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Old 15th May 2007, 05:23   #1 (permalink)
Cha
 
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Recovery technique for windshear?

Hello. My note says, TOGA full power max pitch-up, fly on edge of stall speed.

Understand TOGA Max power part but I don't understand "fly on edge of stall speed" part. Why? if fly on the edge of stall speed, even a slight loss of headwind due to windshear can stall the aircraft I think.

Can somebody explain why "fly on edge of stall speed" please?
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Old 15th May 2007, 05:35   #2 (permalink)
 
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Before any pull-up I would be all for trading any excess terrain clearance for air speed.
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Old 15th May 2007, 08:24   #3 (permalink)
I REALLY SHOULDN'T BE HERE
 
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Windshear technique will vary from type to type but in general terms wings will produce the maximum lift at the angle just below stalling angle.
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Old 15th May 2007, 11:11   #4 (permalink)
 
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TOGA
Keep the configuration you've got
PITCH UP but respect the stick shaker/pusher if fitted.

Thank your lucky stars and do the paperwork.
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Old 15th May 2007, 11:25   #5 (permalink)
 
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Quote:
Can somebody explain why "fly on edge of stall speed" please?
Lift increases as the Angle of Attack in increased .. up to the point of stall!
Maximum Lift occurs just as you approach the stall.

So you increase the pitch attitude until you get initial buffet, or the onset of the stick-shaker, then just ease off the back pressure ever so slightly, and hold it there!

The aim is to maximise lift and avoid ground contact. If you stall the thing during a windshear escape manoeuvre, it just wasn't meant to be!
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Old 15th May 2007, 14:07   #6 (permalink)
 
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Gentlemen (and ladies, if appropriate)
Quote:
Originally Posted by Nubboy
TOGA
Keep the configuration you've got
PITCH UP but respect the stick shaker/pusher if fitted.
Thank your lucky stars and do the paperwork.
While I don't necessarily quarrel with anything said here, I know there have been discussions about "keeping the configuration." Generally, that's probably true - particularly if an encounter is somewhere on final approach. But, and I'm being very careful here, if encountered immediately after takeoff, AND your takeoff flap setting was anything less than the maximum allowable takeoff flap setting - you might want to consider extending flaps to the maximum authorized for takeoff.

During the original discussions with Boeing on this issue, the demonstrations using a B737 simulator showed the "no change of configuration" was appropriate during landing encounters. But, with takeoffs at 5 flaps (B737-200), a greater climb capability was demonstrated by selecting 15 flaps ... and it was achieved without regard to the person flying as all present wanted an opportunity to "give it a try." Yes, there would be additional drag, but the increase in lift more than offsets that drag increase, given the relatively significant increase in climb capability in each case recorded. In fact, I think Old Smokey once alluded to this particular issue in a thread that is now probably over a year old.

Offered just for your consideration.
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Old 15th May 2007, 17:30   #7 (permalink)
 
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Read your manouvres section of the QRH
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Old 15th May 2007, 18:01   #8 (permalink)
 
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For a successful example of how to fly the aircraft accurately, see Windshear Encounter.
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Old 15th May 2007, 20:34   #9 (permalink)
 
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Careful guys, max lift is not achieved just before stall. The "max coefficient of lift" is reached but lift is also a factor of airspeed. Just remember the formula is CL*1/2 rho v sq. It is a dangerous thinking process to believe that lift is at a maximum. Lift is already in decline and drag is increasing dramatically. The one gain is that the thrust vector with high angles of attack is assisting the recovery with bootloads of downward component.

If it were solely to gain the best climb performance to outweigh the degradation, surely a speed closer to the best rate of climb (the speed where the excess of thrust available is at a maximum)i.e. an equivalent improved climb V2 speed depending on config, would be more advantageous. So in my opinion it is a trade-off (as in the final stages of a GPWS encounter) such that if ground contact eventually becomes unavoidable, the slowest possible groundspeed will be achieved thereby minimising the deceleration forces encountered by landing in the high street.

