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.

Check this:

http://aviation-safety.net/database/record.php?id=19940630-0⟨=de

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 :ok:

As far as I remember the mentioned 330 crash shows that there is no pitch protection if in Alt * / Alt acquire mode.

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". :ugh:

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.

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.

The definition of STALL is loss of LIFT, not control.

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. :p

Are we discussing something like "bleeding practice" here? :confused:

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.

@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.

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?)

Is it possible to stall an FBW Airbus in Normal Law ?
 

Such scenario would need to have at least two airspeed indicators to lie in a similar way to trigger the High Speed Protection.

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 ...

No, you are not even close.

The will be no tricky battle at all. Aoa protection overrides all other.

Didn't backofthedrag mention "whilst the pilot is pushing stick full down" ?
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 ... ?

well, a conventional aircraft would have stalled way before, because no human being is able to react so fast and precise as a fbw driven elevator control.

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.

Actually at the stalling angle of attack you have maximum lift! It decreases if you then continue to increase the angle of attack (and of course you have lots of drag).

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

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?

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!!

YES, see my posts #2 and #4

But the wing has not yet stalled! The stall occurs when you increase the angle of attack over the "critical" angle, the airflow breaks down and lift is reduced - the wing has now stalled!

That's because whilst there is AoA protection while airborne, there is no VMCA protection. You will lose control through lack of rudder authority, not because of a wing stall.

Hi Gary,
Did you mean VMCA?

@Gary

That's correct.

Anyhow VMCA isn't that far from Vs.

And in the mentioned A330 accident Toulouse even the top guys didn't have a clue what the aircraft was doing....

Say you're on an approach, fully configured, around one thousand AGL (irrespective of local terrain). Now there is an incipient RA fault that causes the jet to think that it is a 40' RA and the jet goes to flare mode with the Autothrust reducing to idle... Given that scenario I would think that it is possible to stall the jet in Normal Law.

why would the autothrust go to idle? are you doing an autoland with a misbehaving RA?

Stall in Normal Law
 

Very interesting point skidbuggy.
If the pilot doesn't immediately react, AoA will increase and may reach the stall value.
Which AoA the AP would or not maintain is a big question ...

Yes. VMCA. :ugh:

This is not a straw man, it is an argument made entirely of straw.

All aircraft will STALL. Defining the way the controls are managed is irrelevant.

Can we just agree that..... "If an airframe is not STALLED, it is not STALLED"? Trying to parse the needed changes to cause it to stall and how the manufacturer parses his marketing brochure is silly.

The bottom line? If things are going well, and the Airbus is in NORMAL LAW it almost certainly will not STALL. If the aircraft senses it may get into trouble, and changes its flight Law configurations to "NON NORMAL" prior to STALL, people won't be killed in the ensuing crash?

Huh? Would you be kind enough to elaborate? :confused:
Perhaps you were implying an airspeed < 160kts, before the windshear. Why not. But with an airspeed > 160kts, why/how would you "lose valid speed information"?


As per the tests (FAA) on CFIT avoidance:
Yes, the maximum commandable AOA is lower than the AOA corresponding to Cl-max. The difference being the "safety margin" included to let "time" to the flight controls protection to kick in, approaching stall.
IIRC FAA also validated the Airbus "theory" when facing pilots who were "annoyed" that they cannot reach Cl-max ("what if I just need a few feets more to pass that mountain?") :
- with Airbus AoA protection, you can (very easily) maintain alpha-max for as long as needed
- without protection you could reach Cl-max, but also stall, then unload under Cl-max, then pull, then stall... etc. (and that all depends on piloting skills, average may not be sufficient to do it correctly)
That's from memory, but the FAA report is online if you care to more details.

Nobody answered or understood the original question.

The question was actually not very clear.

Supose you are taking off and suddenlly your speed (CApt and FO) goes to 400 Kts.It means you had an ADR 1 and 2 disagreement but since they agree between them , the ADR3 was rejected.So you`ll keep NORMAL LAW.

The high speed proteccion will be activated pulling the nose up to reduce the speed from 400 kts .
Remember that you are taking off , 300 ft , maintaining FLAPS 2. So your real speed is not much higher than 160 kts.

. There will be a conflict here of overspeeding proteccion with stall proteccion.

The original question was:

Can I override the high speed proteccion to avoid the stall?And by the way , which proteccion has precedance?

I got the question , but I don`t know it for sure. I was told once , that in this case your only chance is to immediatelly turn off ADR1 and 2 because in this case you will have a 2 ADR failure , you will go to ALTN LAW and then you can fly the airplane acording to your convenience.

Remember that in ALTN 2 you only have load factor prot.

My 2 cents


A3TWENTY

I tried to stall the A330 in normal law in the sim keeping side stick full back and thrust in idle at 30000ft.

It does not stall , but it will start descending with about 1000FPM.

