PDA

View Full Version : definition of Max wind speed


Dries
28th Feb 2010, 12:55
The other day I was operating into CMN. The wind was 90deg off RWY 35L at 260/30M45. With our company xwind limitation of 38kts (inc. gusts), we attempted 2 approaches before diverting to RAK. What puzzled me was that, while trying to get into CMN, every other traffic around us landed happily, indeed we were the only crew discontinueing their approach. There were some ATRs, 737s and possibly a 320.

This made me wonder what the definition of 'Max' is. Is it synonymous to 'Gust' or could it perhaps mean eg. 'The strongest wind value measured in the last hour.' ? This seemingly subtle difference would make the difference between landing or diverting.

B-HKD
28th Feb 2010, 13:50
And then there is always those that don't give a damn. :E

bfisk
28th Feb 2010, 14:41
Well I think you were right.

Anyway, here's some points: no (or few) airplane manufacturers will put a limit on max crosswind. The max demonstrated crosswind is not to be regarded as a limitation. That means, in turn, that it's up to each operator to assign a max (cross)wind limitation. Therefor, the definition of max (excluding gust, including a percentage or including all the gust, could be 2 minute or instant wind) would also be up to each operator.

Where I fly, the max crosswind component is 30kts (lower limits apply for inexperienced crewmembers and slippery runways, sometimes as low as 5 kts), and max wind is 45kts. These values are including all gusts. Instant winds would be acceptable.

MD11forever
28th Feb 2010, 15:19
ICAO Annex 3, Appendix 3

4.1.2.2 Recommendation.— The mean values of, and
significant variations in, the surface wind direction and speed
for each sensor should be derived and displayed by automated
equipment.

4.1.3 Averaging
4.1.3.1 The averaging period for surface wind obser-
vations shall be:
a) 2 minutes for local routine and special reports and for
wind displays in air traffic services units; and
b) 10 minutes for METAR and SPECI, except that when
the 10-minute period includes a marked discontinuity in
the wind direction and/or speed, only data occurring
since the discontinuity shall be used for obtaining mean
values, hence the time interval in these circumstances
should be correspondingly reduced.


4.1.4 Reporting
4.1.4.1 In local routine and special reports and in
METAR and SPECI, the surface wind direction and speed
shall be reported in steps of 10 degrees true and 1 kilometre
per hour (or 1 knot), respectively. Any observed value which
does not fit the reporting scale in use shall be rounded to the
nearest step in the scale.

c) variations from the mean wind speed (gusts) during the
past 10 minutes shall be reported when the maximum
wind speed exceeds the mean speed by 20 km/h (10 kt)
or more;


In ATTACHMENT D it says more precisely:
2min average wind for Local routine and special report and if wind exceeding the mean speed (2min) by 10kt as max value.
10min average wind for METAR/SPECI and if wind exceeding the mean speed (10min) by 10kt as max value.

PEI_3721
28th Feb 2010, 15:31
There are several threads which have offered a range of views.
Aircraft certification requires that the manufacturer publishes a maximum demonstrated crosswind, which shows that the aircraft may be landed without ‘exceptional’ skill for crosswinds from either side.
If the max demonstrated is considered to be limiting then it is published as a limitation the AFM limitations section.

There are many problems with max demonstrated. Some manufacturers publish additional information such as gusts, more rarely, advice on wet or slippery runway surfaces.
There can be significant errors in wind measurement and reporting.
Opposed to most other ‘speed’ limits in operation, crosswind certification values have no margin for error.
For gusts, there are many unknowns, the duration, direction change, and their effect on the aircraft.

A significant weakness in judging the max acceptable crosswind in operation is that humans tend to overestimate their capabilities, and often underestimate risk. Consider when a ‘max’ crosswind was last flown (if at all), how does the aircraft handle in gusty conditions (pitch, speed, power as well as lat/directional) – how did you find out, what is the runway surface condition, what change in landing distance may be required, etc, etc.


http://www.pprune.org/tech-log/406375-320-crosswind-limits.html

Crosswind Certification - How does it affect you? (http://www.nlr.nl/id%7E5114/lang%7Een.pdf)

Safety aspects of aircraft operations in crosswind. (http://www.nlr-atsi.com/downloads/NLR-TP-2001-217.pdf)

A position paper on crosswind operations. (http://www.vnv-dalpa.nl/commissiepub/4447)

NLR links intermittent / changed.

MD11forever
28th Feb 2010, 15:39
Ah, answers to different questions.
Dries, what are you talking about, the Crosswind limitations (demonstrated whatever) of your aircraft or are you talking about the Wx report?

PappyJ
28th Feb 2010, 16:52
A significant weakness in judging the max acceptable crosswind in operation is that humans tend to overestimate their capabilities, and often underestimate risk. So absolutely correct. :ok:

Dries
6th Mar 2010, 00:07
Thanks for the replies. I am talking about the met report. I am aware that a max demonstrated xwind value is not necessarily a limitation, that's why I phrased my sentence differently in the first place (company x-wind limitation); although I would assume that most companies follow the manufacturer's recommendations and implement these recommendations as limits.

I was trying to discover weather for example W/V 210/18G40 and 210/18M40 have got exactly the same meaning. If my assumption in the first paragraph is correct and 'G' & 'M' share the same definition, many airplanes landed that afternoon while blatantly exceeding their xwind limits. So many of them in fact, that my colleague and I were in the hold wondering whether we had missed something and hence this post...