When can a 1 knot windshear lead to disaster?

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pdw
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Re: When can a 1 knot windshear lead to disaster?

#26 Post by pdw » Wed Oct 11, 2017 9:46 am

When can 1 knot of windshear lead to disaster?
The "windshear" can be stronger than it looks, and here too .. it's a lot more than "1 knot".

It's after the passing, where the backside of a shear is entered after an otherwise steadier groundspeed had already dipped slower solely by the amount of the original shear-strength, that IAS CAN bleed too much/too fast right after (the decreased performance as the knots of that lost-groundspeed must be regained in AS). Even "1 knot" too low / too little in regaining that bleed-loss quick-enough as it happens ... yes "can lead to disaster" ...
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lownslow
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Re: When can a 1 knot windshear lead to disaster?

#27 Post by lownslow » Wed Oct 11, 2017 4:49 pm

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pdw
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Re: When can a 1 knot windshear lead to disaster?

#28 Post by pdw » Wed Oct 18, 2017 11:05 am

I get it ... Who "cares" when you're L O W N S L O W
lownslow wrote: I've always been able to taxi all of my debris off the runway, after all...
I mean, when launching the perrouette off a wingtip there, it seems is not at all life-threatening (the reference / video on previous page) if only the bulb needs replacing on the strobe light ? ... then it's no biggie as long as home in time for dinner.

(Talking about the aerodynamics affecting this competitive incident where nobody gets hurt and isn't offending anyone.)
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Outcomes from High and Slow which have encountered difficulty like THAT (same as this competitive hazzard except from much higher above surface) often are harder/too-hard to talk about:

ie: A slighty bizzarre occurrance during a contact approach after dusk at Cysn Dec6-2016 /6:20pm, is mentionable for this. ( On occasion a shear-situation / potential- "disaster" might be setting up and not forsee-able in advance; and afterwards, there's no obvious gauging how many "knot" until record-searching and some math.)
I witnessed this event from the parking lot:
30kts/3000msl-SSE/SE was given on ADDS earlier and confirmed by crew later that GPS-GroundSpeed was 146kts at an IAS of 175KTS in approaching ~2300agl from North before intitiating a descent/turn right/West. The surface kts were picking up from the East, a serious negative shear lee of Niag-escarpment for ~recip 06 CYSN (now entering a surface downwind / 15kts ENE 6:15pm /Port Weller). Not only 30kt SE headwind (airspeed) lost in the right bank and descent, but also the "15kt" added to that loss, where max roar of engines is heard. (A dozen area/surface wx station histories available for confirmation.) 40-45 knots of decreased performance inside about 13-14sec (turn/descent segment) when only at descent power, an IAS bleedoff-rate about 3kts-per-second or 30kts in ten. Nosedown for speed here expects a usual rise in airspeed, but instead gives rapid bleedoff .. continuing into the"15kts" (a further/rapid loss of relativewind passing over "Port Weller" area) just about where that 90degree right-turn finished at approx circuit height (1300msl) and less.
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The shear might not even look that strong, if one/any is expected at all; ie: in these examples is stronger than thought / understood, which is possibly not so unusual where can-be / more-often helped by an airport's sourrounding terrain ?

In the opening example (initial post of this thread) it works out to probably about a 5kt-plus IPshear-action / 5sec windgust that drives the 'margin above stall' higher than the minimum (ie: the "1/2 knot" suggested there) adhered-to prior. The post-shear power requirement was not / or couldn't apparently be managed as needed to prevent airspeed dropping too far after the duration of the rightXwind/hdwnd gust has passed. Actually it is Post shear where the "1 knot" in ~1/5 of a second replacement is needed to recover adequate speed, so all of 5kts to be regained in a ~one-second period to remain in the competition. If power management restored just 4, then it's "1 knot" short for the necessary rate of replacement needed to remain on the nose-high course as the gust dissipates (back into the slower air). Hanging at that crazy pitch angle (video), the "1/2 knot above" estimate must be about right.
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