Formula for wind shear

[pilot602]

Anybody know the formula to determine the amount (or speed) of windshear you're experiencing after you've identified an encounter? Does one exist? Can you take the 3-degree glideslope forumla (5 x groundspeed or groundspeed /2 + 0) and work backwards?

 

[FlyingNole]

Hey, Im sorry if this comes off as a smartass remark, but once you encounter windshear, does it really matter?

I hit some around 3000ft one day, and let me say, I was more concerned with flying the plane then sitting and figuring out how strong or weak the shear was,etc. When close enough to the ground, hearing the stall horn, and getting a 1500+fpm decent rate while at 100kts (in a 172 trying to keep level flight), there is no time or feasability in figuring that number out. Once we did realize we had just encountered windshear, it was gone...

My .02

 

[pilot602]

In real life .. nope. Do what it takes to keep the airplane doing what you want.


















For an interview .... well that's a different reason.

 

[asl]

On the other hand someone does figure it out, cos the pilots report it to the tower if they experience it on approach, and give it a value

 

[Silverhawk]

Thread Hijack..

Not to change the subject completely but I was out at KPIT watching planes land while studying over the weekend... there was a significant crosswind but nothing overly powerful. Anyhow you pretty much can sit at the end of the runway and watch the entire approach but there was a Midwest MD-?? who was really struggling. The plane was crabbed into the crosswind pretty good but was flopping around and looks like it got a bit too slow or hit some wind shear when the tail basically just looked like it stopped flying and sunk like a rock. I wish I would have gotten a quick video of it but hindsights 20/20.

Has anyone really ever had that happen to them before or does it sound usual when encountering these types of winds? I honestly don't ever think I've seen the tail end of a plane sink like that... they definately threw in some power quick but you could tell it was quite an unstable approach from the start !!

 

[EatSleepFly]

[ QUOTE ]
Anybody know the formula to determine the amount (or speed) of windshear you're experiencing after you've identified an encounter? Does one exist? Can you take the 3-degree glideslope forumla (5 x groundspeed or groundspeed /2 + 0) and work backwards?

[/ QUOTE ]

Have you never been in serious windshear?

That'd be the last thing on my mind. The first being ESCAPE, and get the hell outta there, even if it means bending the power lever forward and torching poor Pratt. I'd rather deadstick it down after burning up the engine escaping the windshear than get pile-driven into the ground by it.

I know, I know....you're looking for something for an interview. There isn't a formula to my knowledge, but even if there was and they asked it on an interview, my answer would still be what I just said above.

 

[Stone Cold]

602,

Are you talking about the +/- reports in the LLWS reports? If you're stabilized in the approach, and you see your airspeed increase or decrease, that is what you report, from my understanding. I don't think there's a formula for it.

Not sure if that's what you're looking for, but that's what I do if I see increases/decreases in airspeed on final that I know are LLWS.

TX

 

[FlyChicaga]

[ QUOTE ]
Anybody know the formula to determine the amount (or speed) of windshear you're experiencing after you've identified an encounter? Does one exist? Can you take the 3-degree glideslope forumla (5 x groundspeed or groundspeed /2 + 0) and work backwards?

[/ QUOTE ]

There is no real formula. I have read many articles and books that relate to windshear, and I've never seen anything mathematical.

However, there are some key points to windshear which may be covered in an interview, such as:
- If thunderstorms are nearby during approach, a positive windshear (speed increasing) should be handled with just as much regard as a negative (speed decreasing) encounter. This is because with microbursts, you will find yourself gaining quite some speed as you first encounter it... should you pull out power at this point to bring your approach speed back to bug, you will compound the situation should you hit the downburst. How you have a negative windshear with power out.
- During a positive windshear encounter (not CB), you should really consider cleaning up the aircraft. Your speed can quickly increase to the point where you will overspeed the gear and flaps.
- During a negative windshear encounter, use full thrust but do not reconfigure. Having the gear down and flaps down can significantly soften the blow should you strike ground.
- During gusty conditions, you will notice airspeed fluctuations, sometimes up to +/- 15 knots. Excessive flucutations or a positive/negative trend that is not reversing over the span of a second or two often means windshear.

Anyways, hope this might help with what you're looking for. Check out "Fly the Wing" and "Weather Flying" which both might give some good info.

 

[pilot602]

[ QUOTE ]
That'd be the last thing on my mind. The first being ESCAPE, and get the hell outta there, even if it means bending the power lever forward and torching poor Pratt. I'd rather deadstick it down after burning up the engine escaping the windshear than get pile-driven into the ground by it.


[/ QUOTE ]

I haven't found anything wither and have decided if I get the question it's going to be a "do whatever it takes to get out (i.e. "fly the aircraft")" kinda answer.

The gouge I've been studying ahs had this pop up once ... i.e. you coming down glidelope at 600fpm your VSI goes to 800fpm with a decrease in IAS -- what just happened (if AWOS/ATIS is calling calm)? Answer "windshear" (I know not much but that's what they're looking for) then how much? Not too concerned with it/about it (the question) just thought I'd throw it out incase someone knew.

edit: Thanks for the info Chicaga.

Back to books. If anyone wants to quiz me on stuff feel free to post away!

 

[pscraig]

I've heard about this formula and have my doubts. I just compare the ATIS winds to what the FMS is telling me, and compare them. Here at IAH the winds are usually 150 at 4-5, but over the marker the FMS may read 200 at 040, so somewhere along the line I expect windshear. I keep the engines spooled up and carry a few extra knots, knowing once I am through the shear everything wil stabilize.

On GA aircraft with a Garmin GPS, you can also configure it to tell you the winds at your current altitude.

 

[FlyChicaga]

[ QUOTE ]
I've heard about this formula and have my doubts. I just compare the ATIS winds to what the FMS is telling me, and compare them. Here at IAH the winds are usually 150 at 4-5, but over the marker the FMS may read 200 at 040, so somewhere along the line I expect windshear. I keep the engines spooled up and carry a few extra knots, knowing once I am through the shear everything wil stabilize.


[/ QUOTE ]

You'll find that probably 90% of the time the winds are 90 degrees different and 15-30 knots stronger at 2000 AGL than at 300 AGL. 300-500 AGL is the "breaking point" where you get to the surface winds. That's why I usually keep my speed at around Vapp + 15 on the ILS, and it will normally (depending on wind direction of course) ease itself down to Vapp by the time I reach 500 feet, without any major power adjustment.

I see a lot of people who try and stabilize at like 2000 AGL, when in reality at 500 AGL the winds might be totally different. Particularly in areas with ground interference, whether it be buildings, hills, mountains, whatever.

The worst I've ever seen is when the winds at 2000 were something like 150 at 68, then tower reported was 180 at 10 gusting to 30. Twas a bit bumpy on approach. I was upset I didn't bring my chocolate milk that day.