Air is a fluid, and an aeroplane flies through it. Whatever the air mass does, the aircraft does too — so a wind blowing across the runway tries to push you sideways on landing, and a wind in cruise quietly carries you off course. Pilots do not fight this by guesswork; they break the wind down into components and solve a small piece of trigonometry called the wind triangle. This article walks through both, with numbers you can check.
Run the figures yourself with the Crosswind Component tool for runway work and the Heading Calculator for cruise — both let you confirm the worked examples below.
How wind affects the aircraft
On the runway, only two things matter: how much of the wind is blowing straight down the runway (the headwind or tailwind, which changes your ground speed at lift-off and touchdown) and how much is blowing across it (the crosswind, which tries to weathervane the nose and drift you off the centreline). In cruise, the whole air mass moves, so a wind from the side pushes your track sideways unless you point the nose partly into it. Both problems are the same idea — decomposing a wind vector — viewed from two angles.
Splitting wind into components
Start with the angle between where the wind is coming from and the runway you are using. Reported winds are the direction the wind blows from, in degrees, and a runway number is its magnetic heading divided by ten (runway 09 points 090°). The angle off is:
Take runway 09 (heading 090°) with the wind reported as 130° at 20 knots. The angle off is 130 − 90 = 40°. Then:
So you would plan for about a 13-knot crosswind from the right and a useful 15-knot headwind. (If θ were greater than 90°, the cosine goes negative — that is a tailwind, and a reason to consider the opposite runway.)
The clock-face rule of thumb
You will not always have a calculator in hand on short final, so pilots memorise a few sine values. The crosswind is the wind speed multiplied by these fractions:
| Angle off runway | Crosswind fraction | Memory aid |
|---|---|---|
| 30° | 0.50 (one half) | sin 30° = 0.5 |
| 45° | ~0.70 | sin 45° ≈ 0.71 |
| 60° | ~0.90 | sin 60° ≈ 0.87 |
| 90° | 1.00 (all of it) | sin 90° = 1 |
So a 30° wind gives you half its speed as crosswind, a 60° wind gives you roughly all of it, and a wind straight across the runway is entirely crosswind. A 20-knot wind 30° off is a tame 10-knot crosswind; the same 20 knots at 60° is a much busier ~17 knots.
The maximum demonstrated crosswind
Every certified aeroplane publishes a maximum demonstrated crosswind component in its Pilot Operating Handbook. The wording is deliberate: it is the strongest crosswind a test pilot demonstrated during certification, not necessarily the most the aircraft can handle. For most light singles it is not a hard limit — it is a proof point. Typical figures sit around 12 to 17 knots for trainers and light GA aircraft (the Cessna 172, for instance, lists about 15 knots). Larger, turbine and transport-category aircraft often publish higher values, and for many of those it is treated as an operating limit. Either way, your own currency and the runway condition usually become the real ceiling well before the book number does.
The wind triangle in cruise
En route, the question flips. You know the course (the track you want over the ground) and you must find the heading to fly so the wind does not push you off it. The wind triangle adds two vectors: your aircraft moving through the air at true airspeed (TAS), and the air mass moving over the ground at the wind speed. The angle you crab into the wind is the wind correction angle (WCA):
Here windAngle is the angle between the wind direction and your course. Suppose you want to fly a course of 360° at a TAS of 120 knots, with the wind from 040° at 25 knots. The wind is 40° off your course, so:
So you point the nose right of course by about 8° to hold a due-north track, and you trade your 120-knot airspeed for a 100-knot ground speed — the wind is costing you a fifth of your speed over the ground. The right-of-track crab makes intuitive sense: the wind is coming from your right-front, so you lean into it.
Putting it to work
On landing, you have two ways to deal with the crosswind. The crab keeps the wings level and the nose turned into the wind, which is comfortable on approach but must be removed before the wheels touch. The sidestep (wing-low) method lowers the upwind wing and uses opposite rudder to keep the nose straight, leaving you aligned with the runway. A common technique is to crab down final and then transition to a sidestep in the flare. Whichever you use, the aim is the same: touch down with no sideways drift and the aeroplane pointed straight down the runway.
Finally, respect gusts. A widely taught rule is to add half the gust factor to your approach speed — for a wind of 15 gusting 25, that is half of 10, so roughly 5 extra knots — buying margin against the airspeed dip that follows a gust. Plan the runway with the Crosswind Component tool, solve the cruise leg with the Heading Calculator, and remember that performance still hinges on the air itself — see Density Altitude Explained and Aircraft Weight & Balance for the rest of the preflight picture.
Frequently asked questions
How do I calculate the crosswind component?
Find the angle between the wind direction and the runway heading, then multiply the wind speed by the sine of that angle. For example, a 20-knot wind 40 degrees off the runway gives 20 x sin(40 degrees), about 12.9 knots of crosswind. The headwind component is the wind speed times the cosine of the same angle.
Is the maximum demonstrated crosswind a legal limit?
For most light general-aviation aircraft, no. It is the highest crosswind a test pilot demonstrated during certification, not a structural or regulatory limit. It tells you the manufacturer proved the aeroplane is controllable at least that far. Some aircraft (and many turbine or transport types) do publish it as a true operating limit, so always check your specific Pilot Operating Handbook.
What is a wind correction angle or crab angle?
It is the angle you turn the nose into the wind so the aeroplane tracks the line you actually want over the ground. In cruise it is often called the crab angle. You compute it from the wind speed, the angle of the wind relative to your course, and your true airspeed.
What is the difference between heading and course?
Course (or track) is the path you want to follow over the ground. Heading is the direction the nose points. In any wind that is not directly on the nose or tail, the two differ by the wind correction angle — you point the nose into the wind to hold the desired ground track.
Why does wind reduce my ground speed even when it is mostly a crosswind?
Crabbing into the wind means part of your airspeed is spent fighting sideways drift rather than moving you forward along course. Ground speed is true airspeed times the cosine of the crab angle, minus any headwind component. So a strong crosswind costs you ground speed twice: through the crab and, usually, through an accompanying headwind component.
Should I crab or sidestep when landing in a crosswind?
Many pilots crab on the approach to track the centreline, then transition to a sidestep (wing-low) just before touchdown so the aircraft is aligned with the runway and tracking straight as the wheels touch. The goal is to land with the longitudinal axis parallel to the runway and no side drift, however you get there.
How should I account for gusts?
A common rule of thumb is to add half the gust factor to your approach speed. If the wind is 15 gusting 25 knots, the gust factor is 10, so add about 5 knots. This gives a margin against the airspeed loss that can follow a sudden gust, at the cost of a slightly longer landing roll.