Minibus (A320) Stall Recovery
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Hi Microburst2002,
According to page 18 of the accident report http://www.bea.aero/docspa/2008/d-la...a081127.en.pdf "At 15 h 45 min 05, the aeroplane was at 2,910 ft altitude and a speed of 99 kt. Pitch angle was 18.6 degrees. The stall warning sounded."
They were in Normal Law, but received false AoA info from frozen sensors.
Those of us with a healthy disrespect of "In normal law this aeroplane has been designed to avoid stall ..." would not go near a pitch of 18 degrees in level flight and IAS of around 100 kts - because it doesn't look right and AoA probes have been known to fail.
If still in normal law, Will you get the stall warning or will it be a "silent" stall?
They were in Normal Law, but received false AoA info from frozen sensors.
Those of us with a healthy disrespect of "In normal law this aeroplane has been designed to avoid stall ..." would not go near a pitch of 18 degrees in level flight and IAS of around 100 kts - because it doesn't look right and AoA probes have been known to fail.
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Originally Posted by Microburst
Will the FCS pitch the a/c down to get the AoA to alpha prot?
RE stall warning: it is not inhibited in normal law, but the threshold is set so high that it will normally not be reached.
Crawling back under my rock ...
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Normal law, unreliable airspeed caused by blocked pitots (the ASI behaves like a VSI) - just after take off, as the aircraft climbs away, the airspeed unreliably indicates a rapid increase which in turn causes the overspeed protection - a PROTECTION thus implying the aircraft is in normal law. Once in the protection you ain't getting out without crashing or forcing it into alternate law. The aircraft, being in normal law and trying to protect from the overspeed will keep raising the nose, into the stall, and then roll on it's back. To force into alternate law consider switching off 2 ADRs.
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By the way: What happens if you fly at VLS at level flight, stick loose, with CONF 3, and then you select flaps zero? clac, clac. Ooops!! Seriously. What will happen?
Since VLS for Conf3 is higher than Vs for Conf1, you won't stall. The low speed protection might however kick in...
Normal law, unreliable airspeed caused by blocked pitots (the ASI behaves like a VSI) - just after take off, as the aircraft climbs away, the airspeed unreliably indicates a rapid increase which in turn causes the overspeed protection - a PROTECTION thus implying the aircraft is in normal law. Once in the protection you ain't getting out without crashing or forcing it into alternate law. The aircraft, being in normal law and trying to protect from the overspeed will keep raising the nose, into the stall, and then roll on it's back. To force into alternate law consider switching off 2 ADRs.
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The topic has veered off plenty from the original question, but it is an interesting discussion. I don't know about anyone else, but I try to view all those Airbus protections with what I consider to be healthy suspicion.
These computers are slow. In a dynamic environment, it's good to remember that physics is the only thing that keeps you alive. If the air stops flowing over the wing, you are ****** and need to act yesterday. I'm not waiting for any computer to kick in when my and others lives depends on it. Although I am sure that all those Airbus engineers are brilliant, airmanship is why we get paid.
In my humble opinion, of course.
These computers are slow. In a dynamic environment, it's good to remember that physics is the only thing that keeps you alive. If the air stops flowing over the wing, you are ****** and need to act yesterday. I'm not waiting for any computer to kick in when my and others lives depends on it. Although I am sure that all those Airbus engineers are brilliant, airmanship is why we get paid.
In my humble opinion, of course.
pennies worth...
The AI FBW logic acts to provide protection away from the stall, but it is just another system and as such has limits that may exceed the design criteria. At that point, even without system failure, the aircraft may achieve a stall. At that point, the fact that it is such a rare and unexpected outcome, and indeed "trained" out of the normal expectation of the crew, results in potential issues arising in the recovery.
The basic recovery for any airplane achieving a loss of control of flightpath remains simple, and is the same for an F4, MiG21 B777 or a C150, or a Pitts S1... aircraft are inherently recoverable without pilot intervention once the thrust pitch couple is removed, except where the FBW has acted in such a manner to put the controls in a non neutral position... i.e., sensor failure, software faults. In the latter case, the pilot may well need to defeat the logic to achieve the inherent longitudinal stability of the airplane. (true to the point of initial departure... ).
