A320 Stall Recovery

[Haroon]

What is the reason of delaying the slats extension so much. Isn't earlier the better? Is it to avoid its pitch up side effect or something else? Thanks.

[Uplinker]

Originally Posted by Haroon

........What is the reason of delaying the slats extension so much. Isn't earlier the better? Is it to avoid its pitch up side effect or something else? Thanks.

The primary objective is to immediately reduce the AoA to get out of the stall, which one should do instinctively by immediately pitching down. Your question almost implies that it would be acceptable to hold the AoA and just wait for the the slats to deploy, ("Flap 1" is Slats only when airborne of course). But no: pitch down must be the instant response.

As soon as you have pitched down, you need to remedy why you stalled - you will have been flying too slowly for the wing, so increase thrust to increase speed. Only then, after you have unstalled and have started increasing your speed, you can get some flap(slat) out to give yourself increased stall margin while you get sorted out.

The reason for increasing speed before getting slats out is to arrest any speed decay, which is probably what led to the stall in the first place. If you put slats out first it will increase drag, reduce speed further and confound the problem. So slats are the last consideration, only after you have sorted the speed out.
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[Jonty]

It’s just to help reduce the chances of a secondary stall during the recovery. The problem is, if you read the QRH, you don’t select flap 1 until after the recovery. You have to look a little deeper into the FCTM, as you have, to discover what Airbus actually want.
there’s a very good video on AirbusWIN about stalling. Well worth a watch.
Also, if you can talk your trainer in to showing you “deterrent buffet” in the sim with the motion on, that’s a real eye opener! How the hell the pilots of AF447 rode that out I will never know!

[vilas]

Airbus has divided stall recovery procedure in twp parts.
1. Recovery from stall.
2. Recovery of flight path
​​​​Recovery from stall is to first reduce the AoAThen level the wings if they are not.
Increase energy by adding required thrust
Check speed brakes retracted.
Slat extension is not a part of stall recovery but is part of flight path recovery.As you start raising the attitude extend the slat to increase stall margin to prevent secondary stall.
While performing the exercise in the simulator three problems are experienced.
First as you lower the nose to reduce AoA it rises as speed increases and if not held with forward pressure it triggers secondary stall warning.
Second as you start start flight path recovery of attitude change is fast it triggers secondary stall warning.
Third as you extend slat there is small margin to VFE so as speed increases rapid nose up triggers stall. What needs to be done is slow nose up but don't stop at 4to 5° but continue to 8 to 10° till trend arrow disappears. Confirm climb and retract slat.

[Haroon]

Your question almost implies that it would be acceptable to hold the AoA and just wait for the the slats to deploy

Not really. Nose down and adding thrust will always be the first two steps. What I meant was that once these two steps have been done then what will be the downside of taking slats before flight path recovery. If slats increase drag they also reduce AoA, so the net effect might not be unfavorable. Following figure might clear what the question is:



As mentioned by Vilas:

as you extend slat there is small margin to VFE

Taking slats at position 1 (in the fig above) will also increase this VFE margin and there will be more time available for the good effect of slats. Obviously the manufacturer has a reason but was just curious about it.

[FlightDetent]

Not sure we ain't trying to read more information than the FCTM text actually carries. For the record, it is

If in clean configuration and below FL 200, during flight path recovery, the flight crew must select FLAPS 1 in order to increase the margin to AOAstall.

By the sound of it, namely the word 'must', this is not a formal certification standard language (AFM). Despite the name change and scope development, the FCTM is genetically an instructor's handbook.

For procedure design in FCOM-ABN (and AFM), the needed linearity could be limiting. Individual items are sequenced by their acuteness, placing 'FLAPS 1' higher up the list simply opens another set of questions.

Other than the above speculation, it is quite probable many skilled people asked and tried, eventually coming to realize this is the optimal way.

[Vessbot]

The text below the box says to do the slats "during" (not after) flight path recovery. In the box, the indentation places it under "when out of stall," at the same level as flight path recovery.

