A320 unreliable speed . . . only if you are a nerd like me
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A320 unreliable speed . . . only if you are a nerd like me
Hi all,
An interesting knowledge gap came up during my sim program at my new employer.
I have seen unreliable speed simulated several times throughout the years on the 320/330 but the exercise has always been the result of ADR faults resulting in loss of speed info (red speed tape on PFD)requiring pitch and power to continue safely flying. Not difficult.
Yesterday I went in to the sim armed with what I thought was sufficient knowledge to cope with the unrel speed exercise but I was wrong.
The instructor simulated blocked pitot tubes at 330 knots climbing thru F150 that went undetected by the ELAC’s and FAC’s. 2 ADR’s gave subtly false IAS and only one gave correct.
Of course the incorrect (Cap) side increased into the red and because the fault was undetected the aircraft remained in normal law and gradually pitched up despite our efforts to lower the nose. At no time did it fault the ADR’s and trigger alt law.
I was under the impression that the speed discrepancy would be detected immediately and the ELACS would trigger the NAV ADR disagree followed by altn law as per the wording of the FCTM.
But in this situation the only exit strategy would have been to switch all 3 ADR’s off almost immediately, BEFORE consulting the QRH. The event unfolded very quickly and my efforts to do the right thing by the QRH got us into trouble.
Can this really happen where a speed difference of 30-40 knots can be undetected and put the crew into a situation where normal law refuses you the ability to manage the pitch attitude?
Regards
An interesting knowledge gap came up during my sim program at my new employer.
I have seen unreliable speed simulated several times throughout the years on the 320/330 but the exercise has always been the result of ADR faults resulting in loss of speed info (red speed tape on PFD)requiring pitch and power to continue safely flying. Not difficult.
Yesterday I went in to the sim armed with what I thought was sufficient knowledge to cope with the unrel speed exercise but I was wrong.
The instructor simulated blocked pitot tubes at 330 knots climbing thru F150 that went undetected by the ELAC’s and FAC’s. 2 ADR’s gave subtly false IAS and only one gave correct.
Of course the incorrect (Cap) side increased into the red and because the fault was undetected the aircraft remained in normal law and gradually pitched up despite our efforts to lower the nose. At no time did it fault the ADR’s and trigger alt law.
I was under the impression that the speed discrepancy would be detected immediately and the ELACS would trigger the NAV ADR disagree followed by altn law as per the wording of the FCTM.
But in this situation the only exit strategy would have been to switch all 3 ADR’s off almost immediately, BEFORE consulting the QRH. The event unfolded very quickly and my efforts to do the right thing by the QRH got us into trouble.
Can this really happen where a speed difference of 30-40 knots can be undetected and put the crew into a situation where normal law refuses you the ability to manage the pitch attitude?
Regards
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Can this really happen where a speed difference of 30-40 knots can be undetected and put the crew into a situation where normal law refuses you the ability to manage the pitch attitude?
If you have totally blocked pitots and are still climbing, then the indicated airspeed will increase (due lower static pressure) and if overspeed protection activates then the aircraft will pitch up uncontrollably in Normal Law.
Don't be afraid to put the aircraft into ALT LAW to regain control (unless you are like the crew of AF447 or QZ 8501).
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i'm not sure if your scenario is possible in real a/c. However, two things should be noted here:
- Hi AoA protection has priority over high speed protection, so you shouldn't stall as long as you stay in Normal Law.
- switching off 2 ADR's is sufficient to force Alternate Law. While it's is not the procedure for UAS (it is for the spurious AoA port), I think it's quite justified in your scenario- after all, you have to stabilize the path first, then troubleshoot...
- Hi AoA protection has priority over high speed protection, so you shouldn't stall as long as you stay in Normal Law.
- switching off 2 ADR's is sufficient to force Alternate Law. While it's is not the procedure for UAS (it is for the spurious AoA port), I think it's quite justified in your scenario- after all, you have to stabilize the path first, then troubleshoot...
