WPIAeroGuy
Posts: 312
Joined: Tue Aug 21, 2007 11:52 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Fri Oct 04, 2019 8:21 pm

Quote from the Leeham article:

The AD and the explanation from Airbus singles out recently changed flight laws in the ELACs for the A321neo to be the root cause for the pitch-up described in the AD. It allows the Pilot to command fast dynamic pitch ups when in Flare mode (which is just before landing) and at a very aft CG. If the pilot keeps commanding the fast pitch-up into the stall region it can overwhelm the present anti-stall algorithm. The FBW software needs a change so this condition does not occur and the Airbus anti-stall function works as described in the Flight Manual. Until this change is fielded 3Q202o there are restrictions to the aircraft’s aft CG limit to avoid the described condition to appear.
-WPIAeroGuy
 
VV
Posts: 947
Joined: Sat Feb 13, 2016 1:03 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Fri Oct 04, 2019 10:14 pm

WPIAeroGuy wrote:
Quote from the Leeham article:

The AD and the explanation from Airbus singles out recently changed flight laws in the ELACs for the A321neo to be the root cause for the pitch-up described in the AD. It allows the Pilot to command fast dynamic pitch ups when in Flare mode (which is just before landing) and at a very aft CG. If the pilot keeps commanding the fast pitch-up into the stall region it can overwhelm the present anti-stall algorithm. The FBW software needs a change so this condition does not occur and the Airbus anti-stall function works as described in the Flight Manual. Until this change is fielded 3Q202o there are restrictions to the aircraft’s aft CG limit to avoid the described condition to appear.



Basically they were saying that you burn all the margins, the speed margin to the Vsr and also the maneuver margin? That a lot of margin to eat into. If I remember well, the speed margin is at least 23% to Vs1g at landing.

I find it interesting.
 
asdf
Posts: 439
Joined: Tue Mar 18, 2014 12:03 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Fri Oct 04, 2019 11:17 pm

WPIAeroGuy wrote:
FluidFlow wrote:
VV wrote:

I do not necessarily agree high pitch is the important parameter for stall. Angle of attack is.

If I understand well, pitch is relative to ground reference whereas angle of attack is relative to the aerodynamic reference.

Therefore a high pitch (theta) during a high climb gradient (gamma) is not necessarily a stall problem as long as the angle of attack (alpha) is below the alpha max.

Perhaps you can explain better your position?


I think in this case it is semantics: An aircraft pitches upwards, so it increases its pitch and AoA, it does not angle up. You could write it increases its angle of attack but that also increases pitch during the flare (pitch is in general really low in this phase, I think 0-10)°. Anyway during flare the aircraft is really slow and engines in general in idle. In this stage of the flight increasing pitch will lead to a reduction in airspeed. Yes AoA is one possible indicator when a stall sets in, this AoA is dynamic and a function of airspeed. If you keep your airspeed and increase your AoA you eventually stall. If you keep your AoA constant and slow down you eventually stall. So if you pitch up and your airspeed drops quick you stall your aircraft when the alpha (pitch) protection does not kick in fast enough because it cant reduce your pitch input.

So pitch up --> airspeed reduced --> AoA increases --> max AoA (Stall angle) reduces --> now if the pitch protection does not kick in fast enough the actual AoA exceeds the max AoA --> Stall. Now if you are slow enough you can stall your aircraft at AoA=0.

This means that during this stage of the flight if you need to go around your pitch up has to be limited controlled by the actual airspeed and the predicted change of airspeed. It probably takes a buffer value into account to how fast the engines produce enough thrust to guarantee climb at a certain pitch and limits the pitch up (rotation) during the spooling up time. This guarantees that at no point during the go around the actual AoA exceeds the max AoA and therefore stalls.


