Wednesday, November 30, 2011

Wind shear

I was reminded about the subtlety of wind shear during an IFR proficiency flight last night. We tend to think of wind shear as the evil microburst that brought down a Delta L-1011, but that is an extreme.

I did an ILS to runway 21 with reported surface wind 050 at 5. The POH says the airplane can stop in the runway available with that tailwind, but other traffic meant that I was only going to do a low approach.

The winds aloft were calm or light (based on GPS groundspeed), but at about 300' AGL I started to lose the glideslope, which is (I like to think) unlike me, and if nothing else embarassing when my friends see it. I tried to adjust the descent rate to catch the glideslope (I got to about 1/2 deflection) but reached Decision Altitude before the glideslope needle centered again. OK, a decent but not great approach.

We went around as planned and entered on left base for runway 3 behind the traffic, and I finally figured it out: I was crabbing to the right while flying a square left base; the wind above 300' was from the southwest, but the surface wind was from the northeast.

Wind shear!

So my ILS problem was the classic response when a headwind shears to a tailwind: I got above the glideslope. Just like the books say.

The shear was weak, not strong, so not enough to keep from completing the approach. In fact, the shear was so weak that I wouldn't have been looking for it even if I had been looking for it! But its effects were measurable, though not dangerous. So there's no point in beating myself for not noticing it.

Yeah, right.



At November 30, 2011 at 11:20 AM , Blogger Mike said...

Nice analysis. Mike


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