A320 OEB Blocked AOA probes
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I'm afraid I disagree. For 'a long time', stick pushers worked on IAS alone, and for an AoA driven pusher to 'malfunction' would require it to 'achieve' a critical AoA in the first place - in which case its 'shout' was probably valid!
The problems are occurring because AoA is now so deeply embedded in the control system that a malfunction has a far more serious effect then ever before. If I recall PGF correctly, a 'sensible'/working AoA was 'voted out' through the logic circuits. The 447 AoA problem was caused - only once a stupid AoA had been achieved - and the IAS logic removed the stall warning. Not really a 'problem' with the AoA sensor itself which I understand worked correctly - again, a problem with its interface with the system. Agreed they should not 'freeze' but equally their effects when so frozen should be easily negated. While AoA represents an excellent performance guide, you do NOT need AoA to fly an aircraft safely under normal circumstances.
The problems are occurring because AoA is now so deeply embedded in the control system that a malfunction has a far more serious effect then ever before. If I recall PGF correctly, a 'sensible'/working AoA was 'voted out' through the logic circuits. The 447 AoA problem was caused - only once a stupid AoA had been achieved - and the IAS logic removed the stall warning. Not really a 'problem' with the AoA sensor itself which I understand worked correctly - again, a problem with its interface with the system. Agreed they should not 'freeze' but equally their effects when so frozen should be easily negated. While AoA represents an excellent performance guide, you do NOT need AoA to fly an aircraft safely under normal circumstances.
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Quite honestly, i was surprised how many systems are affected by a failed AoA probe in a 737. We had quite a few problems with them a few years back and one ended up well past the end of a runway as a result.
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Originally Posted by OK465
(The instructor station also had a one-touch icon to artificially put the sim flight package in Direct for handling purposes only without all the other associated effects of turning off or failing all 3 PRIMS. One-touch, kinda like the suggestions on some of these threads.)
It seems the A400 has such a switch - Is it the red guarded one on the left side on the upper panel ? - I don't know.
Now, following the last incidents with protections going crazy, when Airbus is going to install a similar switch on all types ... ?
/pedantic mode off
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While it's true to say that most scenarios for a stuck/frozen AOA require you to have been at that AOA when the system was working 9just before failure0 it doesn't mean that the warning will be valid. In a similar fashion to the scenario AB has laid out, I can conceive of the AOAs ona pusher equipped aircraft freezing at relatively low speeds (perhaps shortly after TO) at which point you are still well clear of pusher. if the crew was unaware and the system kept treating the AOA as valid 9albeit a constant value0 then as you accelerated and climbed the pusher activation angle would usually drop - an aircraft retracting slats would probably see a major reduction in the pusher AOA at that point, in fact. Which might bring the activation angle BELOW the frozen sensor value, triggering the pusher to fire. The only way to stop it would be to get slow and back in the original config - and slowing down is not something that comes to mind as a reaction to pusher activation.
We typically consider erroneous pusher activation as a potential loss-of-control event, so while it's not quite as bad as the Ab scenario, it'd be pretty bad news. (Its easier to turn off a pusher than to turn off FBW systems, and the pusher is only a small component of the FCS, so you degrade the aircraft less by turning it off. But it wouldn't be fun getting to that point)
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Could someone explain what a modern 'software controlled' flight system would do in the following circumstances?
I am in the ITCZ, in cloud, with moderate turbulence and in significant airframe icing conditions but in a (reasonably) 'steady state' condition of speed and power:-
a) Two Air Data units decide my speed has fallen significantly and I appear to be flying at an unsafe speed OR
b) Two Air Data units decide my AoA has reached levels that are unsafe
So - does the system quickly take control and manouevre my aircraft for me OR does it just alert me to the issue and enable me to fly pitch and power while I sort it out?
I am in the ITCZ, in cloud, with moderate turbulence and in significant airframe icing conditions but in a (reasonably) 'steady state' condition of speed and power:-
a) Two Air Data units decide my speed has fallen significantly and I appear to be flying at an unsafe speed OR
b) Two Air Data units decide my AoA has reached levels that are unsafe
So - does the system quickly take control and manouevre my aircraft for me OR does it just alert me to the issue and enable me to fly pitch and power while I sort it out?