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Oicur12
Recently I had the same situation in a recurrent training sim. The only difference was the fault was set just after lift off. So after setting TOGA and 15 deg nose up, the airspeed rapidly indicated overspeed. ( and consequently triggered high speed protection). The situation unravelled very quickly. It was only in the debrief with the time to consider the scenario that I fully understood what happened. I think it surprised the instructor also. Must say I hope nobody has to face this in a real life situation.
Recently I had the same situation in a recurrent training sim. The only difference was the fault was set just after lift off. So after setting TOGA and 15 deg nose up, the airspeed rapidly indicated overspeed. ( and consequently triggered high speed protection). The situation unravelled very quickly. It was only in the debrief with the time to consider the scenario that I fully understood what happened. I think it surprised the instructor also. Must say I hope nobody has to face this in a real life situation.
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Had it in the sim as well after lift-off, simply switching 2 ADRs off forces alternate law. Completely lost the inexperienced FO though when I asked him to do this. So had to do it myself as pitch was going above 25 degrees with full downstick and 10000 ft/min climb. I realised the problem was the climb making the overspeed worse, but in normal law I could no longer level off. recovery in alternate law was easy.
This machine needs a big shiney red button to eliminate normal law.
Part of the problem was terrain that required a climb, but through the climb ADR 1 and 2 reached overspeed condition, which triggered the protection. this made us climb faster thus creating a bigger "overspeed" and thus a greater reaction from the protection. Making the situation worse.
Instructor was surprised I could not lower nose with full downstick in normal law.
We tried the same scenario without switching ADRs off just to see what could happen, outcome wasn't pretty. Would have hit the ground at 12000 ft per minute if it wasn't for sim freeze.
Anytime the aircraft normal law goes weird just kill 2 ADRs and fly pitch power. It's not just for spurious aoa vanes.
This machine needs a big shiney red button to eliminate normal law.
Part of the problem was terrain that required a climb, but through the climb ADR 1 and 2 reached overspeed condition, which triggered the protection. this made us climb faster thus creating a bigger "overspeed" and thus a greater reaction from the protection. Making the situation worse.
Instructor was surprised I could not lower nose with full downstick in normal law.
We tried the same scenario without switching ADRs off just to see what could happen, outcome wasn't pretty. Would have hit the ground at 12000 ft per minute if it wasn't for sim freeze.
Anytime the aircraft normal law goes weird just kill 2 ADRs and fly pitch power. It's not just for spurious aoa vanes.
Last edited by 737Jock; 5th Dec 2015 at 00:00.
Thread Starter
Thanks for the feedback folks, yes nideed it was a learning experience, albeit a slightly embarrasing one.
"This machine needs a big shiney red button to eliminate normal law"
I agree.
And a big red button that turns off auto trim on final when its gusty!!!!!
cheers
"This machine needs a big shiney red button to eliminate normal law"
I agree.
And a big red button that turns off auto trim on final when its gusty!!!!!
cheers
A few people have argued for the 'big shiny red button' I think having three ADR buttons, neatly out of the way is just fine. I think we all realise that this is not quite the way Airbus intended things however should the above sort of situation start to unfold then the PIC or their delegate would be well within their rights to switch off two ADR's as per the OEB, just don't switch off the IRU's by mistake!
The reason that increasing numbers of aircraft have flight envelope protections (starting with stick pushers, decades ago) is to protect against inadvertent approach to potentially catastrophic flight regimes. In general, pilots may encounter the flight envelope protections because:
1. They have lost awareness of the aircraft's attitude, flight path and/or energy state for human factors reasons or incorrect flight data, or
2. Upset due to an external influence.
In Case 1, de facto the pilot probably does not understand the situation. Providing a 'shiny red button' to immediately disable flight envelope protections may very well be the last thing that we should provide, because it is much more likely that those protections are working correctly than it is that they are the cause of the event.