Stall is dependent on AoA and AoA alone. Speed does not matter. If you reduce speed and maintain AoA your lift will decrease and you’ll no longer be in 1g flight.


last time in checked stall depends on AoA and speed

high speed high AoA no problem
lift will be sufficient

lower speed, same AoA as above
not enough lift -> stall

if you wanted to point outvthat you can have a stall at any speed ( and thus speed doesnt matter on stall) then well, you are right
but then your wording was pretty unfortune to explain that
 
UncleAce
Posts: 1
Joined: Sat Jun 20, 2015 1:58 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 12:07 am

asdf wrote:
WPIAeroGuy wrote:
FluidFlow wrote:

I think in this case it is semantics: An aircraft pitches upwards, so it increases its pitch and AoA, it does not angle up. You could write it increases its angle of attack but that also increases pitch during the flare (pitch is in general really low in this phase, I think 0-10)°. Anyway during flare the aircraft is really slow and engines in general in idle. In this stage of the flight increasing pitch will lead to a reduction in airspeed. Yes AoA is one possible indicator when a stall sets in, this AoA is dynamic and a function of airspeed. If you keep your airspeed and increase your AoA you eventually stall. If you keep your AoA constant and slow down you eventually stall. So if you pitch up and your airspeed drops quick you stall your aircraft when the alpha (pitch) protection does not kick in fast enough because it cant reduce your pitch input.

So pitch up --> airspeed reduced --> AoA increases --> max AoA (Stall angle) reduces --> now if the pitch protection does not kick in fast enough the actual AoA exceeds the max AoA --> Stall. Now if you are slow enough you can stall your aircraft at AoA=0.

This means that during this stage of the flight if you need to go around your pitch up has to be limited controlled by the actual airspeed and the predicted change of airspeed. It probably takes a buffer value into account to how fast the engines produce enough thrust to guarantee climb at a certain pitch and limits the pitch up (rotation) during the spooling up time. This guarantees that at no point during the go around the actual AoA exceeds the max AoA and therefore stalls.


Stall is dependent on AoA and AoA alone. Speed does not matter. If you reduce speed and maintain AoA your lift will decrease and you’ll no longer be in 1g flight.


last time in checked stall depends on AoA and speed

high speed high AoA no problem
lift will be sufficient

lower speed, same AoA as above
not enough lift -> stall

if you wanted to point outvthat you can have a stall at any speed ( and thus speed doesnt matter on stall) then well, you are right
but then your wording was pretty unfortune to explain that


From my understanding as long as you stay above stall speed AoA is all that matters, pitch is irrelevant as long as critical AoA is not exceeded. Also speed does not make a difference as long as the critical AoA is not exceeded and this is a set value (for a particular configuration, flaps slats, gear etc may change this).

Edit: just saw your last sentence, sorry was just trying to clarify
 
WPIAeroGuy
Posts: 312
Joined: Tue Aug 21, 2007 11:52 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 12:47 am

Stall is AoA and only AoA dependent. Stall speed is an analog for measuring AoA and varies with weight. If you exceed critical AoA, regardless of speed you will stall. At low speeds, you need to maintain a higher AoA because your lift coefficient Cl varies approximately linearly with AoA. Lift = Cl*dynamic pressure pressure*wing area. In 1g flight your total lift equals the aircraft weight. In accelerated flight, like a turn, your lift will equal aircraft weight times g force, this you need a higher AoA to maintain lift.

So in the low speed condition you need a high AoA to maintain lift because your dynamic pressure is low. In this condition, with an aft CG and light weight, the pitch rate can be fast enough such that the AoA could exceed the critical AoA limits BEFORE the FBW protections could kick in. Thus, a stall could occur.
-WPIAeroGuy
 
WPIAeroGuy
Posts: 312
Joined: Tue Aug 21, 2007 11:52 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 12:50 am

To add to the above, by definition you cannot stall below the critical AoA even if your speed is below the ‘stall speed’. Stall is the point at which increasing the AoA does not increase lift and is caused by excessive flow separation from the wing. Stall speed is a function of aircraft weight and has nothing to do with flow separation. The same wing at the same airspeed on a lighter airplane would fly just fine below the heavier airplanes stall speed.
-WPIAeroGuy
 
2175301
Posts: 1505
Joined: Wed May 16, 2007 11:19 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 1:10 am

VV wrote:
Okay.

