ALF5071H
Thanks for the lecture on the theory of crosswinds. Do you know of any airline applying stronger crosswind limits than the manufacturers' suggested limits?
The original poster asked for a guide to calculate the crosswind while on final approach and the replies have given a rough way to do so. My post was to suggest that crosswind calculations be used as a 'ball-park figure' only. Perhaps you could say as to what stage on the approach would you go-around, if your 'ball-park figure' exceeds the company's maximum? In my experience the maximum crosswind limit was never without moderate, even severe turbulence and I have twice gone around because of that, rather than the actual wind strength itself. Your observation that a surprising number of pilots only concentrate on the wind strength, is probably because with that strength of wind, we are concentrating on completing the landing safely and have little capacity to consider the other factors that you have stated.

handflown

I'm certainly not better than anybody else. When it comes to mental calculus I always used to be far below average. However, the good thing about mental calculus is that it's easy to improve your own abilities. Every morning after breakfast I take five minutes to practice with a small programme (including crosswind, tailwind, turns, etc.). It helps.

Someone gave me a PM, asking about calculating tailwind components. So here you are:

TAILWIND = WIND SPEED * COS (WIND ANGLE)


Of course the same formula applies to headwind.

I never could get the sins right....

The thread reminded me of a chart we all used to use in the really olden days before we had confusers......

Google got it in one when asked "crosswind component chart"

http://www.nifa.us/judgesmanual/apendixC.pdf

But I guess having a piece of paper in your flight bag is sooooo old-fashioned...back to the trigonometry, chaps and chappesses...

X-Wind Component Determination
 

Geeze fellas, now I know why I was a professional F/E and a Private Pilot. Talk about make things complicated, except that is for the "Clock method". But it seems to me that the best rule to follow would be that if you have any doubts about whether or not it is safe to land in the prevailing conditions, go around and make another approach. It could be that the pilot's "limit" may be less than the published "limit". As for x-wind "limits", either demonstrated or company SOP, if it appears in the Limitations Section of the FM, then it is a limit.

skiesful, et al. Sorry, didn’t intend to lecture (# 19); a sharing of thoughts, an observation of how PPRruNe’s think, and a reminder not to rely on a single data source etc, etc.

“Do you know of any airline applying stronger crosswind limits than the manufacturers' suggested limits?” No.
Although I am familiar with several aircraft types, I don’t have access to operator’s manuals. I suspect that the lack of response to the question in this and other threads, suggest that where operators publish a limit, they use the demonstrated value without additional margin. Hence my concern about understanding what demonstrated means and the errors in wind measurement / reporting.
I agree with the ball-park approach. Where to discontinue the approach probably depends on how a limit is determined and stated by the operator.
A hard rule like JAR-OPS RVR minima in low visibility enables a specific position; a more flexible rule like a stabilised approach (more variables) uses several decision points, i.e. 1000ft, 500ft, threshold, and even touchdown. The latter rule might result in more errors as several parameters have to be assessed - more opportunities for judgement. Even so, flexible rules should still have a final hard limit.

The concept of operator crosswind ‘limits’ appear to be even more flexible still (perhaps due to certification terminology, weak operational knowledge, etc), whereas in reality a hard limit is required (accident / incident statistics).
Comparing manual landings with autoland, the latter has a hard limit applied before an approach. Autoland limits include a safety margin due to difficulties in determining the system’s performance in the conditions. This is not so much a technical issue, (the reliability and consistence can be assessed), but more the variability of the conditions and their effect on the aircraft / autoland system.
To me this is similar to the problems experienced by crews during manual landings; they might miscalculate the crosswind component, or misjudge the effect of gusts;- human fallibility.
So just as the certification process adds a margin for autoland, so too should the operational process for manual landings.

I would recommend a hard limit less than the demonstrated value, but greater than autoland (credit for the positive aspects of pilot judgement). The limit would depend on good knowledge and training to provide the essential skills of judging and flying in such conditions (airmanship, currency).

