Windshear

[xairbusdriver]

There's been several mentions of "Windshear" here and the media in general. Those who rarely get more than a few feet above the surface of the earth tend to think of this in context of changing speeds/directions in that limited domain. It can be dangerous and even destructive on the surface; the only thing we may notice is some tall weeds or limbs getting blown down. However, even just a few hundred meters above ground level, there may be much more severe conditions.

For a great (current as of this posting) example of what I'm talking about, take a look at any Windity display. Mine is at <Mid-South Weather> but it's available on many Templates. Center the map/reporting point on the Memphis, TN area (extreme western TN). Set the display to Wind and Altitude at Surface and the Time 7pm. Not much to see, right? Now, change the Altitude to 300m/1,000ft.

Notice that there is little change in direction, but a massive change in velocity. That is why it's such an important consideration for pilots. This display is computed, of course, and real life info can be much different. But real life info can also be hard to get without newer inertial and GPS navigation systems. And simply finding out that here is a massive change in windspeed/direction while flying through it is sometimes too much, too late. In busy airports, pilots and radar controllers are usually very good at keeping everyone up-to-date. Pity the crew who makes the first approach into some quiet, low traffic, automated-weather-reported location before the days of better computer generated weather models.

What the Windity display mentioned above does not show is the rate of change of the different velocities. A gradual change is not unusual and would not qualify as 'shear'. However, if the change is within a hundred feet of altitude, it can be very rough (usually the first clue) and the actual airspeed can become very erratic. If the shear was not adequately considered before the flight, and the difference is high enough and fast enough, the aircraft can actually find itself very near it's minimum safe flight speed. It would be best if this happens at 1,000 ft AGL or higher. Plenty of time and altitude to recover and even continue the approach. Below 500 ft AGL, things can get very "interesting". Most pilots don't like "interesting" things happening unless they are doing acrobatics and after a thorough before-take-off briefing.

[elagache]

Dear X-Air and WeatherCat "heads in the clouds" types,

For a great (current as of this posting) example of what I'm talking about, take a look at any Windity display. Mine is at <Mid-South Weather> but it's available on many Templates. Center the map/reporting point on the Memphis, TN area (extreme western TN). Set the display to Wind and Altitude at Surface and the Time 7pm. Not much to see, right? Now, change the Altitude to 300m/1,000ft.

Okay, you've made your point.  That is a very impressive change in wind speeds.  I was aware of the phenomena, but never paid attention to it because . . . well, I spend my time very firmly planted on the ground!

Thanks for the lesson in flying weather! 

Cheers, Edouard 

[Blicj11]

Thanks for the explanation Xair.

[Bull Winkus]

Thanks Bus Driver! Very interesting. I did not know that. I'm glad that wind stays up there and not down here! This old planet would be a completely different place if it was always down here.

The message in this poem is just as true today as it was in the year 1400, and just as true in Memphis (or anywhere else in the civilized world) as in Engelond. Note that the verse talks of the effects of Spring and also note that Zephirus is the Greek god of the west wind. Well, that's my segue, and since it's not that obvious I thought I'd mention it.  - Herb

Whan that Aprill, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
Whan Zephirus eek with his sweete breeth
Inspired hath in every holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his halfe cours yronne,
And smale foweles maken melodye,
That slepen al the nyght with open ye
(So priketh hem Nature in hir corages);
Thanne longen folk to goon on pilgrimages
And palmeres for to seken straunge strondes
To ferne halwes, kowthe in sondry londes;
And specially from every shires ende
Of Engelond, to Caunterbury they wende,
The hooly blisful martir for to seke
That hem hath holpen, whan that they were seeke.

- Geoffrey Chaucer
Prologue to The Canterbury Tales
Source: Wikipedia

[Blicj11]

Thanks for the reading challenge Herb. Good exercise.

My favourite Chaucer quote (with modernized spelling): By nature, men love newfangledness.

[Bull Winkus]

My favourite Chaucer quote (with modernized spelling): By nature, men love newfangledness.

Thanks Blick! I didn't know that one, and it's a good one too! So true.

Bus Driver reminded me of the Prologue to The Canterbury Tales with his wind story, since this is April. I'm not that familiar with the rest of the tales, but I, as well as everyone else, was required to commit to memory the entire Prologue in Middle English, and recite it to the rest of the class in my senior year in high school. It carried the same weight as a term paper on our grades. On the day she introduced the challenge, she brought a 33 1/3 RPM recording of a famous orator into the class to give us an audio impression of the task. When the class ended, the rest of the students left in full dismay over what was expected of them. I stayed and asked the teacher if I could record a copy of the presentation. She said it would be fine, so by her last class of the day I had returned with a reel to reel tape recorder and a microphone.

Let me tell you, it was much easier to learn it with the sound, than it would have been just reading the passages! I played it a few times first, then I played a few verses, stopped and recited, then played it again if I hadn't got it. In about an hour, I had it perfect. I could rattle off the whole thing without missing a beat.

On the day of the test, I never gave a hint to the teacher as to whether I was prepared or not, but she must have instinctively knew. She called on every other member of the class first. I was last to get up. Naturally, everyone had sort of learned the words, with terrible pronunciation and no understanding. Then I got up there and rattled it off in close approximation of the original recording, word by delicious word, with every nuance of inflection and with feeling. Jaws dropped and the teacher was beaming!

And you know, ? I can still recite it today at age 68, just like I did that day.

 

[Blicj11]

And you know, ? I can still recite it today at age 68, just like I did that day.

Ha ha. Thanks for sharing how that came to be. Wish you lived closer; we could have some fun in person.

[Bull Winkus]

Yep! Two old codgers reminiscing over our long ago exploits? We might never stop talking. 

 

[Steve]

I am in awe!

I've been on umpteen forums, newsgroups, mailing lists, etc. over the past 34 years, and I can say with certainty that this is the first time I have ever  seen Chaucer quoted. Bravo, Herb! Bravo to you!

[dfw_pilot]

xAir, you flew a lot into PANC, right?

That place has some really wild wind, and sheer to boot. It's one of the few places I go into where the wind at 1200 feet might be 070 at 45 and at 900 feet is 330 at 5. Guys who are always married to their auto-throttles get real excited when they come down through that at about 1'000 when the bottom drops out! All the best,

dfw

(DFW Delta L-1011, 1985. Never Forget.)

[xairbusdriver]

The etymology of "shear" has a lot of negative connotations, well before man left the confines of the ground; a perfect choice for describing what happens when it is ignored.

[dfw_pilot]

haha! Dang auto correct.