The thread was supposed to be about an "excessive" pitch.
Now the debate is about stall.

My understanding is that the stall warning is triggered well below the AoA when stall occurs. If my memory serves me well you have the 1.13 Vsr margin plus the maneuver margin

Now you are talking about a risk of stall. What the heck?


Of course people are now talking about Stall.... as the discussion about pitch has stalled with no new information. Just a logical progression... ;)

Have a great day,
 
VV
Posts: 947
Joined: Sat Feb 13, 2016 1:03 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 8:33 am

2175301 wrote:
VV wrote:
Okay.

The thread was supposed to be about an "excessive" pitch.
Now the debate is about stall.

My understanding is that the stall warning is triggered well below the AoA when stall occurs. If my memory serves me well you have the 1.13 Vsr margin plus the maneuver margin

Now you are talking about a risk of stall. What the heck?


Of course people are now talking about Stall.... as the discussion about pitch has stalled with no new information. Just a logical progression... ;)

Have a great day,


There is something that bothers me in the description of the issue and the way it is discussed here.

Very honestly I haven't read any article or document on the issue and I do not wish to do so.

If the issue is about a risk to get into stall then the issue should have been titled "risk of aircraft stall under certains conditions" with the new flight control software. It is not "excessive pitch".

In my opinion "excessive pitch would just mean that there is a risk the aircraft pitch attitude eats into the pitch attitude margins. However, I do not necessarily see it as a risk to stall the aircraft.

Now everybody seems to agree on a risk of stall. I do not have the feeling there is still a margin to stall.
I am not trying to minimize the issue, but the whole discussion does not feel very right to me.


By the way, changing the control laws is not just programming a new software. There are a whole bunch of tasks that need to be done and completed, Based on my observations in the past I sincerely believe Q32020 for the new software is a little bit optimistic. This said, perhaps they will put an extra effort on this issue and put the necessary resource to complete the work in time.
 
2175301
Posts: 1505
Joined: Wed May 16, 2007 11:19 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Sat Oct 05, 2019 2:49 pm

VV wrote:
By the way, changing the control laws is not just programming a new software. There are a whole bunch of tasks that need to be done and completed, Based on my observations in the past I sincerely believe Q32020 for the new software is a little bit optimistic. This said, perhaps they will put an extra effort on this issue and put the necessary resource to complete the work in time.


I will agree with you that such changes are not so simple... and take time.

I disagree in that I think this is only a software change. But, it is not just a small section of code to be changed. Many things have to be addressed, and addressed right. Adding and changing capabilities to older equipment is often not easy - especially when dealing with safety critical equipment.

Have a great day,
 
FluidFlow
Posts: 292
Joined: Wed Apr 10, 2019 6:39 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Tue Oct 08, 2019 8:24 am

WPIAeroGuy wrote:
Stall is AoA and only AoA dependent. Stall speed is an analog for measuring AoA and varies with weight. If you exceed critical AoA, regardless of speed you will stall. At low speeds, you need to maintain a higher AoA because your lift coefficient Cl varies approximately linearly with AoA. Lift = Cl*dynamic pressure pressure*wing area. In 1g flight your total lift equals the aircraft weight. In accelerated flight, like a turn, your lift will equal aircraft weight times g force, this you need a higher AoA to maintain lift.

So in the low speed condition you need a high AoA to maintain lift because your dynamic pressure is low. In this condition, with an aft CG and light weight, the pitch rate can be fast enough such that the AoA could exceed the critical AoA limits BEFORE the FBW protections could kick in. Thus, a stall could occur.


Can I ask you, what the exact definition of stall is in aviation?

