Humbling sim experience
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Micro:
As I said, you will have to "change the Law" in order to pitch down, otherwise, in Normal Law, the Overspeed Protection will take control.
To those who say that, this is a very unlikely situation, I would like to ask if you think this would be absolutely impossible to happen?
That scenario of the false oversped triggering undue high speed protection, when you are actually flying very slow, is dreadfull. But I believe you can still use manual pitch trim to achieve pitch down.
To those who say that, this is a very unlikely situation, I would like to ask if you think this would be absolutely impossible to happen?
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Originally Posted by A-FLOOR
The pressure variations in the turbulent air will still find their way to the IAS via the static ports, and this is of course also the reason why climbing will give you the impression aircraft is accelerating. What was also significant in the AF447 case is that the turbulence will cover up any other cues that the aircraft has stalled (stall buffet), and the FBW will mask any change in roll response of the flight controls due to the extreme AOA, as I recall that the roll rate response vs. stick deflection is linear in alternate law.
Mach buffet is a violent high frequency shake of the aircraft that is very different from turbulence-induced g-variations. The airplane had left the turbulence before it stalled.
Once stalled, the airplane is effectively uncontrollable in roll, its reponse to a roll input is often opposite to that commanded, at any rate quite different from normal.
Last edited by HazelNuts39; 11th Jun 2012 at 15:47. Reason: correction: Stall replaced by Mach (buffet)
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What was also significant in the AF447 case is that (...) the FBW will mask any change in roll response of the flight controls due to the extreme AOA, as I recall that the roll rate response vs. stick deflection is linear in alternate law. FBW being FBW, it will always try to give you what you are asking of it.
If still AF447 : sorry, this one never get to overspeed.
This tells you how a Boeing would have fared in the same situation: the lack of autotrim on Boeings leaves the THS in the same position unless you conciously tell it to do something else using the trim switches, and this leaves open your avenue of sensing that the aircraft is stalled as you are unable to maintain a nose-up attitude as the aircraft loses airspeed.
- It still remain to be proved that elevators only aren't enough to maintain stall (at a lower AoA than that obtained with the help of the THS, maybe). IIRC knowledgeable peope (of which I'm not) suggested otherwise.
- What do you mean by "the lack of autotrim on Boeings"?
[edit] May I suggest we move to the AF447 thread if I undestood correctly that was the topic at hand? We're hijacking flyburg's thread, here.
Last edited by AlphaZuluRomeo; 11th Jun 2012 at 16:12.
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Mach buffet is a violent high frequency shake of the aircraft that is very different from turbulence-induced g-variations. The airplane had left the turbulence before it stalled.
Once stalled, the airplane is effectively uncontrollable in roll, its reponse to a roll input is often opposite to that commanded, at any rate quite different from normal.
What flight are you talking about
If still AF447 : sorry, this one never get to overspeed.
Same question. If that's AF447, I fail to see anything suggesting any of the crew members ever understood the stall situation.
If still AF447 : sorry, this one never get to overspeed.
Same question. If that's AF447, I fail to see anything suggesting any of the crew members ever understood the stall situation.
Two remarks:
- It still remain to be proved that elevators only aren't enough to maintain stall (at a lower AoA than that obtained with the help of the THS, maybe). IIRC knowledgeable peope (of which I'm not) suggested otherwise.
- It still remain to be proved that elevators only aren't enough to maintain stall (at a lower AoA than that obtained with the help of the THS, maybe). IIRC knowledgeable peope (of which I'm not) suggested otherwise.
- What do you mean by "the lack of autotrim on Boeings"?
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Originally Posted by A-FLOOR
I am talking about the stall buffet. The aircraft was at that point flying too slowly to induce a mach buffet
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[edited] A-FLOOR, I moved my answer to the AF447 thread, here, in order to comply with my own remark re: "hijacking" of the present thread 
Last edited by AlphaZuluRomeo; 11th Jun 2012 at 19:41.
With regards to Airbus vs. Boeing: the Airbus overspeed mode was commanding an increase in pitch through the flight directors, which was followed religiously at least until the aircraft reached its apogee at 38000ft. This continuous backpressure on the stick caused the THS to trim all the way up, leading to the deep stall.
In case of the A340 Zoom Climb it was overspeed protection that send the aircraft into a climb but it did not stall. AoA protection law kicked in and kept it at Alpha_Prot. No stall here, and no alternate law either.
If you are referring to AF447, there was no overspeed event, it was in alternate law 2 due to the ADIRU's signalling Unareliable airspeed (Icing of pitots) and it was sustained NU commands by the manually flying PF that caused the THS to finally support his efforts and go to a significant NU Trim setting. This combined with further mainly NU commands on the sidestick kept it in the stall.
Alpha_Max is a HARD protection which cannot be overriden as long as it's active (not the case in AF447 due to Alt2). High speed protection might win against Alpha_Prot but not against Alpha_Max
It is not THAT easy to fool the Airbus protections. That takes much more finesse and or bad luck (stuck AoA vanes in Perpignan).
@flyburg: Apologies for also taking part in the AF447 thread hijacking ;-)
Last edited by henra; 11th Jun 2012 at 19:52.
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Originally Posted by henra
In case of the A340 Zoom Climb it was overspeed protection
I was referring to the second one where at least the FDR trace shows 2 overspeed events right at the beginning of the incident.
See http://www.bea.aero/docspa/2011/f-zu...f-zu110722.pdf
page 18.
But indeed it seems the pitch up was due to Sidestick input although the AoA starts to increase prior to deflection of the S/S which I attributed to overspeed protection. Looking closer at the traces the pitch only starts to increase after the NU S/S commands.
