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MichaelAdams
1st Aug 2011, 15:01
Hi,

I am trying to understand the difference between gust and wind shear.

According to my knowledge wind shear is a difference in wind speed and direction over a relatively short distance in the atmosphere.

Gust is defined (on the internet) as a sudden increase in speed of the wind.

I can image that the causes, the amplitude and the effect on the aircraft can be different. But I haven't found any resource which explicitly points out the differences.

Any help or pointers to resources is appreciated.

Thanks!
Adams

Microburst2002
1st Aug 2011, 19:16
Good question

In my opinion, the main difference is the non-cyclic character of the Windshear. Gusts, on the contrary, are cyclic:gust-lull-gust-lull. In a windshear, however, there is before the WS and after the WS.

WS has an effect in the IAS of the airplane that lasts enough to make things difficult, specially if the IAS decreases. A gust is always transient, it passes quickly, probably shortly followed by a lull and another gust. IAS fluctuates up and down. Thay make things more difficult, too, when they are intense, but not in the way that a lasting loss of 30 or 40 precious knots can.

I hope I've been helpful

Mad (Flt) Scientist
1st Aug 2011, 20:57
I would have tended to agree with Microburst, whose name does suggest he ought to know something about this. ;-)

However, looking up the "Airplane Upset Recovery Training Aid Revision 2" it, at least, defines windshear as "Wind variations at low altitude". Interestingly it doesn't define "gust" at all. It then contradicts itself by later discussing "Windshear at the boundaries of the jet-stream" which clearly isn't a low altitude phenomenon!

AC120-50 specified "low altitude windshear" which allows for the possibility of it occurring elsewhere. AC00-54 is a bit more useful in defining windshear as "any rapid change in wind direction or velocity". There's an unspoken "sustained" in there somewhere...

westhawk
1st Aug 2011, 23:03
I would characterize "gusts" as a difference in measured wind velocity near the ground such as might be included in a METAR if the velocity exceeds 5 knots of difference with the average over the last minute or whatever the convention might be at the location where the observation is recorded.

"Windshear" could be defined in several ways depending upon the specific context in which the term is used:

1) A low level windshear warning may be issued at an airport if certain threshold values of wind velocity/direction/location differences are detected by airport LLWS detection systems. At so equipped airports in the US, the tower will generally issue the warning when the machine squawks and read off the recorded sensor differences. ( "North field wind 350/30G40, South field wind 270/20G30", etc.)

2) A windshear advisory may be issued due to a pilot report. ("A Falcon jet reported a 15 knot airspeed loss on short final for rwy 21 15 minutes ago.")

3) A GPWS windshear warning. The magic box compares data inputs according to it's programming and generates a warning. Reaction to such warnings vary according to circumstances and policy.

In a very real sense, windshears are the norm. It's just a matter of degree that we're concerned with. If the degree of shearing is considered to be great enough to constitute a threat according to established policy, a warning is issued.

Just as an aside:

Much of the impetus for the funding of such a warning capability as LLWS detection and the WS mode of GPWS can be traced to the DAL 191 crash in DFW. In that case, a microburst. It's one reason why they call the FAA "the tombstone agency!" Not fair perhaps, but then most major aviation legislation in the US can be traced back to singular events.

Mad (Flt) Scientist
2nd Aug 2011, 00:14
@westhawk

In addition to the METAR type of gust, there's also a whole section in Part 25 structural design about "gusts" which defines sudden airmass changes all over the speed/altitude envelope. Depends on what you're discussing - either can mean some specific low altitude concepts, or could apply to more widely defined atmospherics.

Graybeard
2nd Aug 2011, 05:34
For low level windshear, visualize a column of cold air dropping from a storm, and spreading like an inverted mushroom as it hits the ground.

As you fly into it on approach, for example, you get lift from increasing headwind, followed by a downburst, and then a tailwind.