In general, as I understand it is, that if lift becomes 0 you stall (or at least almost 0). Therefore if either of the factors in

Lift = Cl*dynamic pressure pressure*wing area


becomes 0 you stall. That's my understanding of stall. So as dynamic pressure is dependent on flow velocity, if said velocity becomes 0 then there is no lift and a stall.

But I can be wrong what the definition of stall is.
 
mxaxai
Posts: 1154
Joined: Sat Jun 18, 2016 7:29 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Tue Oct 08, 2019 10:41 am

FluidFlow wrote:
Can I ask you, what the exact definition of stall is in aviation?

Perhaps this can help? http://www.aerospaceweb.org/question/ae ... 0136.shtml

In addition, Thin Airfoil Theory doesn't account for the fact that the lift coefficient eventually reaches a maximum and then starts decreasing. The angle of attack at which this maximum is reached is called the stall angle. According to Thin Airfoil Theory, the lift coefficient increases at a constant rate--as the angle of attack α goes up, the lift coefficient (CL) goes up. But in real life, the angle of attack eventually gets so high that the air flow separates from the wing and the wing stalls.

In any discussion of wings and airfoils, you'll find such diagrams of cl over alpha; they are independent of dynamic pressure. (To some extent. Compressibility and Mach effects do play a role.)
 
FluidFlow
Posts: 292
Joined: Wed Apr 10, 2019 6:39 am

Re: A320neo Family 'Excessive Pitch' Issue Thread

Tue Oct 08, 2019 11:04 am

mxaxai wrote:
FluidFlow wrote:
Can I ask you, what the exact definition of stall is in aviation?

Perhaps this can help? http://www.aerospaceweb.org/question/ae ... 0136.shtml

In addition, Thin Airfoil Theory doesn't account for the fact that the lift coefficient eventually reaches a maximum and then starts decreasing. The angle of attack at which this maximum is reached is called the stall angle. According to Thin Airfoil Theory, the lift coefficient increases at a constant rate--as the angle of attack α goes up, the lift coefficient (CL) goes up. But in real life, the angle of attack eventually gets so high that the air flow separates from the wing and the wing stalls.

In any discussion of wings and airfoils, you'll find such diagrams of cl over alpha; they are independent of dynamic pressure. (To some extent. Compressibility and Mach effects do play a role.)


Thanks.

Than my idea of stall was totally wrong. I am used to calculate overall lift and that is AoA and airflow velocity dependent and I always thought stall would be called the moment when lift suddenly drops to near 0 independent of the actual underlying cause.
 
WPIAeroGuy
Posts: 312
Joined: Tue Aug 21, 2007 11:52 pm

Re: A320neo Family 'Excessive Pitch' Issue Thread

Thu Oct 10, 2019 1:54 am

mxaxai wrote:
FluidFlow wrote:
Can I ask you, what the exact definition of stall is in aviation?

Perhaps this can help? http://www.aerospaceweb.org/question/ae ... 0136.shtml

In addition, Thin Airfoil Theory doesn't account for the fact that the lift coefficient eventually reaches a maximum and then starts decreasing. The angle of attack at which this maximum is reached is called the stall angle. According to Thin Airfoil Theory, the lift coefficient increases at a constant rate--as the angle of attack α goes up, the lift coefficient (CL) goes up. But in real life, the angle of attack eventually gets so high that the air flow separates from the wing and the wing stalls.

In any discussion of wings and airfoils, you'll find such diagrams of cl over alpha; they are independent of dynamic pressure. (To some extent. Compressibility and Mach effects do play a role.)


Exactly. The critics angle of attack is the point at which an increase in AoA does not result in an increase in Cl. This is due to flow separation, but it doesn’t mean the flow is totally separated or lift is zero.

The shape of the Cl curve can give some indication as to the abruptness of the stall. Airfoils with sharp peaks can lead to more abrupt stalls and can be dangerous if not incorporated properly.
-WPIAeroGuy

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