I routinely (poor choice of words) stall large and small aircraft in test, and the factors I emphasise in the test brief and test cards is ensuring that the recovery is not complicated by power additions, and that the crew terminate stab trim before reducing speed below Vref. We also conduct test profile training in simulators, however the stall conditions of the simulator are not necessarily indicative of the aircrafts actual dynamics and performance. The ride in the cockpit of a large aircraft in the stall is rather more interesting than the simulator, vertical and lateral accelerations at the seat are considerable.
If you are comfortable with the assumption that your aircraft cannot stall, so be it. Be prepared to recover when it does happen, and ensure that you do not forget the thrust couple and the stab trim position, particularly when you are now trained by AI design to not use a trim system as a matter of routine. At least the B777 and B787 still keeps a thumb in the loop to some extent... FYI, I prefer AI FBW laws when operating normally over TBC's but, when they go squirrelly, there is a simplicity to the TBC product that is nice in a time of high cognitive load.
cheers.
The basic recovery for any airplane achieving a loss of control of flightpath remains simple, and is the same for an F4, MiG21 B777 or a C150, or a Pitts S1... aircraft are inherently recoverable without pilot intervention once the thrust pitch couple is removed, except where the FBW has acted in such a manner to put the controls in a non neutral position... i.e., sensor failure, software faults. In the latter case, the pilot may well need to defeat the logic to achieve the inherent longitudinal stability of the airplane. (true to the point of initial departure... ).
I routinely (poor choice of words) stall large and small aircraft in test, and the factors I emphasise in the test brief and test cards is ensuring that the recovery is not complicated by power additions, and that the crew terminate stab trim before reducing speed below Vref. We also conduct test profile training in simulators, however the stall conditions of the simulator are not necessarily indicative of the aircrafts actual dynamics and performance. The ride in the cockpit of a large aircraft in the stall is rather more interesting than the simulator, vertical and lateral accelerations at the seat are considerable.
If you are comfortable with the assumption that your aircraft cannot stall, so be it. Be prepared to recover when it does happen, and ensure that you do not forget the thrust couple and the stab trim position, particularly when you are now trained by AI design to not use a trim system as a matter of routine. At least the B777 and B787 still keeps a thumb in the loop to some extent... FYI, I prefer AI FBW laws when operating normally over TBC's but, when they go squirrelly, there is a simplicity to the TBC product that is nice in a time of high cognitive load.
cheers.
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lift & drag
In the OP it was suggested that selecting F1 after the fact would be moot. Now am I correct in assuming that selecting F1 --during-- the stall might make things even worse because the airflow that should generate lift has already seperated and hence the flaps are only increasing drag?
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drfaust
Some of us in our faith or hostility to protections are forgetting that they come in to effect beyond the normal flight envelope. A professional pilot should never experience these protections through acts of commission or omission. It's like money in fixed deposit which you are not going to break but only take comfort from the fact that it is there. so like the SFO accident "Yah the speed was falling but we thought ATHR will correct" is plain bull****.
Some of us in our faith or hostility to protections are forgetting that they come in to effect beyond the normal flight envelope. A professional pilot should never experience these protections through acts of commission or omission. It's like money in fixed deposit which you are not going to break but only take comfort from the fact that it is there. so like the SFO accident "Yah the speed was falling but we thought ATHR will correct" is plain bull****.
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shorthauler
F1 there is no flap. It is only slat. The point is it doesn't help recovery from stall. The basic technique before they got obsessed with preventing loss of height, reduce AoA.
F1 there is no flap. It is only slat. The point is it doesn't help recovery from stall. The basic technique before they got obsessed with preventing loss of height, reduce AoA.
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It won't stall - AoA protection has priority over all other protections....
Just remember, you're flying an aeroplane not a text book.
Last edited by WhyByFlier; 4th Sep 2014 at 06:55.
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@WhyByFlyer
I am in no way saying that the Bus can't be stalled. Laws of physics apply equally to all aircraft.
However, regarding your scenario:
The AoA protection (as the name implies) is based on angle of attack, not on airspeed (regardless of whatever the FAC calculates and presents on PFD) and it should be able to ovrride the high speed protection. What happened to you in the sim is (hopefully!) just a simulator glitch. If the real a/c behaves the same way, then we have a serious problem...
I do agree however, that switching off 2 ADR's is the quickest way to get into ALTN Law, anytime the FBW system decides to go AWOL...
I am in no way saying that the Bus can't be stalled. Laws of physics apply equally to all aircraft.
