R2000
This is a curious source of confusion for many pilots, including (and in my experience, in particular) some very experienced ones.
Here's how to do it:
First of all use the clock-face rule of thumb: 15 degrees = 25%, 30->50% 45->75% etc.
In the same way that one needs to consider how many degrees off the
nose the wind is for x-wind component, for headwind, we need to consider the angular distance between the wind and the
wingtip.
For instance, consider lining up on runway 27 somewhere or other. If the wind was 240/10, then the wind is 30 degrees (or thereabouts!) from the nose, so 10x0.5=5 kts x-wind.
The wind is 60 degrees from the wingtips, however, which are pointing N/S - 360, 180 degrees. So in this case, the clock-face r.o.t tells us consider all of the wind as h-wind = 10kts. As pilotmike correctly points out, the real number is closer to 90%, but never mind!
Another example, still on rwy 27:
Wind 340/20
X-wind 20 kts (70 degrees from nose, so 1 x 20=20)
H-Wind 6-7 kts (20 degrees from wingtip, so 1/3 x 20 = 6 and a bit)
You'll also notice that in the above example 70+20=90, and in the first one 60+30=90. No big surprise for those who have a recollection of trigonometry!
In my experience it works, more quickly and accurately than the captains calculator. That he's forgotten how to use.
sin/cos on your calculator, are, of course, fine if you have one handy, and the time to use it.
Hope this helps
CC