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Flagon
Details would require you clarifying deeper the exact issues, but in principle I would say:
a) This is a double failure, and (from your post) determined as such by the systems. Therefore Flt Ctrl laws will usually degrade e.g. to Altn Law, protections reduce i.e. you will be able to sort it out if you correctly assess the situation
b) If 2 ADRs determine they have a valid, and similar AoA, they will be believed and vote "out" the 3rd (potentially correct) AoA, and determine this as a single failure, hence you may not be able to directly override the ensuing protections. A major part of the unreliable airspeed drill is to potentially resolve the issue of the lone ASI being the correct one...
The 2nd scenario is essentially the recent OEB. However, the probability of 2 AoA probes going into error, in a similar fashion is low (very low). The issue the OEB raised is where the AoA probes, say "freeze", and you now alter the environment such that the frozen AoA value goes from "safe" to "hazard" e.g. reflecting reduced buffet AoA at high M number. What the OEB does is give you a course of action to degrade the Flt Ctrl laws.
It should be emphasised this has occurred once (only) in all the years of Airbus FBW Ops, and in the one case it occurred, in fact the system did drop itself into Altn Law IIRC? Easy to slam the system as a result, but as we have seen, obscure, or even simple but un-envisaged, failures in more classic designs result in (near) accidents (737 AMS, BOH etc.)
Details would require you clarifying deeper the exact issues, but in principle I would say:
a) This is a double failure, and (from your post) determined as such by the systems. Therefore Flt Ctrl laws will usually degrade e.g. to Altn Law, protections reduce i.e. you will be able to sort it out if you correctly assess the situation
b) If 2 ADRs determine they have a valid, and similar AoA, they will be believed and vote "out" the 3rd (potentially correct) AoA, and determine this as a single failure, hence you may not be able to directly override the ensuing protections. A major part of the unreliable airspeed drill is to potentially resolve the issue of the lone ASI being the correct one...
The 2nd scenario is essentially the recent OEB. However, the probability of 2 AoA probes going into error, in a similar fashion is low (very low). The issue the OEB raised is where the AoA probes, say "freeze", and you now alter the environment such that the frozen AoA value goes from "safe" to "hazard" e.g. reflecting reduced buffet AoA at high M number. What the OEB does is give you a course of action to degrade the Flt Ctrl laws.
It should be emphasised this has occurred once (only) in all the years of Airbus FBW Ops, and in the one case it occurred, in fact the system did drop itself into Altn Law IIRC? Easy to slam the system as a result, but as we have seen, obscure, or even simple but un-envisaged, failures in more classic designs result in (near) accidents (737 AMS, BOH etc.)
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Flagon
Turbulence, ITCZ, airframe icing do not alter flight control laws. Like any other aircraft there is a system of redundancy to mitigate the effect of failure of a device. There are three ADRs which collect AoA info from three AoA sensors also they collect pressure data from Pitot/static probes . Normally the averaged reading of AoA info from ADR 1+2 is converted to speed as Valpha prot and Valpha max and displayed on the speed tape but these are not speeds but AoAs. If one ADR malfunctions and its reading strays it is automatically rejected and flight control laws do not change. This is termed as self detected failure. But if two ADRs fail at the same value which is different from the only correct ADR and is between these protection values then Alpha protection will be triggered and aircraft will maintain this speed(AoA) by doing whatever it takes normally pitch down but can also pitch up. Since it is treated as single failure normal law prevails and pilot cannot over ride it. So the OEB/AD tells you to get into alternate law and override. Pitot failures are also treated as ADR failures and handled similarly when single failure but when not self detected pilot has to switch of the faulty ones and fly with the correct one or has to diagnose all are faulty and apply memory items if required and/or apply paper procedure. Recognising unreliable speed is the tricky part.The ECAM will not tell you that. Erroneous AOA data can make system act on its own and with erroneous M/CAS data you must be able recognise and act immediately on your own.
