Is it possible to stall an Airbus fly-by -wire aircraft in Normal Law ?
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Is it possible to stall an Airbus fly-by -wire aircraft in Normal Law ?
If PF has unreliable speed indications and overspeed protection comes in , forcing the nose-up , could the aircraft stall whilst the pilot is pushing stick full down whilst the overspeed protection is forcing the nose up regardless of inputs due to the wrong speed being sensed.
Or does AofA protection over-rule overspeed protection or would now alternate law without protections be the case.
Please forgive me if this has been covered and I've failed to find it and forgive my abysmal ignorance if the answer is obvious and I cannot see it.
I have 9,000 + hours in command of Airbus fly-by-wire and have endured over 40 simulator sessions where my lack of knowledge has been shoved down my throat, and am on my 8th Airbus company so I am fully prepared for more abuse in my seeking for knowledge.
Or does AofA protection over-rule overspeed protection or would now alternate law without protections be the case.
Please forgive me if this has been covered and I've failed to find it and forgive my abysmal ignorance if the answer is obvious and I cannot see it.
I have 9,000 + hours in command of Airbus fly-by-wire and have endured over 40 simulator sessions where my lack of knowledge has been shoved down my throat, and am on my 8th Airbus company so I am fully prepared for more abuse in my seeking for knowledge.
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This is how I understand it - In this case you are in Alternate law, so the pilot can overide the "protection" (high speed stability). Check the FCOM 01.27.30 Page3 on the old FCOM layout 
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Maybe its a "Catch 22" situation: You can't stall a Airbus FBW aircraft in "Normal Law" because as soon as a Airbus FWB aircraft being flow in "Normal Law" starts to stalls its automaticlly switches to "Alternate Law". 
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Not at all - it is the "ADR DISAGREE" status that triggers Alternate Law 2, not the stall or overspeed warnings. Based on what I've been told, one should not hear the stall warning in Normal Law before the protections limit the aircraft AoA, but it is possible to hear the overspeed warning before the AoA protections take effect.
Somewhere back in the very early AF447 threads it was said that it was possible to stall an A320 in Normal Law, but it required a very specific combination of (IIRC) gear down and asymmetric flaps - possibly a couple of other things.
The short answer is in Normal Law the AoA should never extend beyond the safe parameters in all but a single specific (and extremely unlikely) circumstances. This is different from the older A300/A310 generation, where the limits were more basic and it was possible to end up "fighting" the aircraft, which may be what the OP was thinking of.
A UAS situation that lasts for more than (IIRC) 30 seconds engages and latches Alternate Law 2, where all protections can be overridden by pilot input.
Somewhere back in the very early AF447 threads it was said that it was possible to stall an A320 in Normal Law, but it required a very specific combination of (IIRC) gear down and asymmetric flaps - possibly a couple of other things.
The short answer is in Normal Law the AoA should never extend beyond the safe parameters in all but a single specific (and extremely unlikely) circumstances. This is different from the older A300/A310 generation, where the limits were more basic and it was possible to end up "fighting" the aircraft, which may be what the OP was thinking of.
A UAS situation that lasts for more than (IIRC) 30 seconds engages and latches Alternate Law 2, where all protections can be overridden by pilot input.
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Any aircraft will STALL. A control Law is method. The definition of STALL is loss of control, so what you are saying is this aircraft will not STALL because we define it so.
Of course it will STALL in NORMAL LAW. Some will say NO, but there isn't time to change the "Rules", or the "definition of the method" in sufficient time to prevent STALL in any of a number of circumstances. Turbulence, for one. WIND SHEAR for another.
It is no more than saying, "if flown correctly" an airplane will not STALL.
Or, "I'm wearing red, so I am safe."
It is a fool's advertising Pitch, and they are just as dead.
It is pretending one has something when one doesn't that traps people into problems.
Of course it will STALL in NORMAL LAW. Some will say NO, but there isn't time to change the "Rules", or the "definition of the method" in sufficient time to prevent STALL in any of a number of circumstances. Turbulence, for one. WIND SHEAR for another.
It is no more than saying, "if flown correctly" an airplane will not STALL.
Or, "I'm wearing red, so I am safe."
It is a fool's advertising Pitch, and they are just as dead.
It is pretending one has something when one doesn't that traps people into problems.
Could you not stall an A330 in Normal law if you got it into a particular approach attitude, and once stable, poll the throttles to idle while trying to hold the nose up?
No, I don't want to try this. I doubt anyone does.
As I understand the control logic, an input to "counter to the nose down pitching moment associated with power reduction" would be called for, but so also would a command "don't stall so pitch the nose down to avoid stall."
Are you sure the latter gets priority? I would guess it does.
