Negative windshear!

[Olendirk]

Hey!Could somebody explain me:

1.What a negative windshear is?And what happens to the TAS and the GS, what actions do we have to do in a approach, and how can I detect them ?

2. The same fpr the positive windshear!

Thanks!

OD

PS: Denmark is nice, come and visit !:ok:

[Piltdown Man]

Positive windsheer is an increase in IAS (and GS), generally caused by increasing tailwind with reducing height or by flying through rising air. Negative windsheer the opposite and seemingly the most common, most often reported by Nigel in his 757. Tell-tale signs of windsheer - unusual power settings on approach; from a given pitch (or attitude) and power setting a changing IAS or a given power setting and constant pitch a changing attitude. Together with all of the above, vertical speed may not correspond with what you would normally expect. I'm sure that there are bits in books on this... Hope this helps.

[Stoney X]

I don't agree with Piltdown Man on this one. It is my understanding that when a positive windshear is experienced your IAS increases, which is an increased headwind, not tailwind.

Imagine an aircraft on an approach flying down the glideslope. If an increased headwind is experienced, a positive windshear, the IAS increases. Initially the GS remains constant due to inertia. Increased airspeed, increased lift, and the rate of descend decreases. But the pilot allows to the aircraft to correct itself by not changing power or trim so the IAS gradually decreases back to the original value and the GS also decreases. Net result is same IAS, decreased GS and aircraft above glideslope. If a decreased headwind (or increased tailwind) is experienced, a negative windshear, then initially IAS decreases, aircraft slows and descends at a greater rate, with the same initial GS. IAS will then start to increase back to the original value. Net result is same IAS, increased GS and the aircraft below the glideslope.

So to answer the question, a sudden decrease in IAS is an indication of negative windshear and visa versa. As for corrective action that depends. Back to the glideslope, aircraft experiences positive windshear, so the pilot pushes the nose down and pulls the power a bit to maintain glideslope. As the aircraft starts to respond it passes out of the windshear back into the original headwind (or even a tailwind, i.e. a microburst) and the pilot now has the aircraft losing speed and height and he's just pulled the power. Not good. Hopefully a more experienced pilot than I will give the correct actions to take.

Regards
Stoney X

[OneIn60rule]

I'll have to agree with Stoney on this.

Positive Wshear is a Headwind because you will now get a LOT more lift and it will also increase the IAS.


Negative Wshear is basically a tailwind, you lose a lot of lift and IAS decreases.


If you start losing lift, raise nose, increase power.

If you start to gain lift, decrease power, pitch down.

I'm not going to say that I'm 100% on this, been a while since I did pof/perf.

Things I am certain of: Negative Wshear= Tailwind
Positive= Headwind.

[Olof]

I've heard that when being faced with a (negative) WS on final you should initiate a goaround with a pitch up just below the stick shaker (no, not on the cessna :E ). Anyone who knows more about the actual procedures that are to be applied when you're exposed to low level WS (i.e landing config on final) ?

[Olendirk]

Thanks for the answers!

[moggiee]

I've heard that when being faced with a (negative) WS on final you should initiate a goaround with a pitch up just below the stick shaker (no, not on the cessna ). Anyone who knows more about the actual procedures that are to be applied when you're exposed to low level WS (i.e landing config on final) ?

same!

[Piltdown Man]

I have done my reading and yes, I have it arse about face. Positive Windshear is performance decreasing shear, therefore Negative Windshear increases your performance. My guides to recognition however, remain valid. But what I really should have made clear is that air movements should really be viewed as a three dimensional phenomena.

Olof - I can't agree with you, it just depends.

The techniques for an escape manoeuvre are generally apply full power (which often means full power/thrust lever movement to give more than Take Off power) set flaps to Go-around and pull to half a nano-tadge before the stick shaker.

OneIn60rule - I think you are nearly right.


Either way, windshear is unpleasant stuff and should really be avoided.

[Pilot Pete]

Windshear is a sudden change in direction and/ or speed of the wind between two 'layers' (think as you descend towards an airfield you are descending through lots of horizontal 'layers' of wind).

Those above now have it the right way round, positive is where there is a sudden increase in the headwind component, negative is when there is a sudden decrease in the headwind component. I would dispute the claim that negative is 'tailwind' though, as the way I understand it it is just a loss of headwind component (could still be a headwind, just suddenly not as strong).

Positive shear in a big jet will have you removing a handful of power to try to maintain your airspeed and the glideslope. It will vary depending on exactly how far down the approach you are; if you are still at 2500' and established on the glide you can pull a bit more power off than if you are lower on the approach (remember big fan engines take a while to spool up so if you need to go around you don't want the levers shut before you start on a 1/2nm final!!) The key is to anticipate and take quick corrective action which will limit the divergence.

The negative shear will have you adding a handful of power briskly and lowering the nose as this is potentially more serious (remember we fly a bug speed for the flap setting, so if you were on this speed and then experienced negative shear your airspeed is going to drop below your reference speed and start eating into your stall margin protection. Also remember that if the aircraft is slowing you may have several hundred tons of 'big jet' inertia wanting to keep decellerating, so you will need a 'handful', not only to stop the decelleration, but to accellerate that bulk back to the bug speed for the flap setting.) All through this you are using the pitch to maintain the glideslope and the power to get/ maintain the speed. The pitch changes are again going to be working against you (pitch up to generate the required lift from a 'slower' wing) which means you need more thrust again! It comes pretty natural to do it, the anticipation is the key; don't let it diverge too much and the corrective actions will be smaller.

If you are unlucky (!) enough to fly a jet with underslung engines, each power change creates a pitching moment which you again may have to overcome depending on the shear.

Windshear knowledge and local knowledge assist in the anticipation. For instance Tenerife South is a classic when landing on 08 with a windspeed of over 20kts. You get a positive shear of about 10-15kts (which will get the inexperienced pulling the levers right back! I know, I've done it and got my knuckles verbally slapped!) followed by negative shear a little further down the approach. Point and Power works very well with fairly aggressive movements on the thrust levers to 'nail' the speed where you want it and 'positive' control inputs on the column to point the machine where you want it to go (maintain the glide).


A windshear go around (flown if you are not stable or want to discontinue the approach in windshear conditions) is different to a normal go around in that we do not alter the config of the aircraft during the initial go around actions. This is so as not to endager the aircraft by way of compounding a problem by putting some flap away, or getting the gear up early only to be 'dumped' on by a downdraft (or negative shear) and end up touching the ground in the go around! The standard call would be "Windshear Go-Around" The PF pushes the switches and rotates the aircraft to the go around attitude as the power increases, but the PNF does not alter the flap setting (normally we would go from landing flap 30 to flap 20 at the start of the go around to remove some drag) and puts the gear up on the "positive rate/ climb" call. This stays down as well until you are out of the windshear (remember you may get one then t'other as you climb away) and then we would complete the gear and flap retraction.

Hope this helps.

PP

[B727-200]

Pilot Pete

Please check your PM's

Many thanks,

B727-200