So it's not preventing you from doing it as soon as the stall is broken, thrust is increased, and speedbrakes are in. (However it can not be done before those things, for obvious reasons)

In your profile view diagram, at position 1, if you're in a configuration that's ready for flight path recovery (thrust is increased and speedbrakes are in) at position 1, then you can already be doing the flight path recovery there, which effectively makes position 2 be the same place as position 1.

[FlightDetent]

Originally Posted by Vessbot

(However it can not be done before those things, for obvious reasons)

And those are?

Same as checking the speedbrakes retracted. To my uninformed opinion doing that first does no harm but is unnecessary before the ND input.

Having the choice of good, better and optimal, the procedure designers must take the last one.

Really, if there was a better place for F1 or SPD brakes it would had been there.

The printed procedure is linear by necessity. Also it is compiled with a fallible human in mind, assuming the pilot might remember only some of it and execute it imperfectly.
​​​​​​
There are critical actions and supportive tasks, listed in order of priority.

To illustrate, certain airline had a nice PA for passangers after the PIC's 'Brace for impact call'

​​​​​​"Heads down, stay down - grab your ankles!" Shuffling that order devaluates it.





​​​​​​

[Haroon]

In your profile view diagram, at position 1, if you're in a configuration that's ready for flight path recovery (thrust is increased and speedbrakes are in) at position 1, then you can already be doing the flight path recovery there, which effectively makes position 2 be the same place as position 1.

Not really, the way I look at it is this:




After adding thrust it will take some time for speed to be sufficient for flight path recovery. If we recover the path with insufficient speed a secondary warning might come. So the point of sufficient speed will be the point of flight path recovery and as per the book, that will be the time to take slats. Soon after adding thrust would go against the text of the book.

Other than the above speculation, it is quite probable many skilled people asked and tried, eventually coming to realize this is the optimal way .... Having the choice of good, better and optimal, the procedure designers must take the last one.

Yes of course, the idea for the discussion is not to challenge the optimum way suggested by the manufacturer. The answer I am looking for is not related to understanding of stall or how to recover from it. I am in search for "you can say" an aerodynamic logic related to taking slats during speed increase. Which factors made the manufacturer delay this step until flight path recovery. Could it be the initial drag of slats that might slow the overall speed increase or something else?

Note: I guess the statement in my initial post "Isn't earlier the better" might have created a confusion here, sorry about that.

Thankyou

[FlightDetent]

Roger. I was discussing overruling non-aerodynamic factors that may even be easier to ​​​come up with. To point out a chance of no independently sufficent aero reason behind this all.

Quite likely a mix.

For your angle: Stall is high AoA and comes irrespective of speed.

At the extremes there would be a

(A) high energy accelerated stall from an abrupt manoeuvre where inertia and angle of incidence create the excess AoA.

and (B) a low energy stall, way behind the wrong side of power curve.

Both have in common the need for
- pitch down to reduce AoA
- wings level to reduce loading

next, we fly out with key parameters if applicable
- assure sufficient energy (thrust) ​​​​
- effective wing (slats)
- low drag (spoilers)
- obstacle control (flightpath)

There need to be only one procedure for both A and B. Obviously slats don't work for A and would delay the recovery actions. The 2+4 above don't really mix.

For the secondary 4 items, I have deliberately used alphabetic order.

Try to sequence them to fit both A and B (or any mid-range case) at the same time so that you'd be happy about it.

Don't get me wrong, my hand is itchy about F1 too, but we practice slow speed stalls at FL100. As well I tend to reach for the speedbrake lever before the thrust, when levelling the wings. But writing a procedure needs to be more responsible than that.

Try hard as I might, I cannot justify any other sequence than the printed one.

[Haroon]

Thank you for your response,.

[vilas]

Originally Posted by FlightDetent

Roger. I was discussing overruling non-aerodynamic factors that may even be easier to ​​​come up with. To point out a chance of no independently sufficent aero reason behind this all.

Quite likely a mix.

For your angle: Stall is high AoA and comes irrespective of speed.

