacturer proved, and usually beyond what you have practiced, at the moment the airplane is slowest and closest to the ground.
Decide with three numbers side by side: the computed crosswind, the gust value and the POH figure. A personal-minimums crosswind is the more useful number day to day, and it is allowed to sit well below the POH value. Then take the whole picture into Go / No-Go.
Picking the Runway
At a non-towered field the runway choice is yours, and the smallest crosswind is not always the right answer: a shorter runway, a displaced threshold, rising terrain off the departure end or a NOTAM’d closure can all outrank a couple of knots of wind.
Density altitude belongs in the same call, since thin air lengthens the roll on the same runway the crosswind is pushing you off of. Read the density altitude guide, and check current weather for the trend rather than the snapshot.
Every Runway, From the Live METAR
The crosswind calculator splits the current wind for every runway end at your airport, flags the best one and checks the gust against your aircraft’s limit.
Keep Learning
Frequently Asked Questions
What is a crosswind component?
The crosswind component is the part of the wind that blows across the runway, perpendicular to your direction of travel. The rest of the wind is the headwind (or tailwind) component, blowing along the runway. You care about the crosswind because it is what pushes the airplane sideways during takeoff and landing.
How do you calculate the crosswind component?
Find the angle between the wind and the runway, then: crosswind = wind speed × sine(angle), and headwind = wind speed × cosine(angle). Example: a 30° angle with a 20-knot wind gives a crosswind of 20 × sin 30° = 10 knots and a headwind of 20 × cos 30° ≈ 17 knots. A crosswind calculator or a wind-component chart does this instantly.
What is the crosswind clock rule of thumb?
Treat the wind angle like minutes on a clock face: 15° ≈ a quarter of the wind, 30° ≈ a half, 45° ≈ three-quarters, and 60° or more ≈ the full wind speed as crosswind. It is close enough for a quick mental check in the run-up area before you commit to a runway.
Is maximum demonstrated crosswind a limit?
It is the strongest crosswind a test pilot demonstrated during the aircraft’s certification, published in the POH. It is not a regulatory limitation, but it is a strong guideline, and exceeding it means you are operating beyond what the manufacturer proved and your own demonstrated skill. Weigh it, your personal minimums and gusts together when deciding.
True wind or magnetic wind: which do I use?
Runway numbers and tower or ATIS winds are referenced to magnetic north, so they match directly. Winds in a METAR or a forecast are referenced to true north. If you mix the two, correct for local magnetic variation first, or the angle and your crosswind will be off.
How do gusts change the crosswind number?
Run the calculation twice: once at the steady wind and once at the gust. The gust figure is the one to check against your limit, because the airplane has to be controllable at the worst moment of the landing, not the average one. The gust factor, gust speed minus steady speed, is also what many pilots add part of to approach speed; use your POH and your own procedure.
How much crosswind is too much?
There is no universal number. It depends on the aircraft, your proficiency, the gust spread and the runway. Build a personal-minimums crosswind limit, check it against the POH max demonstrated value, and add margin for gusts. When in doubt, pick a more favorable runway or wait.
Sources
- FAA-H-8083-3C, Airplane Flying Handbook — crosswind takeoff and landing technique
- 14 CFR §91.103 — preflight action: runway lengths and performance data