The AF447 report is required reading to those of us flying advanced aircraft. But so is the report into the recent RAF A330 Voyager tanker upset, caused by a loose object pushing the stick full forward. In the latter case, the captain, at a loss to understand the behaviour of the aircraft, considered switching off the ADRs. However, the high speed protection was active at that moment and protecting the aircraft. Had the ADRs been switched off, or a 'shiny red button' been used, the aircraft would almost certainly have been lost due to grossly exceeding design speeds.
Be careful what you wish for, and remember that those devising emergency procedures and OEBs had much more time and information to come up with the best response to such events than a pilot would have in the unlikely event of such a malfunction.
1. They have lost awareness of the aircraft's attitude, flight path and/or energy state for human factors reasons or incorrect flight data, or
2. Upset due to an external influence.
In Case 1, de facto the pilot probably does not understand the situation. Providing a 'shiny red button' to immediately disable flight envelope protections may very well be the last thing that we should provide, because it is much more likely that those protections are working correctly than it is that they are the cause of the event.
The AF447 report is required reading to those of us flying advanced aircraft. But so is the report into the recent RAF A330 Voyager tanker upset, caused by a loose object pushing the stick full forward. In the latter case, the captain, at a loss to understand the behaviour of the aircraft, considered switching off the ADRs. However, the high speed protection was active at that moment and protecting the aircraft. Had the ADRs been switched off, or a 'shiny red button' been used, the aircraft would almost certainly have been lost due to grossly exceeding design speeds.
Be careful what you wish for, and remember that those devising emergency procedures and OEBs had much more time and information to come up with the best response to such events than a pilot would have in the unlikely event of such a malfunction.
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ExV238
Suggest you stay well away from B777 & B787 aircraft because they have a "shiny red button" (called PFC disconnect switch on the overhead panel in the flight deck) and the aircraft design utilises envelope protection in all of its functionality rather than envelope limiting. (see para 11.3 for an explanation http://www.davi.ws/avionics/TheAvion...ook_Cap_11.pdf
Providing a 'shiny red button' to immediately disable flight envelope protections may very well be the last thing that we should provide, .....
Had the ADRs been switched off, or a 'shiny red button' been used, the aircraft would almost certainly have been lost due to grossly exceeding design speeds.
Had the ADRs been switched off, or a 'shiny red button' been used, the aircraft would almost certainly have been lost due to grossly exceeding design speeds.
Situational Awareness
ExV238 has just summed up what appears to me to be a major problem with today's pilots: They are entirely to content to let the airplane do what the computers tell it to do and they do not recognize when they are telling it to do the wrong thing. AF447 is probably the ultimate example of this.
Goldenrivett,
Indeed, the PFC switch is in an equivalent position to the ADR buttons in the A320/330. However, the point is that the Boeing would not have been preventing an over speed in an upset such as the one I mentioned and might have exceeded VD regardless of the status of the flight control system. Therefore, the use and effect of operating that switch in the protection regime is very different. This is a philosophical difference between Airbus and Boeing that has been thrashed around for years on this forum and elsewhere.
However, the 'protection' and 'limitation' terms are misnomers. Both designs provide protection In different forms, but the Airbus does not limit the operational flight envelope (despite what some here might say!)
Whilst I'd love a go in the 777/787 for comparison (although I've flown the sim and tried the protections), I remain to be convinced that it's superior to my 'Bus!
Indeed, the PFC switch is in an equivalent position to the ADR buttons in the A320/330. However, the point is that the Boeing would not have been preventing an over speed in an upset such as the one I mentioned and might have exceeded VD regardless of the status of the flight control system. Therefore, the use and effect of operating that switch in the protection regime is very different. This is a philosophical difference between Airbus and Boeing that has been thrashed around for years on this forum and elsewhere.
However, the 'protection' and 'limitation' terms are misnomers. Both designs provide protection In different forms, but the Airbus does not limit the operational flight envelope (despite what some here might say!)