At the extremes there would be a

(A) high energy accelerated stall from an abrupt manoeuvre where inertia and angle of incidence create the excess AoA.

and (B) a low energy stall, way behind the wrong side of power curve.

Both have in common the need for
- pitch down to reduce AoA
- wings level to reduce loading

next, we fly out with key parameters if applicable
- assure sufficient energy (thrust) ​​​​
- effective wing (slats)
- low drag (spoilers)
- obstacle control (flightpath)

There need to be only one procedure for both A and B. Obviously slats don't work for A and would delay the recovery actions. The 2+4 above don't really mix.

For the secondary 4 items, I have deliberately used alphabetic order.

Try to sequence them to fit both A and B (or any mid-range case) at the same time so that you'd be happy about it.

Don't get me wrong, my hand is itchy about F1 too, but we practice slow speed stalls at FL100. As well I tend to reach for the speedbrake lever before the thrust, when levelling the wings. But writing a procedure needs to be more responsible than that.

Try hard as I might, I cannot justify any other sequence than the printed one.
​

This procedure when it was changed from the old one it has been discussed by Airbus with customer airlines in a conference with airlines asking them clarifications. So all is catered for.

[OSDriver]

During play time at the end of a simulator session we once tried the slow speed stall scenario recovery with flaps 1 as the first step. And of course it worked very well indeed. That said, it was just idle curiosity and we all still do the airbus prescribed stall recovery every six months of course.

[8314]

20000 being max alt for extension. Gives you better AOA margins in recovery.

[CMpilot1]

Originally Posted by Haroon

What is the reason of delaying the slats extension so much. Isn't earlier the better? Is it to avoid its pitch up side effect or something else? Thanks.

I don’t see a reason to delay the slat extension while recovering from a clean stall. Please have a look at the VLS before slat extension and after slat extension. You will see the VLS dropping by at least 30 knots or more after the slats extend. Now, you may recall that VLS is a factor of VS(1.28 VS in clean configuration and 1.23VS after retraction of one step of flaps indicating it’s relationship with the stall speed). However, on extension of slats, there is a tendency for the airplane to pitch up that could lead to a secondary stall. If prompt action is taken to prevent this pitch up tendency, I don’t see why you can’t extend the slats and make use of the additional safety margin it provides from stall speed

[vilas]

Slat extension is not a part of stall recovery but of flight path recovery. It's clearly spelt out by Airbus.

[CMpilot1]

Originally Posted by vilas

Slat extension is not a part of stall recovery but of flight path recovery. It's clearly spelt out by Airbus.

FCTM also spells out that SLATS and FLAPS have a different impact on the lift coefficient obtained for a given AOA. Both Slats and FLAPs create an increase in the maximum lift coefficient. (ref: FCTM PR-AEP-MISC P26/34). Again, per FCTM definition stall is a condition in aerodynamics where the AOA increases beyond a point such that the lift begins to decrease, won't it be prudent to make use of a device asap that will aid in increasing airplane's maximum lift coefficient and complement the stall recovery process.

[Goldenrivett]

Originally Posted by CMpilot1

won't it be prudent to make use of a device asap that will aid in increasing airplane's maximum lift coefficient and complement the stall recovery process.

The stall recovery procedure is the same for all altitudes.
The flight path recovery procedure is different if below FL200.
Flight path recovery begins once the pilot starts to increase pitch attitude.
The note about avoiding impact with the ground, especially applies below FL200.

[vilas]

Originally Posted by CMpilot1

FCTM also spells out that SLATS and FLAPS have a different impact on the lift coefficient obtained for a given AOA. Both Slats and FLAPs create an increase in the maximum lift coefficient. (ref: FCTM PR-AEP-MISC P26/34). Again, per FCTM definition stall is a condition in aerodynamics where the AOA increases beyond a point such that the lift begins to decrease, won't it be prudent to make use of a device asap that will aid in increasing airplane's maximum lift coefficient and complement the stall recovery process.

The test pilots know better. Don't they?

[CMpilot1]

Originally Posted by vilas

The test pilots know better. Don't they?

I have quoted FCTM...not sure what your comment implies