Wind and the TAF

I just read this TAF for Watertown International Airport (KART):

KART 	121738Z 121818 19008KT P6SM SKC
FM0600 17006KT P6SM SCT250 WS015/23035KT
FM1400 19012KT P6SM BKN250 WS015/23045KT

The tricky parts are the phrases “WS015/23035KT” and “WS015/23045KT” — those might be common out in the prairies, but I don’t see that kind of thing often in TAFs around the Great Lakes. The “WS” stands for “wind shear”. The following number is the altitude of the shear layer above ground level (1,500 feet in both cases), followed by the wind direction and speed at that altitude.

So starting at 06:00z tomorrow morning (that’s 01:00 EST), the wind will be from 170 degrees true at 6 knots on the ground, but from 230 degrees true at 35 knots just 1,500 feet up; from 14:00z (09:00 EST), the wind will be from 190 at 12 knots on the ground, but from 230 at 45 knots 1,500 feet up.

What does that mean, practically speaking? As you approach to land on runway 25 at 10:00 am local time tomorrow morning, you’ll be facing a headwind of 43 knots until 1,500 feet AGL, at which point the headwind will drop abruptly to about 8 knots — that means that your airspeed will suddenly drop by 35 knots as you descend through the shear layer, until your plane has time to reestablish its trimmed airspeed. If you’re approaching at 80 knots calibrated airspeed, you’ll suddenly find yourself at 45 knots with your nose swinging hard towards the ground trying to make up the missing speed (you’ll probably also be in moderate-to-severe turbulence). In a light aircraft, you may have room to recover at 1,500 feet; in something heavier, like a commuter turboprop, I’m not so sure.

When you take off from runway 25, exactly the opposite will happen. As you climb through the shear layer (and turbulence), your airspeed will suddenly increase by 35 knots, and the nose will shoot up to the sky to try to regain the plane’s trimmed airspeed. For a brief time, the climb rate will be spectacular, but you’ll have to make sure that you get the nose down before the extra speed decays on you and leaves you nose-high and slow.

The exact effect will depend on how thin the transition layer is and how fast the plane is descending or climbing. A slow descent or climb, or a thicker transition layer, will give more time for a gradual adjustment.

Anyone for some touch-and-goes at Watertown tomorrow?

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4 Responses to Wind and the TAF

  1. phil says:

    very informative blog.

    btw – I like the idea of cruising LOP (wide open throttle style), but the 172 does not have enough temp guages to assure me.

  2. david says:

    Thanks for the comment, phil.

    The Warrior has the same problem with lack of temperature instrumentation: just oil temp and EGT (single cylinder). Unless you always fly full rich, though, that’s a problem no matter where you put the mixture. The normal ROP lean setting is not too far about peak cylinder head temperatures, so you’re even more likely to have a hot cylinder there than you are flying LOP-WOT.

  3. Pingback: Rants and Revelations » I feel better about that decision

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