In my limit, if the manufacturer does not specify any demonstration of gusts, then either gusts are limiting or factored, e.g. wind + half the gust. An absolute gust value might not be as important as reminding crews to think about the consequences of the risks from gusts.
More generally a safety margin of 60% of any wind speed error, or a combination with direction error (and calculation) could be used. This probably results in a 5 kt reduction from the demonstrated value (ball-park figure).
When to apply it? Earlier rather than later. Although wind measurement is historic (up to two mins), which might encourage a later assessment, this would result in less time for a well considered judgment and greater pressure to continue the approach (plan continuation bias; ‘press-on-it is’ – the closer you get the more you continue). In difficult conditions, turbulence, gusts, etc, crews should avoid increasing workload at a late stage of the approach where greater attention must be allocated to the flying tasks, thus a late consideration of a limit has greater risk. I suggest that the decision is made at 1000ft; this a consistent point with any RVR limit (if applicable) and a stabilised approach (IMC). An earlier decision also provides opportunity to refocus on the missed approach procedure, thus avoiding the rush of manoeuvring due to last minute decisions.

A lengthy view originating from a short question, but that’s like most safety issues.

Refs: Crosswind Certification - How does it affect you?
Safety aspects of aircraft operations in crosswind.

Clock Method!!! My favourite!:D

Simple, does the job.

If I need a reasonably accurate figure, I use the back of my CR-5 circular computer. All sorts of good stuff on that thing, needs no batteries & it fits in a shirt pocket too!

I always have a chart to hand, similar to the one posted above, which I can quickly read off the X-wind and Headwind/Tailwind components. Very useful.

The simplest solution?
 

Lets keep in mind that the original request specified that you where "ON FINAL APPROACH", which probably means that you have a few other priorities besides working out the crosswind.

I had a good look at that sine tables once and found a, in my opininon, very simple system that seems to work. It is accurate within 2.5%, which, given the somewhat inaccurate wind reporting system that we have to live with, it's accuracy may be regarded as well within any practical limits.

Lets have a brief look at the cine tables, (and then forget about them):

Sine 0 degress is: 0

10 : 0.173648
20 : 0. 34202
30 : 0.5
40 : 0.642788
50 : 0.766044
60 : 0.866025
70 : 0.939693
80 : 0.984808
90 : 1


From a practical point of view, these figures are very interesting and,-
quite useless.

You will agree
That for 20 degrees: 0.34202 is very close to 0.3333 which is = 1/3
That for 40 degrees: 0.642788 is very close to 0.66666 which is = 2/3
That for 50 degrees: 0.766044 is very close to 0.75 which is = 3/4
That for 60 degrees. 0.866025 is very close to 0.9, so maybe you could multiply your given main wind by.9 and just round DOWN, ( thats veeeery close, no?)

So our new, final and complete chart could rather be:



20: 1/3
30: 1/2
40. 2/3
50: 3/4
60: .9


And thats ALL.
All above 60 degrees would be 1, or full wind strength.
A crosswind of 10 degrees is IMO if little interest anywhere below 100kts.

If you just could keep in mind the figures: 1/3, 1/2, 2/3, 3/4, .9 and the fact that your "1/3" starts at 20 degrees, you should IMHO have a perfectly adequate personal crosswind calcualtion system.

NB! Keep in mind that we, as pilots, normally seem to constantly underestimate our crosswind component.

Its more than a bit touchy to do so, but if you were to ask your highly experienced friend and collegue: You are landing on runway 18, tower reports wind 230/30. What is you effective crosswind component?
You'll find that more often than not the answer would be, "somewhere around 12-13 knots".(!)
But of course you would know better.

Finally, I think that with a little bit of cooperation, an even simpler, better system for crosswind calculation may be found, but for now, I'd very much appreciate anyones opinion of the simple procedure suggested above.

Stay safe.

Or even fewer numbers...
 

0.5, 0.7 & 0.9 for 30, 45 & 60 degrees (or 60, 45, 30 for headwind) angle between track and wind. It's close enough for government work (and short finals on a "nasty" night).

PM

With the exception of asuweb, no-one has acknowledged that the single piece of paper with a X-wind component chart on it is easiest, quickest and most accurate method.

I'm really puzzled by this.

