Is it possible to stall an Airbus fly-by -wire aircraft in Normal Law ?
Actually at the stalling angle of attack you have maximum lift!
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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.
Last edited by Gary Lager; 28th Sep 2011 at 14:58.
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@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....
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....
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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.
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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 ...
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 ...
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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?
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?
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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"?
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?
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.
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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
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
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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
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.
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.
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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.
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.
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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?
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?
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The bottom line? If things are going well, and the Airbus is in NORMAL LAW it almost certainly will not STALL.
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.
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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.
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.