What is windshear

[oicur12]

Does anybody have a definition of windshear. We often report to ATC windshear on approach when really it was probably just turbulence. Different authorities define it differently - FAA,CASA,CAD etc.

But what is it. 15 knots or greater sounds familiar but over what period of time.

any thoughts.

And while i am in a posing question mood, what is freezing fog.

Does temp below fzg combined with fog constitute fzg fog. A meteorologist would say no, it depends on things like the moisture content of the air and the liklyhood of super cooled water droplets.

But from an aviation point of view (A330 ground ice shedding proc) when do we consider fzg fog?

[Virtual Reality]

For windshear, try this link:

http://en.wikipedia.org/wiki/Wind_shear

For freezing fog, try this link:

http://www.theweatherprediction.com/habyhints/359/


Happy reading......

[AirRabbit]

Here's another reference for you - not that you're necessarily interested in wind shear training in a simulator - but it does give some good information on the subject...

http://www.faa.gov/safety/programs_i...edia/03-05.pdf

[SIUYA]

ICAO Windshear guidelines

oicur12.............

From ICAO 1987 Windshear publication (ICAO Circular 186-AN/122):

Windshear, being the change of wind vector from one point in space to another, is given by the vector difference between the winds at the two points, which itself is a vector(having both speed and direction). The intensity of the shear is calculated by dividing the magnitude of the vector difference between the two points by the distance between them, using consistent units.The calculation of the shear may be done graphically using the triangle of velocities or by subtraction of the components of the two wind vectors either manually or by computer or by trigonometry.

The ICAO guidelines for unacceptable flight path degradations in windshear are:

Takeoff and approach:

• + or - 15kts IAS
• + or - 500 FPM V/S
• + or - 5 degrees pitch attitude

and:

Approach:

• + or - 1 dot glideslope displacement
• unusual thrust lever position for a significant period of time

Hope this helps.

[Check Airman]

Windshear, being the change of wind vector from one point in space to another, is given by the vector difference between the winds at the two points, which itself is a vector(having both speed and direction). The intensity of the shear is calculated by dividing the magnitude of the vector difference between the two points by the distance between them, using consistent units.The calculation of the shear may be done graphically using the triangle of velocities or by subtraction of the components of the two wind vectors either manually or by computer or by trigonometry.



That hurt my brain

[rada76]

IMO Windshear is often reported incorrectly.

A turbulent approach where windspeed is constantly jumping up and down 10 kts is very different from an approach where true windshear gives a clear gain OR loss of 15+ Kts and the speed holds for some 5 or more seconds.

I have been asked many times by ATC if I experienced windshear during turbulent conditions? My reply is "Negative, just a bit turbulent" Then I hear the following aircraft report Windshear

Why do I post this? I am no sky god and I respect the views of fellow pilots but if we all shout 'windshear' during turbulent weather.... When there really is Windshear and it has been correctly reported on ATIS or other, we may fall into the 'cry wolf' situation and then get caught out when true windshear hits us hard!

[ppppilot]

IMO Windshear is often reported incorrectly.

Totally agree.

I have been asked many times by ATC if I experienced windshear during turbulent conditions? My reply is "Negative, just a bit turbulent" Then I hear the following aircraft report Windshear

I do the same

[111boy]

In answer to your question about freezing fog, its just a little less cool than super cool fog....

[Putt]

The 1977 edition of the Jeppeson annual contains an excellent review of the Eastern Airlines 727 crash at JFK due to a downburst cell over the end of runway 22. A professor at the University of Chicago conducted a study of downburst cells and published the study results around that same timeframe.
I had the opportunity to work with Paul Kadlec, Chief Meterologist at Continental Airlines, in 1976. I spent a lot of time riding jumpseat with CO and others, talking with crew about the use of weather radar.
Paul is a great guy, also a BG in the USAF. He studied the CO 727 crash at Denver and develoed a theory about micro bursts which he believed caused that accident. Theory had to do with warm, dry climates and a temp/dewpoint spread of 40 degrees.
I really enjoyed the time I spent in the "front office" in '76. I also flew with Frontier, Hughes Airwest, Western, Hawaiian, Mexicana, Avianca, Air Micronesia and All Nippon.
Putt