Crosswind component
The FAA tests this with a chart, and generations of pilots have read the answer off a nomogram without being shown that it is a right triangle. It is two lines of trigonometry. The part worth a page is not the arithmetic — it is that the runway and the wind are very often measured against different norths.
Optional. Plan on the gust, not the mean.
Crosswind
kt
kt
Landing downwind unless you use the other end.
- Angle off the runway
- Crosswind in the gust
- kt
- Wind used, magnetic
Without the variation correction this would have read kt of crosswind — a difference of kt, from doing nothing but comparing two bearings measured against different norths.
crosswind = V · sin θ
headwind = V · cos θ
- V
- wind speed
- θ
- angle between the wind and the runway
A negative headwind is a tailwind, which is why this page keeps it as one signed number rather than two boxes. The components are the legs of a right triangle whose hypotenuse is the wind, so they always satisfy crosswind² + headwind² = V² — a test asserts it at every angle.
The trap: two different norths
Runway numbers are magnetic. Runway 27 points at 270° magnetic, not 270° true, and the number is repainted when the variation drifts far enough to matter.
The wind in a METAR or a TAF is true. The wind from a tower, ATIS or AWOS is magnetic. So exactly half the wind reports you meet are in the same reference as the runway number, and half are not, and nothing on either one tells you which.
Subtract one from the other without noticing and the angle is out by the local variation — which runs past 15° over much of the western United States and most of the UK. The worked example above, at 14° east, turns 10 kt of crosswind into 5.5. That is not a rounding difference; it is the difference between a landing you were relaxed about and one you were not.
Doing it without the page
The sines of the angles that matter are worth carrying in your head, because they are almost embarrassingly round:
| Angle off | Fraction of the wind | Exact |
|---|---|---|
| 15° | a quarter | 0.26 |
| 30° | a half | 0.50 |
| 45° | about two thirds | 0.71 |
| 60° | nearly all of it | 0.87 |
| 90° | all of it | 1.00 |
Thirty degrees is a half. That single fact covers most of the decisions anybody actually makes at a hold, and the worst error in the whole table is four per cent of the wind speed — smaller than the resolution the wind was reported at.
What is different about a glider
A crosswind is three separate problems on a gliding day, and only one of them is the landing.
On the ground run a glider has one wheel, a long wing and no power to fix a swing with. The into-wind wing wants to lift long before you have aileron authority to stop it, which is why the wing runner holds the downwind tip and why a crosswind launch gets called off at a lower number than an aeroplane's.
On a winch launch the cable pulls you along the ground track while the wind pushes you off it, and the two do not resolve the way they do in free flight. Clubs that winch have their own crosswind limits and they are not the same as the landing ones.
On the approach the arithmetic is the ordinary one, but the margin is not: you are landing once, with no option to go around, on an airfield you may have picked ten minutes ago. That asymmetry is worth more than any number on this page.
What this assumes, and what it can't know
No limits, only components. Demonstrated crosswind is per type and sits in your flight manual, and for a glider it varies by launch method as well. This page says what the wind is doing; whether it is acceptable is a question about your aircraft, your currency and your airfield.
The reference is yours to get right. The page applies a variation you supply, to a wind you tell it is true-referenced. It has no way to detect a mismatch you have not flagged, and the answer will look perfectly reasonable while being wrong by the variation.
Reported wind is an average over a point. It was measured somewhere on the field, at some height, over some minutes. Curl off a hangar, a treeline or a ridge is real, local and not in the report.
Not yet checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.