As I say just my thoughts only. I hope I never have to test my theory anyway.
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Old 15th May 2007, 21:06   #10 (permalink)
 
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flying with intermittent stick shaker is only suggested when impact with the ground is likely.

i miss the airbus.
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Old 15th May 2007, 21:37   #11 (permalink)
 
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You don't mention which aeroplane this is for, so at risk of leading this thread off at a tangent......
For the B737 the windshear escape manuoevre is TOGA, keep the configuration you are in, and pitch up to 15 (Which approximates to Vy IIRC)
For the GPWS avoiding ground contact manuoevre it's as above except you pitch up to 20 (Which approximates to Vx)
I would be surprised if your type asks you to go to the edge of the stall as you will have no protection in the event of any gusts.
Quote:
i miss the airbus.
If I was in windshear so would I
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Old 15th May 2007, 23:47   #12 (permalink)
 
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"surely a speed closer to the best rate of climb (the speed where the excess of thrust available is at a maximum)"

You really need to be careful that you don't confuse the most fundamental of differences between best rate and best angle.

In a GPWS encounter, you want best angle of climb without any question. The maximum "up" for the minimum distance.

For a windshear encounter, I would be generally inclined towards Stick Flyings idea. Best rate would be much better as it will give you the best protection against a downdraft.

BUT!!

At the kind of speeds that you are going to encounter windshear, best rate is not an option. You are way too slow already. Best rate of climb speed in a jet is significantly higher than best angle. The most suitable option remaining is to trade any excess speed over and above best angle of climb speed and turn it (albeit it briefly) into improved climb performance. This way you maximise the short term capability of the aircraft to outclimb the shear or downdraft even though it's long term climb rate may have been adversely affected. If you can not trade enough airspeed for an improved climb, well it's just not your day.

The majority of machinery I have seen has generally the same principle. Trade airspeed for climb ability all the way down to the minimum safe airspeed which is usually to stick shaker or Alpha Floor.

Remember, your immediate concerns are short term.
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Old 16th May 2007, 02:50   #13 (permalink)
 
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Quote:
If it were solely to gain the best climb performance to outweigh the degradation, surely a speed closer to the best rate of climb (the speed where the excess of thrust available is at a maximum)i.e. an equivalent improved climb V2 speed depending on config, would be more advantageous.
That MIGHT be true if you were in steady-state conditions. HOWEVER, wind shear is NOT a steady-state condition!

What you need in windshear is ALTITUDE! If you are above Vx, Vy, or V2 (whichever is stuck in your mind right now), you want to trade any excess kinetic energy (airspeed) for ALTITUDE! Pulling aggressively to the stick shaker/"edge of stall"/CLmax is the way to do that IMMEDIATELY!

Generally, you will not react to a windshear that INCREASES airspeed with anything other than a power reduction, unless you have anticipated it and recognize it as it is happening. So, when the airspeed REDUCTION happens (or if the reduction happens BEFORE any increase), you are already in a disadvantaged condition.

Don't second-guess your FHB procedures in this situation. REACT -- NOW!
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Old 16th May 2007, 03:29   #14 (permalink)
Cha
 
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Thanks

Hi Everyone. Thanks for your posts.

My theory note says that "fly on the edge of stall" and the note is not talking about any particular aircraft type.

Lift will increase at hight A.O.A. but then decrease as soon as the angle exceeds the critical A.O.A.

I don't think "fly on the edge of stall" is right in any aircraft type.
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Old 16th May 2007, 09:26   #15 (permalink)
 
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Cha,

What your material might mean when it says 'fly on the edge of stall' is that you should reduce the airpseed as much as possible. This in essence is perfectly accurate. You must trade as much airspeed as possible for improved climb capability.

You are still confusing the long and short term scenarios I think. Whatever you speed is upon encountering windshear, your target airspeed immediately becomes the speed just above stall speed. The only way to achieve this target is by pulling back aggresively, thereby improving your climb performance dramaticaly for the next few seconds at least. Modern aircraft have stick shakers, etc to advise you that you are getting very close to the stall and are good indicators of when you have reached the limit of your speed/altitude trade.