That considered, the significant deceleration, that close to stall margin could change AoA rather quickly into stall before the nose down commands kick in ... or, maybe the computer is that fast, and the nose dumps to avoid stall so you crash nose first thanks to that input.
Are we discussing something like "bleeding practice" here?
No, I don't want to try this. I doubt anyone does.
As I understand the control logic, an input to "counter to the nose down pitching moment associated with power reduction" would be called for, but so also would a command "don't stall so pitch the nose down to avoid stall."
Are you sure the latter gets priority? I would guess it does.
That considered, the significant deceleration, that close to stall margin could change AoA rather quickly into stall before the nose down commands kick in ... or, maybe the computer is that fast, and the nose dumps to avoid stall so you crash nose first thanks to that input.
Are we discussing something like "bleeding practice" here?
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No, completly impossible. If you would want to try very suicidal attitudes, aircraft would go into alternate law. If you would lose part of your flight controls (ailerons, elevators) you would also fall into alternate law. You could hang it on a very tall crane and let it fall with zero forward speed, but then it would be still in ground mode and not normal law...
Or you could get into a heavy windshear (say 100kts within a second or so), so that the aircrafts authority to pitch down wouldn't be enough, but split seconds before stall you would lose valid speed information and crash in alternate law, too.
Or you could get into a heavy windshear (say 100kts within a second or so), so that the aircrafts authority to pitch down wouldn't be enough, but split seconds before stall you would lose valid speed information and crash in alternate law, too.
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@LW_50:
The infamous A320 video illustrates the point here. With insufficient thrust/speed to climb, the aircraft's systems hold Alpha Max.
Capt. Asseline asserted correctly that the aircraft would not allow him to climb, but the investigation discovered that this was due to the reasons above (i.e insufficient airspeed due to A/THR disengage and poor approach technique), proving in the process that Landing Mode was not triggered.
The infamous A320 video illustrates the point here. With insufficient thrust/speed to climb, the aircraft's systems hold Alpha Max.
Capt. Asseline asserted correctly that the aircraft would not allow him to climb, but the investigation discovered that this was due to the reasons above (i.e insufficient airspeed due to A/THR disengage and poor approach technique), proving in the process that Landing Mode was not triggered.
Dani:
Not suggesting suicidal attitude, but I appreciate your attempt to explain this to me.
What you suggests to me is that the robot will bail out on you before you stall, then hand it to you as you stall? (WTF?) (On second thought, Wind Shear is a real bugger no matter what features your aircraft has ...)
The idea I have is that if you were flying in an approach configuration, and with stall AoA being about 1.3 of your current AoA, would a rapid reduction of throttle to minimum/idle induce a rapid enough deceleration (I do understand inertia and momentum) of sufficient magnitude to induce a stall?
What you (and Dozy) are telling me (I think) is that
IF that were to occur (call it a condition considered in system design)
THEN the Normal Law stall protection would keep adjusting the pitch (regardless of SS input) to seek an AoA greater than (continuously computed) stall AoA, and by design the computerized flight control system has suffient rate authority to do so, even in that bizarre circumstance.
You could liken it to being on approach and eating a whole flock of geese all at once in both engines ... and the engines failing in spectacular fashion. So there you are, going down, not having a Hudson River as Sully did, and being prevented from stalling by the robot until you hit earth.
What chance of a flare before impact, I wonder ... not a good situation, for any pilot in any plane, of course.
For purposes of discussion: coupled approaches, and A/P, and Landing Mode laws are known to exist, but let's leave them out of this for the moment.
My idea is to understand what the Normal Law can or will do for you while it is active ... once in Alt Law, the question in the Opening Post becomes irrelevant, since it was asked regarding Normal Law.
Do I understand the system correctly (or close enough?)
Not suggesting suicidal attitude, but I appreciate your attempt to explain this to me.
What you suggests to me is that the robot will bail out on you before you stall, then hand it to you as you stall? (WTF?) (On second thought, Wind Shear is a real bugger no matter what features your aircraft has ...)
The idea I have is that if you were flying in an approach configuration, and with stall AoA being about 1.3 of your current AoA, would a rapid reduction of throttle to minimum/idle induce a rapid enough deceleration (I do understand inertia and momentum) of sufficient magnitude to induce a stall?
What you (and Dozy) are telling me (I think) is that
IF that were to occur (call it a condition considered in system design)
THEN the Normal Law stall protection would keep adjusting the pitch (regardless of SS input) to seek an AoA greater than (continuously computed) stall AoA, and by design the computerized flight control system has suffient rate authority to do so, even in that bizarre circumstance.
You could liken it to being on approach and eating a whole flock of geese all at once in both engines ... and the engines failing in spectacular fashion. So there you are, going down, not having a Hudson River as Sully did, and being prevented from stalling by the robot until you hit earth.
