Wind shear
 

Can any of the fly-boys out there help me right with this one please:

When you report wind shear to the controller, you report it as either a gain or loss of airspeed and sometimes drift tendencies. So, what exactly do you mean by gaining speed or loosing speed because of wind shear? Take an example - you are established on a 10 nm final at 3000 ft above the aerodrome and down to 1500 ft, you have a headwind of say 50 knots, now al of a sudden as you pass the wind shearline at 1500 ft, the headwind component falls to 15 knots down to touchdown. As you hit 1500ft in your aircraft, what exactly happens ? I guess from my understanding you will overshoot the glidepath (go above it) and the normal reaction would be to reduce power to intercept the GP and you end up slow descending through and below the GP and land short? Is that correct? and my really big question is, what speed (Indicated , True or ground speed)do you lose or gain????
I cannot imagine it to be indicated because there is no change in dynamic pressure when you reach 1500ft, and yes I can see why your grounspeed change ( because of the headwind dropping off ) , but how can that situation change your Indicated and therefore True airspeed? The fact that there is only a change in wind speed and direction alone cannot change your TAS or IAS,can it???? IAS is obviously what you see corrected for instrument error only as far as I can remember and TAS is IAS corrected for temperature and density. So at 1500ft in my example, how does a sudden drop off in wind speed change your TAS or IAS??? I have no problem with the groundspeed bit, as your GS changes by the TAS being corrected for the new wind component, so no problem there. If you report wind shear then to me, do you mean that you either gained or lost groundspeed??? I can see why wind shear is a problem, basically an unstable approach either landing short or overshooting because of the power changes required etc. I guess it comes back to the age old question again of using either power or elevator to make attitude or speed changes on the approach. So what technique do you actually use if say in this case you gain some GS on short final ?? Dont touch the power??? or use elevators??? and by making no power adjustments you will probably gain speed again until you establish the GP again and take up a constant descent rate???

Hope you can help me out of my confusion here!
Thanks
Fart

At low levels, IAS=TAS (because the pressure at the low level has not fallen significantly from ISA, hence the two are roughly similar)

Soo..At 2000' the headwind is 50 kt. You are maintaining 150kt (say) so your groundspeed is 100kt. Now say the headwind drops to 30kt. The inertia of the aircraft is still the same, so you are still doing 100kt groundspeed, but your airspeed drops to 130kt. Solution POWER (sorry, thrust!) to recover back to your chosen speed. You may dip below the glideslope for a short mo as well.

This is the reason behind adding a few knots to Vref, so that when your speed drops, it should not drop below you minimum approach speed. This is why putting it on the ATIS helps, as we are able to plan for it.

CCCCCoooo...ugh

[This message has been edited by Cough (edited 18 May 2001).]

Wind loss:
Initially:
Groundspeed remains the same (inertia)
IAS drops
Rate of descent remains the same
Then:
Groundspeed drops due to IAS drop
IAS remains lower
Rate of descent increases due to groundspeed drop (and lift reduction due to the lower IAS), resulting in a dip below glideslope
Stabilizing:
IAS will return to initial value (same power setting)
Groundspeed will follow IAS
Rate of descent stabilizes again

This would be the situation if no power is added. Now because the airspeed loss and dip below glideslope can be dangerous, we're not gonna just sit there and look at it, we have to add power to minimize airspeed loss and a dip below GS. Then, once the stabilization phase sets in, reduce thrust to more or less the original position.

So fart, your assumption was incorrect that a loss of headwind will cause you to go above the GS.

If windshear is severe, a go-around should be performed since it may be impossible to get back in a stabilized approach config due to large thrust and elevator actions.

On the airbus the aircraft continiously monitors the actual ground speed and winds aloft, and makes comparisons to the pilot input of the runways surface wind.

The aircrafts autothrust system automatically keeps a safe speed for you in "mild/moderate" conditions, severe conditions (windshear warning) - go around.

I have previously talked about the airbus autothrust in more details in this post that was along similar lines http://www.pprune.org/ubb/NonCGI/For...ML/001938.html

:rolleyes:

Fart, you have described a 35Kt "negative" shear, i.e you just LOST 35kt of energy. At 1500aal you would probably have to go around with a shear that big. As Iz explains, you will end up with a high rate of descent close to the ground. Only if the windshear is "positive" i.e an increase in wind speed or energy will you balloon high above the profile. So perhaps thinking of the wind in terms of positive or negative energy (more headwind = more lift) will help your understanding of shear. Hope that helps

The more airflow that passes over a wing the more lift it creates (By creating a low pressure area above the wing), the less the airflow the less the lift. Imagine your doing 160kts IAS on approach (this includes any headwind, in this case 50Kts). If you suddenly lose 35kts of this headwind then you have lost 35kts of airflow over the wing. Remember less airflow = less lift, and you will go below the G/s.

and the Q of whether to use elevators or thrust when on approach?

When on a desired flight path, elevators fly the PATH, thrust flys the SPEED. This applies during cruise and approach.

Whereas when thrust is at the limits, elevators will then fly the SPEED. This applies during climb (thrust at 'max') or descend (thrust at 'idle').

HS

A 15 knot shear is defined as "Severe". A 35 knot undershoot shear at 1500' would probably result in a smoking hole in the ground.

Thanks everybody for helping out. I am beginning to understand the problem and hopefully next time I will be able to give you better advise when you encounter wind shear.
I think it would be nice for you guys out there to maybe better explain yourselfs to the controller next time you encounter wind shear, for instance you can say: " WE lost 15 knots IAS and went below the GP" for example.
I obviously dont expect you to do that when you are trying to land the aircraft, but maybe afterwards to the ground controller.
I again do appreciate the information and will make sure the controllers at our unit will do a better job in future.
Thanks
Fart

If the wind changes during the long period ,let's say 1 hr ,change of 50kt head wind to 15kts head wind during 1 hr is not a significant change it is very common in enroute flt. You don't have to compensate thrust for that wind change. But Windshear is a different story. Windshear is a sudden change of wind velocity or direction. Aircraft(mass) has an inertia.
Even though there is a sudden change of wind from head wind to tail wind that aircraft wants to maintain that condition like a stone fixed in the air. In that case tail wind will change the atmosphere around the aircraft. Head wind to tail wind. It will reduce the IAS suddenly. If you don't adjust thrust ias of the a/c will increase eventually. Reverse will happen if wind changes from tail to head wind.
It is a problem of inertia not the stabilized condition.
Hope this will be a help for your question.
See you around.
I had had that same question in my ppl period.

I have some concern with comments involving "sudden" changes.

One must recall that the ASI is a differential pressure gauge - no more - no less, and subject to a host of pneumatic and airflow "errors". In general, in rapidly varying conditions, the instrument tells a great many fibs in respect of airspeed as calibrated.

One should only draw conclusions based on mean changes over a (short, but reasonable) period of time. Thus, when the instrument is fluctuating wildly, the "real" airspeed value is not - the instrument's gyrations reflect instrument and system problems, not the actual airspeed.

All well and good mate but when it happens at 500´as is often the case here in LIS then one tends to believe the guage. Especially when the donks are spooling up like mad, the mdlc has fully retracted and the sink rate is heading for 1000´down.
Windshear will and has killed.
Thats why we like to know and why we tell ATC when it happens.

Fart if you have a flight sim at home, you can reproduce both under and over shoot shear and it works, just set up changing winds at various low alt.

FE Hoppy,

I have no dispute with your observation of what we do in the real world. But the fact remains that the instrument is a poor reflection of the reality. We are, to some extent, adrift on a sea of presumptions for which the only hard rule is "Don't crash"

Very true John.