Turbulence, ITCZ, airframe icing do not alter flight control laws. Like any other aircraft there is a system of redundancy to mitigate the effect of failure of a device. There are three ADRs which collect AoA info from three AoA sensors also they collect pressure data from Pitot/static probes . Normally the averaged reading of AoA info from ADR 1+2 is converted to speed as Valpha prot and Valpha max and displayed on the speed tape but these are not speeds but AoAs. If one ADR malfunctions and its reading strays it is automatically rejected and flight control laws do not change. This is termed as self detected failure. But if two ADRs fail at the same value which is different from the only correct ADR and is between these protection values then Alpha protection will be triggered and aircraft will maintain this speed(AoA) by doing whatever it takes normally pitch down but can also pitch up. Since it is treated as single failure normal law prevails and pilot cannot over ride it. So the OEB/AD tells you to get into alternate law and override. Pitot failures are also treated as ADR failures and handled similarly when single failure but when not self detected pilot has to switch of the faulty ones and fly with the correct one or has to diagnose all are faulty and apply memory items if required and/or apply paper procedure. Recognising unreliable speed is the tricky part.The ECAM will not tell you that. Erroneous AOA data can make system act on its own and with erroneous M/CAS data you must be able recognise and act immediately on your own.
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[vilas - not quite sure why you posted "Turbulence, ITCZ, airframe icing do not alter flight control laws." - they certainly alter 'flight'.[
Why, then, does AB not offer the option to the crew to decide what is erroneous and what to do in the 2 ADR case rather than decide on its own course of action which might be wrong?
Anyone with other FBW system experience to comment?
Why, then, does AB not offer the option to the crew to decide what is erroneous and what to do in the 2 ADR case rather than decide on its own course of action which might be wrong?
Anyone with other FBW system experience to comment?
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Flagon
I posted Turbulence, ITCZ, airframe icing do not alter flight control laws because they don't. What changes the laws is failure of sensors or probes due to what ever reason. Every day aircrafts negotiate these conditions successfully. These are specific cases of equipment failure(anti ice) which can be rectified. Putting FBW technology in the dock is like throwing out the baby with bath water.
I posted Turbulence, ITCZ, airframe icing do not alter flight control laws because they don't. What changes the laws is failure of sensors or probes due to what ever reason. Every day aircrafts negotiate these conditions successfully. These are specific cases of equipment failure(anti ice) which can be rectified. Putting FBW technology in the dock is like throwing out the baby with bath water.
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Originally Posted by NoD
It should be emphasised this has occurred once (only) in all the years of Airbus FBW Ops
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vilas - "I posted Turbulence, ITCZ, airframe icing do not alter flight control laws because they don't." - are you then under the impression that someone thought they did?
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It is actually the third known case of AOA sensors blocking at a similar value
Goodrich Pitot and Thales AoA sensors seems to be the proper combination for Airbii.
Last edited by A33Zab; 11th Jan 2015 at 18:40.
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If icing of AoA sensors and Pitot/static probes have caused a few problems in some million hours the most logical thing to do is to improve the heating of those probes which should not require a technological marvel. Just because some people and many of them are non airbus pilots who are not comfortable with the idea of hard protections you introduce a Kill protection switch which will bring in the human element with human error. If a few aeroplanes are lost because of the operation of that killer switch at the wrong time then what? This cure may prove deadlier than the disease. In the interim period a method is given to override them. I do not agree you need to go in direct law at the drop of a hat. Don't bring in AF447 the only switch it needed was to switch the pilots with competent once. I say that with sadness because they lost their lives and there may be reasons why they were so.
Last edited by vilas; 12th Jan 2015 at 05:44.
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If a few aeroplanes are lost because of the operation of that killer switch at the wrong time then what? This cure may prove deadlier than the disease.
However, given the extremely low number of incidents caused by the hard protections there is probably no regulatory need to change anything and apparently the insurance companies are quite happy as it is as well, otherwise they would require higher premiums.
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Denti
Boeing FBW and AB FBW are based on different philosophies. AB is maximum automation with auto trim and alternate law in degraded mode. In case of double failure of ADRs or IRs when computers cannot self detect the job is given to the pilot, the direct law comes in as a short term phase and alternate law is recovered after pilot identifies and switches off the faulty component. The other case is in A320 where direct law comes in as a flare mode of alternate law after gear is down to help you land. Since there is no Q or any tactile feed back in the stick as is the case in conventional aircraft or Boeing FBW the direct law handling is different than in normal and alternate law. So there is a an interim procedure to enable you override protections, giving a switch to induce direct law in complicated situation for long term may not be a good idea and nor is it required if pilots follow correct procedures. Auto trim in alternate law driving the stab to its limit has happened as a result of mishandling and was pilot induced. Airbus philosophy has its own followers and is quite popular amongst them.