Reactive windshear systems, such as in GPWS computers, sense the change in performance once you have entered the shear, and give guidance for escape.

Predictive (really, Forward Looking) windshear systems imbedded in the plane's weather radar use doppler shift detection to identify microbursts up to two miles ahead.

Escape or avoid.

GB

MD83FO
2nd Aug 2011, 06:39
In the middle east its very common to have temperature inversion windshear at night. Such as a wind difference of 30 knots at 500' and less than 10 kts on the surface. Without any gusts, turbulence Or clouds.

westhawk
2nd Aug 2011, 08:21
Mad (Flt) Scientist:

Yes, I seem to recall that "vertical gusts" or similar terminology is used in both part 25 and part 23 certification requirements. Unless I'm missing the mark, this is primarily concerning the establishment of airspeeds at which up and down drafts of a an arbitrarily established velocity (50 fps up and 30 fps down seems familiar?) will not cause an exceedence of structural load limitations. I also recall seeing some reference to gust factor application somewhere in the certification flight test guide, perhaps in reference to takeoff and landing performance flight tests?

Since these particular uses of the term "gust" are not normally encountered in day to day line flying, I omitted them from my earlier post. However I'll concede that knowing how certain flight limitations and recommended speeds published in the AFM were established is useful. If for no other reason, a pilot aware of certification requirements and testing methodology seems more likely to have a proper respect for limitations and recommendations included in the AFM.

westhawk
2nd Aug 2011, 08:41
Hey Graybeard!

I see you're from SoCal too. We don't have many microbursts around here, but we still get our share of windshear don't we? Nothing like Burbank or Ontario for a good time when the Santa Anas get spooled up. A bit ironic that the only place I ever had to hold for a windshear warning was at DFW. A big +CB right over the airport. Being familiar with the DAL 191 accident made the wait seem prudent and not even an inconvenience.

And thanks for bringing up the RADAR predictive feature. That function is yet another tool in the bag.

Prince Niccolo M
2nd Aug 2011, 09:18
I think that windshear is a steady phenomenon whereas gusts are unsteady - in other words I think they are distinguished by their time characteristics.

ICAO publish a Doc 9817 "Manual on Low level Wind Shear", which I do not have. :{

Microburst2002
2nd Aug 2011, 10:17
Agree

isent the post two times o I erase this one, please be so kind to read the following for more on WS.

Microburst2002
2nd Aug 2011, 10:18
Agree

gusts make it bumpy, you go in and out of eddies, updrafts, downdrafts... IAS will fluctuate. The airplane's energy state remains approximately the same.

WS make it scary, you pass from a headwind to a tailwind, (or the opposite, less scary but tricky) and in the new air mass you will stay. IAS will change, and only will recover with time, depending on the mass of the airplane and the airspeed change. In extreme cases, time is so long that the airplane will crash before it can regain climb performance. airplane's energy state changes (decreasing in the worst case, increasing in the tricky case).

The WS case has to do with inertia. airplanes "inertial speed" (if I am allowed to invent terms) is GS. If the wind speed changes suddenly from a 20 kt headwind to a 20 kt tailwind, GS will still be the same, initially, due to airplane's inertia, and then gradually be increased as the new tailwind airmass "carries" the airplane within it, accelerating it. therefore IAS will instantly be reduced in 40 kt and it should tend to gradually increase until reaching the old value. However this can be difficult since performance will be drastically degraded, L/D ratio will be reduced putting you inside the reverse command region, may be making you fly at the verge of stall and then you are facing a difficult decision: stall recovery (at the expense of precious height) or rate or minimising height loss (at the expense of reduced L/D ratio) while we wait till the WS is over and we cross our fingers

Graybeard
2nd Aug 2011, 14:50
NCAR, National Center for Atmospheric Research set up their first NextGen Wx radar, back in the 1970s, NE of Denver to study microbursts. One locale had far more microbursts than anywhere else within range. Guess where they put the new DIA airport?

GB