In the text overspeed protection is also not explicitely mentioned, therefore I come to agree to your statement and stand corrected.
Last edited by henra; 11th Jun 2012 at 22:07.
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Ahem, sorry but...
"Au cours de l’événement, la protection haute vitesse ne s’est pas activée."
=> "Throughout the event, the high speed protection did not activate."
Footnote 4, page 3.
"Au cours de l’événement, la protection haute vitesse ne s’est pas activée."
=> "Throughout the event, the high speed protection did not activate."
Footnote 4, page 3.
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Yes, static ports can get iced over in flight.
That may be true.
However, the static is very sensitive to airflow variations. Put a minor obstruction on the skin somewhat upstream and the PEC can alter substantially from what the AFM suggests.
Keep in mind that a lot of effort in the certification test program goes into establishing the PEC information for the AFM.
That may be true.
However, the static is very sensitive to airflow variations. Put a minor obstruction on the skin somewhat upstream and the PEC can alter substantially from what the AFM suggests.
Keep in mind that a lot of effort in the certification test program goes into establishing the PEC information for the AFM.
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Use of THS
Good thoughts re use of THS as recourse of last resort. One perhaps could file this away under "what to do when all of the usual drills and memory items aren't working" file. I suspect however that the crew of AF447 never really knew that they were in a stall, nor what their real AoA was in the last minutes of flight......
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I know of no Boeing type which has an Airbus type autotrim system that automatically trims the THS to relieve pitch input. In manual flight, even the 777 and 787 have to be trimmed manually for airspeed using the trim switches.
So, if speed is wrong, if speed is overspeed (for the computers, I mean) what would happen in a B777. Its protections are different tha those of airbus. No uncommanded pitch up or down, but there are forces induced in the control column to cue the pilot. In this case it would mis,ead the pilot, right? It would be too stiff for pushing, I deem...
Computers are computers. They have a big problem. What happens when they are doing the wrong thing without detecting it is tthe wrong thing.
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Just a small point of order here. Boeing offers the AOA in all of it's current airplanes. Delta and American are the only airlines that I know for sure have the AOA in the 737NG and 777 series aircraft.
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"Computers are computers" - garbage in/garbage out.
However, the fact remains that accident rates continue to fall, not only per million flights, but in absolute terms. Yes, we need to continue to develop the software models, but the argument that flying is more dangerous because of increased computerisation is simply false. I am not convinced it's even possible to argue that aircraft with computerised protections are more dangerous than those without.
However, the fact remains that accident rates continue to fall, not only per million flights, but in absolute terms. Yes, we need to continue to develop the software models, but the argument that flying is more dangerous because of increased computerisation is simply false. I am not convinced it's even possible to argue that aircraft with computerised protections are more dangerous than those without.
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Absolutely.
I'm talking about the different nature of computers as opposed to mechanical systems.
When computers are involved, wrong data that can't be dismissed by the computers as wrong, can make the computer make very nasty things. The more complex the system is, the more involved one computer in the system and the more systems interface that computer, the worse effects such a malfunction will have.
Wether it is a Boeing or an Airbus, computers are computers.
Computers are protected against that in various manners, like "watchdogs", "voting", control and monitoring dual channels and "dissimilar redundancy". When a computer is not working properly, it is very important that the computer is declared invalid, either by itself or by other computers, so that its outputs are neglected.
The simultaneous and identical freezing of all three pitot tubes will make all three computers be in error, which makes impossible to detect by voting. So it is necessary some added function to avoid that situation, such as probe ice detectors or a "reasonableness check of IAS" using data other than pitot, I think there is already something like that invented.
I'm still curious about what would happen in a B777
I'm talking about the different nature of computers as opposed to mechanical systems.
When computers are involved, wrong data that can't be dismissed by the computers as wrong, can make the computer make very nasty things. The more complex the system is, the more involved one computer in the system and the more systems interface that computer, the worse effects such a malfunction will have.
Wether it is a Boeing or an Airbus, computers are computers.
Computers are protected against that in various manners, like "watchdogs", "voting", control and monitoring dual channels and "dissimilar redundancy". When a computer is not working properly, it is very important that the computer is declared invalid, either by itself or by other computers, so that its outputs are neglected.
The simultaneous and identical freezing of all three pitot tubes will make all three computers be in error, which makes impossible to detect by voting. So it is necessary some added function to avoid that situation, such as probe ice detectors or a "reasonableness check of IAS" using data other than pitot, I think there is already something like that invented.
I'm still curious about what would happen in a B777
Guess I need to give my glasses a good cleaning
On a more serious note:
Is there any documented case of an overspeed protection induced Pitch Up in an A330/340? Apart from this one where I wrongly assumed it was the overspped protection that initiated the climb I'm not aware of any other suspected case on these types. Did I oversee something?
Has it really never been used in anger?
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MEIKLEOUR #59
AVEROW #72
If " The THS is NEVER NORMALLY moved by the Crew in flight... " it might tend to be forgotten.
If a car with hydraulic brakes has a hydraulic leak, the hand brake is a separate system which can be used... And it should be used, just occasionally, to remind the driver that it is still there.
( A family car had a hydraulic leak recently. Luckily the young driver DID remember. All was well...)
AVEROW #72
If " The THS is NEVER NORMALLY moved by the Crew in flight... " it might tend to be forgotten.
If a car with hydraulic brakes has a hydraulic leak, the hand brake is a separate system which can be used... And it should be used, just occasionally, to remind the driver that it is still there.
( A family car had a hydraulic leak recently. Luckily the young driver DID remember. All was well...)