If you are still encountering windshear at this stage, God has decided that this will be your last flight and there is no other technique that would have saved you.

Remember, that windshear (in the negative sense, i.e. suddenly reduced airspeed) is almost always a sudden, once off encounter. It is very rare and highly unlikely that you would suffer a severe shear which reduces your airspeed from say 160 knots to 140 knots and then have that followed a few seconds later by another drop from 140 knots to 120 knots. If you do come across this scenario, make a quick transmission to ATC to ask them to tell your wife that you love her.


The danger that exists is that during a windhsear encounter, your descent rate during an approach suddenly becomes dangerously high with risk of ground contact prior to the runway threshold, or your climb rate during takeoff suddenly becomes a descent rate.

It is obvious I'm sure that attempting to accelerate to better long term climb speed is not an option as this will involve a further unacceptable increase in your descent rate.

If you try to maintain the speed that you have been left with after the shear, you will have to wait quite a while (a good few seconds at least) before the effect of full power changes your flight path from a descending one to a climbing one. Just imagine being on an approach at 160 knots with 50% N1 descending at 700 fpm. Suddenly you speed drops to 145 knots and your rate of descent increases to 2000 fpm, so you apply full power and try to maintain the same speed. Now, imagine how long it will take for your 2000 fpm descent to be reduced, stopped and then changed to a climb. It could well be too late!

If you pull back aggressively and reduce your airspeed all the way to stick shaker, you will immediately limit your descent rate as much as possible and may even get yourself climbing. (This would still be the case even if you did not add any power at all, albeit short lived.) By the time you have reached stick shaker, you will have achieved full takeoff power. Now you are in the same position as in the above paragraph (steady speed and full power) but you are starting from a shallow descent or possibly even a shallow climb. The effect of full power will be felt instantly and if He allows it, you will climb away for another day.

Stick shaker speed is clearly not the most efficient long term climb speed, but your immediate concerns are only short-lived.

Make sense??? That's the way it was explained to me anyway!!
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Old 16th May 2007, 10:47   #16 (permalink)
 
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I've had numerous 'discusions' in the flight deck about the sensible option (airmanship) of carrying considerable speed to the threshold - most a/c types ops manuals allow max 20Kts on Vref which is in itself a reasonable buffer against some of the severe windshear .....

.. I'd rather float and land long down a 3500m runway than risk dropping out the sky on finals!
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Old 16th May 2007, 13:22   #17 (permalink)
 
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Quote:
Remember, that windshear (in the negative sense, i.e. suddenly reduced airspeed) is almost always a sudden, once off encounter. It is very rare and highly unlikely that you would suffer a severe shear which reduces your airspeed from say 160 knots to 140 knots and then have that followed a few seconds later by another drop from 140 knots to 120 knots.
I wouldn't be quite so sure about that... I remember doing a windshear exercise in the sim using a *real* recorded windshear event: There were two or three really nasty bits of shear, as well as considerable rolling moments. From start to finish, the loss of airspeed was >100kts as you went from flying into the outflow from a chain of microbursts to having the outflow behind you. Very sobering.

We has another go, letting the A/P fly it (very impressive) and with the sim set CAVOK, so you could see the ground/buildings/trees rushing past... This has made me even more determined to *avoid* the stuff at any cost rather than fly through it.

Quote:
I've had numerous 'discusions' in the flight deck about the sensible option (airmanship) of carrying considerable speed to the threshold - most a/c types ops manuals allow max 20Kts on Vref which is in itself a reasonable buffer against some of the severe windshear .....

.. I'd rather float and land long down a 3500m runway than risk dropping out the sky on finals!
Reference my previous paragraph, 'severe windshear' is something you really shouldn't be exposing your aircraft to, IMHO.

I think the term 'windshear' is used when often people really mean 'wind gradient' or 'low-level turbulence'. I would contend that you don't want a very large addition to your airspeed over the threshold, you want it further back in the approach, bleeding gradually off to touchdown. I think the FBW Airbuses have this protection? (GS min?). There are many more incidences of aircraft going off the end of runways than there are of them burying themselves in the approach lighting (AF at YYZ, SWA at Chicago etc.)