However, regarding your scenario:
The AoA protection (as the name implies) is based on angle of attack, not on airspeed (regardless of whatever the FAC calculates and presents on PFD) and it should be able to ovrride the high speed protection. What happened to you in the sim is (hopefully!) just a simulator glitch. If the real a/c behaves the same way, then we have a serious problem...
I do agree however, that switching off 2 ADR's is the quickest way to get into ALTN Law, anytime the FBW system decides to go AWOL...
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Side stick
You are correct. AoA info of two ADRs is averaged and used for calulation of Valpha prot and VMAX and converted to speed for display and it definitely has priority over everything else. Also it is phase advanced to cater for rapid increase in angle of attack so you don't have to think yesterday but still in dynamic environment it may be found short.
You are correct. AoA info of two ADRs is averaged and used for calulation of Valpha prot and VMAX and converted to speed for display and it definitely has priority over everything else. Also it is phase advanced to cater for rapid increase in angle of attack so you don't have to think yesterday but still in dynamic environment it may be found short.
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Not quite - they got as far as Stall Warning, but that is fired before entering the stall regime itself. Of course you'll get the warning in the event of unusually changeable wind conditions if you let the AoA deteriorate to that extent.
I would hope not, because the FCOM is very clear on the matter, and always has been.
UAS condition takes at most 1 second for the EFCS to detect, and will cause instant reversion to Alternate Law and suspension of hard protections.
Compared to modern tech yes, but the CPU types involved range between 10,000 and 25,000 operations per second, which is perfectly adequate - and then some - for the kind of calculations we're talking about. In High AoA Protection Mode the systems are also designed to log trends, which in effect provides a crude "look-ahead" function to orient the aircraft accordingly.
All this is a bit of a sideline though - the first two answers pretty much nailed it.
Compared to modern tech yes, but the CPU types involved range between 10,000 and 25,000 operations per second, which is perfectly adequate - and then some - for the kind of calculations we're talking about. In High AoA Protection Mode the systems are also designed to log trends, which in effect provides a crude "look-ahead" function to orient the aircraft accordingly.
All this is a bit of a sideline though - the first two answers pretty much nailed it.
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What if all ADRs give the same wrong indication after the pitots have been left covered from maintainance or hitting a flock of birds? Then the system assumes all are correct, incorrectly.
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DOZY
Some of your posts give real insight into AB FBW design of flight controls. off course it takes a software expert like you to understand it so well. According to me since it was conceived in the 80s Airbus wanted to keep it simple to use like a calculator but in a dynamic environment it is very difficult to decide how much is too much or too little. Since every one is not a software expert it leaves certain areas which are not well understood or for that matter cannot be completely understood which gives rise to fear resulting in suspicion of the systems and lack of faith. That is what I see in numerous posts.I had flown B707, B747, A300, A310 before I flew the A320 and I did enjoy the FBW.
Some of your posts give real insight into AB FBW design of flight controls. off course it takes a software expert like you to understand it so well. According to me since it was conceived in the 80s Airbus wanted to keep it simple to use like a calculator but in a dynamic environment it is very difficult to decide how much is too much or too little. Since every one is not a software expert it leaves certain areas which are not well understood or for that matter cannot be completely understood which gives rise to fear resulting in suspicion of the systems and lack of faith. That is what I see in numerous posts.I had flown B707, B747, A300, A310 before I flew the A320 and I did enjoy the FBW.
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I have not read all the replys, sorry.
The reason you select Flaps 1 below 20,000 ft and not above is that the aircraft is not certified for any flap or slat extension above 20,000ft.
It's that simple.
The reason you select Flaps 1 below 20,000 ft and not above is that the aircraft is not certified for any flap or slat extension above 20,000ft.
It's that simple.
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The odds of a birdstrike blocking all three pitot tubes simultaneously and with the same density is infinitessimally small, and in any case you'd notice a very definite change in the reading - even in the case of AF447 there was enough time gap inbetween the three pitot tubes clogging for the systems to detect UAS DISAGREE and drop to Alternate Law. Remember the systems are checking and cross-checking tens or hundreds of times per second.
Heh. I do (a much simpler kind of) Software Engineering for a living - I wouldn't call myself an expert by any means!
but in a dynamic environment it is very difficult to decide how much is too much or too little.
Since every one is not a software expert it leaves certain areas which are not well understood or for that matter cannot be completely understood which gives rise to fear resulting in suspicion of the systems and lack of faith. That is what I see in numerous posts.