Calculating crosswind

[Ian_Wannabe]

Hi all,

I've never been too quick with mental maths and something I hear very often is bugging me about crosswind calculations.

If someone says the wind is 'x' degrees off the runway at say 20kts, how do you mentally work out the actual crosswind?

[ShyTorque]

Look up the term "Clock Code".

[worrab]

In the aeroplane the precise numerical value can be less of an issue. For your example, if the wind is more than 45 degrees to the runway then the cross-wind is pretty much 20kt. At 30 degrees it's a half (ie 10kt) and less than around 20 degrees off the nose shouldn't affect things too badly.

[18greens]

Or if you have a mathematical bent you can use sine calculation. The Dalton computer (or a clock) is a visual representation of the Sine wave.


Using your 20 kt example: the rule of thumb is 30 deg off is Half the xwind (10 kts ) , more than 60 degrees is all of it (20 kts) . For the rest you can interpolate. I tried to put it in a table but i've discovered tables are hard to do in PPRUNE.


Degrees off ______SIN______Resultant xwind

0__________________0_________0*20=_______0kts
30_______________0.5______0.5*20 =____10kts
45_________________0.707_____0.707*20=____14kts
60_______________0.866____0.866 * 20= __17kts
90_________________1_________1*20=_______20kts

[Piltdown Man]

Just remember three numbers...

Reworking 18greens numbers, you just have to remember .5, .7 & .9. These factors apply for 30, 45 & 60 degrees difference between wind and track. It's close enough for any flying.

PM

[Whiskey Kilo Wanderer]

Sixths

One cheap and cheerful method is to assume the cross wind component is one sixth of the wind value for every ten degrees off the runway heading.

At thirty degrees you have three sixths, or a half of the wind speed, pretty much as mentioned above.

Anything over sixty degrees off runway heading and you can assume that it's all cross wind.

[Desert185]

30 degrees is half. 45 degrees is 70%. More than that, just use all of it. Much of the time its moving around and gusting, so a few knots either way really won't matter, and it will mostly be slightly different by the time you cross the threshold. Close enough for spam can work. Keep it simple, decide, fly the machine, enjoy.

[packo1848]

I find the clock code method the easiest:

Think of the face of a clock divided into portions of 1 hr -

0 degrees = 0 crosswind (of course)
15 degrees (15 mins) = a quarter
30 degrees (30 mins) = half
45 degrees (45 mins) = 3 quarters
60 degrees (60 mins) = full.

An easy rule of thumb and if you look at 18 greens' sin table it is pretty accurate and any error is on the side of caution.

[hvogt]

I've simply learned the rounded sines for angles of whole ten degrees off by heart long ago and this works perfectly well for me. If this is to much mental arithmetic for you, you might try the following rule of thumb instead:

CWC ≈ [(wind angle / 100) + 0.2] x wind speed

Example:

RWY 27, wind 31012KT

1. step: wind angle is 310 - 270 = 40 degrees
2. step: 40 degrees/100 = 0.4
3. step: 0.4 + 0.2 = 0.6
4. step: CWC ≈ 0.6 x 12 kt ≈ 7 kt

[GipsyMagpie]

Packo's way is as taught to RAF pilots and it is simple maths. You shouldn't be wasting precious thinking time airborne with 0.6!

[Maoraigh1]

I've never calculated. I go by the windsock or what the airfield radio says - and then decide if it is possible by the aircraft angle on short final.
Very often the wind is gusting and varying direction. You're going to touch down in a drop in the wind.

[BullHughes]

Degrees off the nose, divide it by sixty and then its that fraction of the wind strength.

10 degrees = 1/6
20 degrees = 2/6 = 1/3 etc...

Easy enough to do mentally when you recieve the wind strength on final, and the clock code method as described above can be even simpler depending on personal preference.

These methods will also work for headwind/tailwind components if you calculate degrees off abeam.

[Andy_P]

As a new comer to the sport, I remember 5-7-9, which is the amount of crosswind for 10 knots. Coincides with 30-45-60

[Ian_Wannabe]

Much clearer, thanks everyone.

[18greens]

The amazing thing,( for possibly the first time in pprune) we all gave the same answer.

And now I know what the clock code is I can dispense with carrying all those 3 digit sine values around in my head.

[Mach Jump]

The amazing thing,( for possibly the first time in PPRuNe) we all gave the same answer.

It surely is a miracle!

I recommend the clock code too!

MJ

[Whopity]

We all have Square Douglas Protractors, nicely calibrated and with a square grid. If N is the runway direction just mark off the wind direction relative to the runway, and the strength using the calibration marks. Now check how many squares abeam that is. There is you crosswind, you don't even need to draw it just use your fingers..

[Heston]

How about "if you think you need to calculate the crosswind, then its too much"?


...no not really - I use the clock code too

[Pace]

You could in fact ignore all that and just see what the aircraft tells you down approach?

what will a number tell you? That its within the demonstrated X wind component? So if its three KTS above its no go?

What do you do if its a gusting wind take the gust factor as your X wind component when on actually landing you may not have that gust factor?

Do you have enough control authority to maintain the centre line down the approach!

i think we all know when its a no go for our experience levels and the aircraft and a number is just an indication.

i Think I would look at the actuals and TAFS before taking off and look at the potential X wind component then before taking to the air as part of my destination decision as well as looking at my alternatives.

demonstrated is just that and I have landed in double the demonstrated before now.

OK just adding another ingredient into the mix (stirring )

Pace

[Pace]

Point is you should make go no go decisions before you leave depending on your experience and the aircraft and not be making calculations when it might be better to concentrate on flying the aircraft

i don't know how you guys work weather but I check the metars and TAFS look at various weather charts and pick out problems areas which I then go into detail and plan accordingly. It could be forecast fog? Usually take on more fuel and look at a number of diversions! It could be packed isobars a new set of problems not just crosswind but shear. Where is the airport? in hills where shear will be worse? Are winds fairly down the runway or 90 degrees across? What is the gust factor? What is the airport location? are there hangers or factory buildings which will effect me and a host of other variables!

So its not for me a matter of Oh my god these winds are strong I better calculate the X wind component down final approach and Oh my God its 2 KTS over the demonstrated That is neither here or there.
Better to feel the air can you maintain centreline on the approach with the control inputs available. If not have you worked an alternative with a a better wind runway direction?

pace