A320

safetypee, what I was working through was trying to determine how the robot prioritizes conflicting demands in Normal Law. Let's not pretend that the pilots will just sit there and watch it all happen as the plane falls ... my question regards what inputs the pilot can make that won't be countered by the robot while still in Normal Law, with protections active. (Wasn't Sully in Normal Law the way to the water in his A320?)

The Alpha Protect seems to have a priority above most every thing else, in pitch, to include side stick inputs. (Ground avoidance maneuver/emergency pull up a case in point here).

From a design philosophy, this makes basic sense, per Davies little sound byte along the lines of "if you have a choice between stall and something else, do something else."

Which takes me back to the geese in the intakes on approach:

The robot will try to keep you from stalling.

If the pilot selects a speed X knots faster than stall during that short descent, (similar to what Sully did), he can fly his way to the ground, knowing he has some cushion for the end game.

Need to review what I have about the landing laws, I think there are some pieces in there that will fill in my blanks.

thanks for your kind reply.

KBPsen: that's what I thought, AoA holds the ace of trump. Thanks.

Or maybe you did not take enough time to read and understand each reply ...

Okay, don't take it too serious...

Manufactor (M)
Customer (C)

M: Sir, may I introduce our absolutely crash proof airplane?
C: What does it mean "crash proof"?

M: Well, as you and everybody else knows, more than 75% accidents are pilot's error.
C: Okay, and what's your solution?

M: Good question:). We have several features in our highly sophisticated machines, that simply overrule the pilot if he is doing something wrong.

C: Please get more in detail.

M: Well, for example, if a pilot doesn't react to overspeed, the aircraft takes care automatically.

C: How about that?

M: The plane simply raises the nose, climbs away to get rid of the overspeed.

C: That's great. Tell me more, please.

M: Well, in the opposite, if one of these monkeys gets too slow, the plane will automatically gain speed by lowering the nose.

C: Cool, so we don't need expensive training 'cause the plane is so forgiving?

M: Indeed, that's the point. In addition, if you want these overpaid guys to convert to another type of our production line, just give them a DVD, that's it.


:ugh:

The unlikely case of both rad alts giving an incorrect height and causing the aircraft to go to idle power and flare, but at the wrong height (mentioned a few posts ago) when on approach was covered by an OEB issued after the Turkish 737 crash at Sciphol. It is still valid and requires immediate disconnection of autothrust to avoid a stall. Although technically in normal law, I am not sure how normal law alters when FLARE mode (NOT the flare law pitch down 2 degrees/8 secs) is activated by the autopilot?

The FCOM says that A of A protection takes priority over other protections so if an overspeed was also indicated, one would expect the stall prot to work? or would normal law degrade?

Amen to that. Has wings, can stall. Cannae change the laws of physics...

Yep. Although I suspect there are actually ways to do it - but that you'd have to try really hard.

Once things are broken, however, particularly air data sensors, all bets are off. Perpignan is probably the classic - 2 stuck AOAs, correct one voted out. Until SW AOA reached at which point normal law decides something isn't right, bails out and hand over to the real pilots who then mess up the recovery with a secondary stall.

No, it didn't stall in Normal Law, or even at the transition - Normal Law bailed a few seconds before. Like you say - that's entirely irrelevant and doesn't get normal law off the hook.

The point is that if things are broken you can go from
1. normal-law-protected to
2. un-protected at the edge of the envelope to
3. stalled
in a matter of seconds. In various scenarios (several I can think of, and there will be more). Arguing about whether or not stall can happen in (1) deflects attention from where it needs to be, namely can you handle (2) and stop it turning into (3), and also recover from (3) if necessary.

1. DALLAS....with nothing broken

In relation to alpha protection (FCOM 1 for A319/320):

Under normal law, when the angle of attack becomes greater than alpha prot, the system switches elevator control from normal mode to a protection mode, in which the angle of attack is proportional to sidestick deflection. That is, in the alpha prot range, from alpha prot to alpha max, the sidestick commands alpha directly. However, the angle of attack will not exceed alpha max, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the
sidestick, the angle of attack returns to alpha prot and stays there.

This protection against stall and windshear has priority over all other protections. The autopilot disconnects at alpha prot + 1deg.

Valpha prot, Valpha floor, Valpha max vary according to the weight and the configuration.

To deactivate the angle of attack protection, the pilot must push the sidestick :
– More than 8deg forward, or
– More than 0.5deg forward for at least 0.5 second, when alpha < alpha max.
In addition, below 200 feet, the angle of attack protection is also deactivated, when :
– Sidestick deflection is less than half nose-up, and
– Actual alpha is less than alpha prot – 2deg.

Strangely the A321 does not have the second paragraph regarding below 200 feet...

I think the bottom line is that while Normal Law does give stall protection under the vast majority of situations, multiple probe faults and/or external violent events can put you out of Normal Law...

In Normal Law you can't Stall, but you can still hit the ground maintaining pretty close to max CL. If you tried the same scenarios in a different type of aircraft, I suspect at best the outcome would be no better.