Wind shear

[Toby]

I went to the local FSDO seminar last night, and the topic was spring and summer flying. One of the areas the SPM covered was wind shear. His advice, for someone coming in for a landing and encountering wind shear near the ground that results in a drastic drop in airspeed and possibly a tailwind, was to fly at Vx while doing a go-around. His thought was that it is better to go at Vx than Vy in that situation, because you want to gain the most altitude in the least distance in order to get out of the wind shear level.

I had an experience with wind shear one day last summer when I was taking off. I don't remember anymore how far above the ground I was, but it wasn't far -- maybe 100 feet? If this is an important thing to know, I'll go back and look in my saved posts. But I had a tailwind, and the airspeed dropped to zero, and then swung back and forth. I was climbing, of course. My instinct was to level the pitch a little and climb more slowly. I wanted to get through the shear level, but I didn't want to stall, so I let myself climb more slowly.

This is the opposite of what I heard last night. Do any of you have comments or experience with this?

 

[John J]

Toby;

Your instinct to "level the pitch and climb more slowly" worked very well in your given situation. It sounds that you had an "increasing tail wind" componet. wind shear close to the ground encounters involve so many complexities, terrain fetures, ie, trees, hills,buildings etc. If I knowor encounter an increasing tailwind componet I keep the speed at vy. The reason is to maintain an "Angle of Attack " that will not allow the plane to stall. With increasing tail wind componet to maintain a constant angle of attack i have to lower the nose to maintain the speed. My plane does not have the power to "pull it through' the shear so I fly at vy or if it is really bad even faster for I want to keep the angle of attack constant and not stall the airplane. Wind shear can be a very scary and to this day I remind myself not to pull the nose up too quick for loss of airspeed on a go-around can make a simple proceedure become very scary. Your instint to lower the nose of the plane,( and angle of attack) made your go-around work out well and that is the most important thing.

John J

 

[RotaryWingBob]

I'm pretty much with you on this Toby -- I'd rather carry the additional airspeed. It seems to me that they were talking about a very specific form of wind shear (ie. one you could climb out of). My experience is that some of the nastier shears are caused by gusts at low levels -- I don't see how a Vx climb is useful under that circumstance.

The worst I've encountered was an attempted landing at Williamsport, PA. The tower reported gusts to 32 kt. I gave it up after my turn from base to final when my ASI was all over the map, recovered with a normal go around, and said adios, we'll try this again another day... I had a CFI friend on board who made no comments about my technique.

Hmmm. I just thought about what I did -- normal go around in a Cherokee is to go to full throttle, flaps up to 25 degrees, Vx climb, flaps to 10 degrees, Vy climb. So I guess what they said is appropriate for a go around anyway.

 

[grattonja]

Thinking about your post, I agree with both what you did and what they are recommending.

In a tailwind situation, no way would I want to be as close to stall speed as Vx gives you. I would want a slightly flatter climb. Ditto a really gusty situation where you can easily have wind disappear at the wrong time or shear to a really strong tailwind and take away your lift 100 or 200 feet off the ground.

But... You want to get as much distance between you and terra firma as possibly quickly on a go-around, and that sounds like what they are basically teaching.

So... Seems to me that bottom line is, you gotta call it as you see it in the left seat. You did the right thing for your circumstances, keeping it at Vy with a tailwind shear. A shearing headwind to crosswind might call for Vx to get the heck out of dodge.

Jim G

 

[Toby]

Yes, I can see it both ways, too. I also know that he was talking about approaching to land, not taking off. Maybe if you come down into the shear level, and you're still very close to the top of it, you want to get the heck out of it as quickly as possible. Maybe if the shear is coming from the side you can do Vx and accomplish that. You also have power to add in that situation, since presumably you've throttled back.

Taking off would be different, because you have all the power in (no leeway) and you're pitched up in a climb.

Thanks for helping me think it out!

 

[gismo]

Vx is going to give you the best chance of flying out of the shear, but I just can't imagine any situation other than a one way runway where climbing at Vy wouldn't also work just fine in the kind of planes we fly. This has little or nothing to do with power, rather it's the ratio of weight to drag which is much lower on a Skyhawk than a Learjet (which might need Vx or slower to make it). With high drag and low weight, your airplane will equalize thrust and drag fairly close within a few seconds of encountering a shear and once that happens you can climb just fine at Vy or Vx. In something slipperier and heavier equlibrium may not be reached for tens of seconds or more and during that time you really want to be at minimum sink speed which is usually between Vx and Vs so Vx might actually be too fast.

 

[fgcason]

Yes, I can see it both ways, too. I also know that he was talking about approaching to land, not taking off. Maybe if you come down into the shear level, and you're still very close to the top of it, you want to get the heck out of it as quickly as possible. Maybe if the shear is coming from the side you can do Vx and accomplish that. You also have power to add in that situation, since presumably you've throttled back.

Just a thought to consider: Depending on the conditions and what you're experiencing, once you're below the shear level, you might reconsider climbing back up through that level again. Sometimes it might be better to regain the energy and stay down there and put it on the runway. Excluding obvious grevious bodily harm, I'm quite sure I wouldn't go back up through a known shear level until I had the plane under control again. Behind the plane and scrambling for Vx and simultaneously adding another round of questionable undefined shear doesn't sound overly safe IMO. YMMV wildly so act accordingly.

Both times I hit significant windshear on final, climbing back out of it wasn't much of an option.
The first time was in the Cherokee 180 coming in to land on the farm in the Catskills. There were some entertaining winds coming over the mountain that I was already weary of. I just cleared the 120ish ft trees at the end of the runway and suddenly got pushed down hard enough to lift charts and everything else not tied down to the roof. Full throttle Vx and ride it down cause there wasn't much else to do except wait and not run into anything in the meantime. Pucker factor was off scale high..then I entered ground effect...
The second time was in a CE172 when the COS area was experiencing a low level jet type situation with significant wind change from the SFC(10kts and turbulent) vs 2000AGL(glass smooth 35-40kts). I just turned final around 400AGL@75kts. (and yes, that's extremely way seriously too fast for generic final but that was the shear safety buffer for the day) There were a few good solid unfriendly bumps and the world suddenly slowed down way too much. Instinct was full throttle and get the nose down. I distinctly recall a windscreen full of ground and the ASI accelerating through 55 kts. Climbing back up into that didn't strike me as a particularly brilliant plan...

Taking off would be different, because you have all the power in (no leeway) and you're pitched up in a climb.

That's where I drop the nose to level and use whatever ground clearance I have to my advantage to gain energy and continue. Level acceleration is the plan in general but I'm not beyond trading altitude for energy. It would be real silly to stall at 200AGL when I could descend to 50AGL and continue safely. IMHO: Empty air at anything AGL not involving stationary objects is fair game if required.

Thought before action...if one has time.

Safe flying...