Crosswind component
Works out the gust as well as the steady wind, and converts a METAR wind to magnetic — because runway numbers are magnetic and METAR winds are not.
Two things a crosswind calculator usually gets wrong
The arithmetic is a sine and a cosine. What varies between calculators is which wind they put into it, and most of them pick the wrong one twice.
The gust is the number, not the mean
A wind reported as 12 gusting 22 is a 22 knot wind for planning purposes. The gust is what arrives in the last twenty feet, and at thirty degrees off it is 11 knots of crosswind against 6 in the steady wind — nearly double.
Planning on the mean and meeting the gust in the flare is how a comfortable approach turns into an uncomfortable one. Both numbers are on screen here, and the gust is the one held up against your limit.
A METAR wind is not a magnetic wind
Runway numbers are magnetic. Winds from an ATIS or a tower are magnetic too — that is on purpose, so you can compare them with a runway without thinking.
A METAR or TAF wind is referenced to true north. Put one straight against a runway number and the angle is out by the local magnetic variation: 10 to 20 degrees across much of North America, more at high latitudes.
At 15 degrees of variation, what looks like a 30 degree crosswind is really 15 degrees, and the component is half what you calculated. It can also go the other way and be double. Either way it is not a rounding error, and almost nothing asks you which kind of wind you typed in.
The demonstrated crosswind is not a limit
On most light aircraft it is the strongest crosswind a test pilot happened to meet during certification — not a structural limit, and not a number the aeroplane was tested to destruction at. It is still the most useful figure you have, because the manufacturer stands behind it.
So this tool reports it as what it is. Exceeding it is a decision made deliberately on the ground, weighed against your own currency and the runway in front of you. Some aircraft do have a real maximum; yours will say which.
The rule of thumb, and why it works
Thirty degrees off gives you half the wind as crosswind, forty-five about seventy percent, sixty or more effectively all of it. That falls straight out of the sine — sin 30° is exactly 0.5 — and it is worth carrying so you can sanity-check any calculator, this one included.
Questions pilots actually ask
Should I use the steady wind or the gust?
The gust. It is the one that arrives at the flare, and it is often close to double the steady component. A wind of 12 gusting 22 at thirty degrees off gives 6 knots of crosswind steady and 11 in the gust.
Planning on the mean and meeting the gust in the last twenty feet is how a manageable approach becomes an unmanageable one. This tool computes both and holds the gust up against your limit, because that is the number the decision rests on.
Why does the tool ask whether the wind is magnetic or true?
Because runway numbers are magnetic, and not every wind report is.
A wind read from an ATIS or given by a tower is magnetic — that is deliberate, so it can be compared straight against a runway number. But the wind in a METAR or TAF is referenced to true north.
Feed a METAR wind into a runway number without converting and the angle is wrong by the local magnetic variation, which is 10 to 20 degrees across much of North America and larger at high latitudes. At 15 degrees of variation a 30-degree crosswind becomes a 15-degree one, which halves the component. That is not a rounding error.
Which way does the variation go?
Easterly variation is subtracted from a true direction to get magnetic; westerly is added. Enter easterly as a positive number here and westerly as negative.
Getting the sign backwards is worse than not correcting at all, because it doubles the error instead of removing it. The tool shows you both the direction you entered and the direction it used, so you can see the conversion happen.
Is the demonstrated crosswind a limit I cannot exceed?
On most light aircraft, no — and this is worth being precise about. The demonstrated crosswind is the strongest crosswind a test pilot happened to encounter during certification. It is not a structural or aerodynamic limit, and the aircraft was not tested to failure at it.
It is still the most useful number you have, because it is a figure the manufacturer stands behind. Exceeding it is a decision to make deliberately on the ground, with reference to your own currency and the runway in front of you — not something to discover in the flare.
Some aircraft do have a genuine maximum crosswind limitation. Yours will say which it is.
Why flag the tailwind separately?
Because it is the more dangerous of the two and it has a much lower ceiling. Many aircraft have a hard tailwind limitation of 10 knots or less, well below any crosswind figure, and landing distance grows sharply with even a small tailwind.
If a runway shows a tailwind component the tool says so plainly and points you at the reciprocal, because that is almost always the answer.
What is the thirty degree rule of thumb?
Wind thirty degrees off the runway gives roughly half its speed as crosswind, forty-five degrees gives about seventy percent, and sixty degrees or more is very nearly all of it.
Those come straight out of the sine: sin 30° is exactly 0.5, sin 45° is 0.707, sin 60° is 0.866. The rule of thumb is good, and it is worth knowing so you can sanity-check any calculator including this one.
Notes
A cross-check, not an authority. This is trigonometry on the numbers you typed. It cannot know about runway condition, gradient, obstacles, your currency, or a gust that was not reported.
No aircraft data is shipped. The demonstrated crosswind comes from your flight manual, for the same reason the weight and balance tool ships no envelopes. The FAA's account of the wind components sits in chapter 11 of the Pilot's Handbook of Aeronautical Knowledge.
Wind reference handling follows the standard convention: METAR and TAF winds are true north referenced, ATIS and tower winds are magnetic, and runway designators are magnetic. Easterly variation is subtracted from a true direction and westerly added.
The tool reports a tailwind component prominently because it is the more dangerous of the two and usually has a much lower limit. Tell me if something here disagrees with your manual.