What is it about complex (to one degree or another) mental arithmetic, or using a calculator, PDA, laptop or whatever, that makes it so much preferable to use that, rather than just to read the X-wind component off a chart?

It takes 4.5 seconds, including the time needed to pull it out of the chart pocket, and it's accurate. All you have to work out mentally, at a moment of higher stress than usual, is the angle of the wind to the runway heading. There is nothing to remember, except where you put the chart.

glance down at the RNAV box, NAV 3, then see the wind, drift angle etc, no need for complex/simple arithmetic. ;)

As for sine tables etc, come on. Just guestimate it, it changes all the way down the approach anyway if thats where you're using it.



Is the wind just off the nose by about 20 degrees? ok, so use about 1/3 of wind speed. 45 degrees? use about 2/3 wind speed. then up to 90 degrees? use all of it.

onb, I am sure you are right about the piece of paper, but if you had a piece of paper to remind you of all you needed to know before lunch today, then your cockpit would soon look like my great grandma's parlor :p

I am kinda surprised that trigonometry and simple vector mechanics seems such a black art to so many 'pilots AND navigators' ..., but then I guess there's more than one way of skinning a cat or landing an aeroplane thesedays - well that's what my great granny says when she discusses it with me :}.

Anyway, I am sure there's a few reading the thread who are grateful for the revision lessons on rules of thumb :ok:

I am sure what we see in this thread shows that with some parts of the syllabus at least, the ATPL Theory multichoice questions in the exams don't quite test understanding as rigorously as might be preferable.

We all know our angles at least. Personally I've then taught my kids to work out sine's, cosines and tangents from geometric first principles (right angle triangles - measure and calculate opposite/hypotenuse etc.) and plot them on a graph to get a better feel for it all - plot their own sine curve, get to know the shape of it, how the gradient reduces rapidly for those angles over 60 - they now associate the values with something they understand and can visualise in their mind's eye, rather than risk regurgitating poorly learned number lists incorrectly e.g. is it sine or cosine I need? Is sin60 .866 or is cos60 .5 ?? Is it sin or cos that is 0 at 0? And why's tan45 1?

Hopefully if they ever take to the air, and one day find themselves trying to make sense out of their Breitling watchface bezel and crosswinds, they'll soon sort themselves out :8.

Why make something complicated when it can be staggeringly simple - no sums are needed.

1. When on final look at your DI/HSI and find the reported wind direction round the outside of it.
2. Mentally drop a line from the wind direction until it hits the horizontal centre line.
3. The proportion of the centre line between this intersection and the centre is the proportion of the reported wind that's across the runway.

(As originally described a bit earlier by low n' slow)

If you do the same thing horizontally you get the headwind.

This is as accurate as you want to make it and takes about 2 seconds.

HFD

(BTW, if you want to know what the components are before you reach final just turn another compass rose (RBI/CDI?) to the runway track and use that)

Erm, your method still requires a calculation, because effectively all you are doing is finding the number of degrees off runway heading. You still need to apply that figure (whether calculated or using your HSI method) to the reported wind speed to get the x-wind.

Personally, I'm with the piece of paper. Where I fly, we are load limited, and evey knot of wind makes a large difference, hence a piece of paper with both headwind and crosswind component.

mini-jumbo:
IME people find it easy to judge ratios/proportions but impossible to do "proper" sums. In fact I can't remember more than a handful of (admittedly, private) pilots over the years who've managed to give me more than a random number when I've asked them for the crosswind component.

(FWIW the same method can be used for drift and gs calcs if you know MaxDrift)

HFD

Best Rule of Thumb
 

When you're on short final and have about 10 seconds when the tower gives you the wind direction/speed, the best rule of thumb was:

For each 10 degrees of X-wind take one sixth (1/6) of the crosswind value.

eg landing on runway 04 with x-wind of 12 kts from 060

Wind = 20 degrees from right so 2/6 or 1/3 of 12 = 4 knots x-wind.

With a bit of practice you can do any permutation in your head in well under 5 seconds!!!

HFD, fair point. I guess for some people visualisation, ratios etc works better.