Boeing FBW and AB FBW are based on different philosophies. AB is maximum automation with auto trim and alternate law in degraded mode. In case of double failure of ADRs or IRs when computers cannot self detect the job is given to the pilot, the direct law comes in as a short term phase and alternate law is recovered after pilot identifies and switches off the faulty component. The other case is in A320 where direct law comes in as a flare mode of alternate law after gear is down to help you land. Since there is no Q or any tactile feed back in the stick as is the case in conventional aircraft or Boeing FBW the direct law handling is different than in normal and alternate law. So there is a an interim procedure to enable you override protections, giving a switch to induce direct law in complicated situation for long term may not be a good idea and nor is it required if pilots follow correct procedures. Auto trim in alternate law driving the stab to its limit has happened as a result of mishandling and was pilot induced. Airbus philosophy has its own followers and is quite popular amongst them.
Last edited by vilas; 12th Jan 2015 at 08:36.
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Originally Posted by vilas
If icing of AoA sensors and Pitot/static probes have caused a few problems in some million hours the most logical thing to do is to improve the heating of those probes which should not require a technological marvel.
Just because some people and many of them are non airbus pilots who are not comfortable with the idea of hard protections you introduce a Kill protection switch which will bring in the human element with human error.
If a few aeroplanes are lost because of the operation of that killer switch at the wrong time then what? This cure may prove deadlier than the disease.
As far as I know the unprotected 737 don't fall at a greater rate that the protected 320 then why ?
In the interim period a method is given to override them.
As a manufacturer you have the responsibility to make it simple, not to go and play with unguarded switches on the overhead panel in the middle of a loss of control.
I do not agree you need to go in direct law at the drop of a hat.
Don't bring in AF447 the only switch it needed was to switch the pilots with competent once.
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CONF iture
Perpignan was the result of wrong maintenance (washing) procedure. Just like the 757 or 767 I am not sure they failed to remove the masking tape from static and it resulted in fatal accident of the non FBW aircraft and Boeing shelled out a hefty sum for faulty design.
As far as 737 is concerned why has Boeing forsaken manufacturing of conventional aeroplanes and switched to FBW you can ask them. 737MAX was launched not because Boeing wanted to but their biggest customer threatened to switch to AB Neo.
whether the procedure stays or not time will tell because OEBs are not permanent changes and if it stays the alternate law meets the purpose and does not require direct law. AF447 I quoted with regret and only because you are fond of closing your eyes to crystal clear evidence of incorrect UAS and stall recovery procedure by the crew to shift the entire blame to the machine and It has nothing to do with my competence. There are any number of pilots out there who would have done what was required.
Perpignan was the result of wrong maintenance (washing) procedure. Just like the 757 or 767 I am not sure they failed to remove the masking tape from static and it resulted in fatal accident of the non FBW aircraft and Boeing shelled out a hefty sum for faulty design.
As far as 737 is concerned why has Boeing forsaken manufacturing of conventional aeroplanes and switched to FBW you can ask them. 737MAX was launched not because Boeing wanted to but their biggest customer threatened to switch to AB Neo.
whether the procedure stays or not time will tell because OEBs are not permanent changes and if it stays the alternate law meets the purpose and does not require direct law. AF447 I quoted with regret and only because you are fond of closing your eyes to crystal clear evidence of incorrect UAS and stall recovery procedure by the crew to shift the entire blame to the machine and It has nothing to do with my competence. There are any number of pilots out there who would have done what was required.
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Originally Posted by vilas
Perpignan was the result of wrong maintenance (washing) procedure.
Just like the 757 or 767 I am not sure they failed to remove the masking tape from static and it resulted in fatal accident of the non FBW aircraft and Boeing shelled out a hefty sum for faulty design.
You tell me ...
If there are faulty design on B products, is it good enough to keep silent on deficiencies of the A ones ?
As far as 737 is concerned why has Boeing forsaken manufacturing of conventional aeroplanes and switched to FBW you can ask them. 737MAX was launched not because Boeing wanted to but their biggest customer threatened to switch to AB Neo.
AF447 I quoted with regret and only because you are fond of closing your eyes to crystal clear evidence of incorrect UAS and stall recovery procedure by the crew to shift the entire blame to the machine
Actually you're the one blaming this crew 100% whatever the type they would have crashed :
AF447 wasn't going any where other than where it went. Put another two in front with same level of competence in a 747 and it will reach the same destination with another 150 Pax to give company.