Looking at it from a practical POV: If it's gusty on the approach, say 30G50 with lulls to 15kts, the maximum loss of airspeed will be 35kts. For an aircraft with a nominal Vref of 120kts @1.3Vs using Vref+10 = 130kts, you have a margin of 38kts. This is the worst possible case for a *momentary* reduction in airspeed. You also have the engines available to counteract this loss, so the chances of actually stalling the aircraft are very slim. If you land deep into a wet runway at Vref+25 the 'plane will go quite a way before you come to a halt, maybe beyond the end... I'm not a great believer in extra airspeed "for Mum" as I think you are just trading one thing for something else, potentially just as dangerous. If it's really that bad: go somewhere else.

Last edited by FullWings; 16th May 2007 at 14:21.
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Old 16th May 2007, 14:51   #18 (permalink)
 
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Fullwings,

I agree extreme windshear is a possibility. I said it was very rare, not impossible.

The >100kts shear in the sim you mention is relating to a microbusrt. A phenomenon which, it is generally accepted, will create conditions outside the capability of most if not all aircraft. Hence the advice to avoid them entirely. There isn't much point in including them in this discussion as there really isn't any decent way out. If you are at final approach speed and you encounter >100kt shear, you're dead, and quickly. (I accept that the 100 knot reduction comes just after a significant increase in airspeed as you enter the outflow, so you are unlikely to be at final approach speed)

In my mind and for my posts, I have considered windshear to be a sudden and lasting change in wind direction or speed which results in a sustained loss or gain of airspeed. Low level turbulence does not fall into that category.

Anyhoo, very interesting thread so far!! Keep it up everyone!
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Old 16th May 2007, 15:38   #19 (permalink)
 
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I 100% agree - avoid it at all cost.

The trouble is how many times do you hear on the ATIS 'windshear has been reported' ? 99% of the time its just wind gradient and gusts, what any one with an ounce of airmanship would expect in gusty conditions and is typically harmless if reacted to with thrust ...

Most of the 'at risk airports' now days have elaborate windshear monitoring equipment .. ie. If I heard 'windshear has been reported' going into say HKG I would be questioning ATC of the exact details and consider the situation carefully before commencing an approach .....

Re. a/c types - I've flown FBW Airbus and Boeing ... Its one of the few times I would prefer the Airbus high alpha protection, I know if I pull the stick right back to the stops its giving me the best rate of climb without stalling ..... The PLI's on the modern Beoengs is of dubious value in these scenarios ...
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Old 16th May 2007, 19:55   #20 (permalink)
 
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An Paddy Eile,

Quote:
...There isn't much point in including them [microbursts] in this discussion as there really isn't any decent way out.
I'm not trying to pick an argument as your posts have been making eminent sense - but I would say that most airline training for "windshear" is based on these sort of encounters. Obviously, you are looking to avoid this scenario if at all possible but if you are in the wrong place at the wrong time, early recognition and prompt decisive action just might save the day. *Any* way out is acceptable at that point. Modern aircraft, twins especially, have an incredible power-to-weight ratio when at normal landing weights and might survive an encounter which would have brought down something from an earlier generation.

As for surviving a 100kt negative shear, if it happened just like *THAT* I agree it's not looking good. In real life it takes a little bit longer and if you're at TOGA power trading speed for altitude or vice-versa, you are adding energy into the equation at quite a rate. You may be forced to descend to convert some potential energy to kinetic, even bounce off the ground (runway, hopefully) but every second on firewall power increases the chances of survival dramatically.

Triggering the GPWS/EGPWS on a gusty day can be more of a nuisance; my operator's SOPs allow you to disregard these warnings under specific (pre-briefed) circumstances. Recovering from a full-blown windshear go-around can be almost as problematic as what caused it in the first place...

Anyway, good discussion, as you say.

Last edited by FullWings; 16th May 2007 at 20:13.
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