What chance of a flare before impact, I wonder ... not a good situation, for any pilot in any plane, of course.
For purposes of discussion: coupled approaches, and A/P, and Landing Mode laws are known to exist, but let's leave them out of this for the moment.
My idea is to understand what the Normal Law can or will do for you while it is active ... once in Alt Law, the question in the Opening Post becomes irrelevant, since it was asked regarding Normal Law.
Do I understand the system correctly (or close enough?)
Last edited by Lonewolf_50; 27th Sep 2011 at 18:38.
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Is it possible to stall an FBW Airbus in Normal Law ?
Originally Posted by backofthedrag
If PF has unreliable speed indications and overspeed protection comes in , forcing the nose-up , could the aircraft stall whilst the pilot is pushing stick full down whilst the overspeed protection is forcing the nose up regardless of inputs due to the wrong speed being sensed.
Or does AofA protection over-rule overspeed protection or would now alternate law without protections be the case.
Or does AofA protection over-rule overspeed protection or would now alternate law without protections be the case.
But then, as Normal Law is maintained, and as you mention, AoA protection would enter the game.
IMO the Airbus would not stall in such situation, but as long as the overspeed indication remains, a tricky battle between 2 protections could take place ...
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Originally Posted by Lonewolf_50
Do I understand the system correctly (or close enough?)
Originally Posted by CONF iture
IMO the Airbus would not stall in such situation, but as long as the overspeed indication remains, a tricky battle between 2 protections could take place ..
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Originally Posted by KBPsen
The will be no tricky battle at all. Aoa protection overrides all other.
As soon AoA protection is left behind, High Speed Protection kicks in again ...
If not a tricky battle, a tricky and 'interesting' situation.
Only Airbus through flight testing could properly answer the question.
Would they have to force Direct Law in to get rid of all protections ?
But then, would they tell ... ?
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the robot will bail out on you before you stall, then hand it to you as you stall?
You are - in a sense - right that you can crash an Airbus (it has been proven several times), but it is not possible to stall in normal law.
Lonewolf, I think that you are trying too hard to find an adverse scenario, not to say that there isn’t one.
First you might consider that a rapid thrust reduction gives a nose down pitching moment (not withstanding any basic C* demands);
and second, it’s possible that the control law incorporates a pitch rate or alpha rate term which advances the protective control. Even quite modest stall warning and stick pusher systems in conventional aircraft have rate advancement for warnings and activations.
In addition, even in the most adverse of scenarios it might be possible to experience a temporary over-swing of the alpha protect value, but this is not the aerodynamic stall alpha or loss of control.
First you might consider that a rapid thrust reduction gives a nose down pitching moment (not withstanding any basic C* demands);
and second, it’s possible that the control law incorporates a pitch rate or alpha rate term which advances the protective control. Even quite modest stall warning and stick pusher systems in conventional aircraft have rate advancement for warnings and activations.
In addition, even in the most adverse of scenarios it might be possible to experience a temporary over-swing of the alpha protect value, but this is not the aerodynamic stall alpha or loss of control.
The definition of STALL is loss of LIFT, not control.
The actual stall is identified by one or more of:-
Heavy airframe buffet (depending on type)
Nose pitching down
A/c descending
Possible wing drop
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AOA for max lift
A related question is what percentage of maximum aerodynamic lift capability does the Airbus normal law allow the pilot to command? It is my understanding that alpha-max as chosen by the control law designers to be the maximum commandable AOA is actually lower than the AOA corresponding to Cl-max. Seems to me that this system leaves some lift capability on the table and does not allow the pilot to command max lift. Anyone who knows the details care to comment?
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There is a small note in the FCOM 3 that states that in situations of full power being applied with an engine shutdown (still in normal law) that loss of control may be reached in low speed situations before control laws activate.
Also, was there not an A330 crash in France at one stage (1994) when high pitch on rotation was selected during a test flight with an engine shutdown. FCU altitude was set low and the high rate of vertical speed resulted in an almost immediate ALT* after lift off. The speed decayed and the aircraft low speed protections didn't activate due to being in 'capture' mode resulting in a stall from normal law.
1994 A330 test flight crash - Wikipedia, the free encyclopedia
You can stall any aircraft!!
Also, was there not an A330 crash in France at one stage (1994) when high pitch on rotation was selected during a test flight with an engine shutdown. FCU altitude was set low and the high rate of vertical speed resulted in an almost immediate ALT* after lift off. The speed decayed and the aircraft low speed protections didn't activate due to being in 'capture' mode resulting in a stall from normal law.
1994 A330 test flight crash - Wikipedia, the free encyclopedia
You can stall any aircraft!!