Whilst I'd love a go in the 777/787 for comparison (although I've flown the sim and tried the protections), I remain to be convinced that it's superior to my 'Bus!
Ed,
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sorry but I think you misunderstand. Unreliable airspeed does not trigger the OEB actions.
But the protections will happily stall the aircraft into an unrecoverable state in case of an unreliable airspeed which senses an overspeed on 2 ADR's.
Unreliable airspeed and you think it's a brilliant idea to just shut down 2 ADR's completely at random losing all the airspeed data.
I'll have that shiny red button any time of the day.
That A330 captain could have turned off 2 ADR's, but he didn't. Why would he have pushed the shiny red button?
In any case it would be a guarded switch that requires confirmation of both pilots, versus one pilot just electing to switch off some ADR's........
But the protections will happily stall the aircraft into an unrecoverable state in case of an unreliable airspeed which senses an overspeed on 2 ADR's.
Unreliable airspeed and you think it's a brilliant idea to just shut down 2 ADR's completely at random losing all the airspeed data.
I'll have that shiny red button any time of the day.
That A330 captain could have turned off 2 ADR's, but he didn't. Why would he have pushed the shiny red button?
In any case it would be a guarded switch that requires confirmation of both pilots, versus one pilot just electing to switch off some ADR's........
737,
If it's me you're responding to, then, with respect, I think you've misunderstood me! Perhaps I didn't make express myself very well. These are important points, so I'll answer them.
Note that I refer to emergency procedures AND OEBs; it may be thread drift (expansion?) but I was talking about more general flight control issues than just unreliable airspeed. My point is, indeed, that we need to be very careful about responding 'creatively' to an emergency by turning off ADRs/PFCs/stick pushers or whatever protection systems the aircraft has, in preference to following published procedures. There are echoes of the Staines Trident crash here; on that occasion the stick pusher was trying to protect the aircraft but was dumped, presumably because the crew's mental model of their predicament was wrong. Oh, and stick pushers were still relatively new technology and mistrusted by some.
My point re the A330 was that the pilot considered turning off the ADRs. On this occasion he didn't, but others in similar conditions might have done.
I know of no technical reason why a correctly loaded Airbus (or comparable Boeing, Embraer, etc etc) would enter an unrecoverable stall if the correct stall recovery actions are taken. Such things are tested in all flight control modes and wouldn't pass certification if this were so.
This is a really interesting subject, and there will always be differences of opinion on flight envelope protection. There are differences of approach across the manufacturers yet all of them are endorsed by the certification agencies, which shows that there's still a healthy opportunity for technology to develop and move forward.
If it's me you're responding to, then, with respect, I think you've misunderstood me! Perhaps I didn't make express myself very well. These are important points, so I'll answer them.
Note that I refer to emergency procedures AND OEBs; it may be thread drift (expansion?) but I was talking about more general flight control issues than just unreliable airspeed. My point is, indeed, that we need to be very careful about responding 'creatively' to an emergency by turning off ADRs/PFCs/stick pushers or whatever protection systems the aircraft has, in preference to following published procedures. There are echoes of the Staines Trident crash here; on that occasion the stick pusher was trying to protect the aircraft but was dumped, presumably because the crew's mental model of their predicament was wrong. Oh, and stick pushers were still relatively new technology and mistrusted by some.
My point re the A330 was that the pilot considered turning off the ADRs. On this occasion he didn't, but others in similar conditions might have done.
I know of no technical reason why a correctly loaded Airbus (or comparable Boeing, Embraer, etc etc) would enter an unrecoverable stall if the correct stall recovery actions are taken. Such things are tested in all flight control modes and wouldn't pass certification if this were so.
This is a really interesting subject, and there will always be differences of opinion on flight envelope protection. There are differences of approach across the manufacturers yet all of them are endorsed by the certification agencies, which shows that there's still a healthy opportunity for technology to develop and move forward.
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737 Jock and OP
Hi Guys,
Just so I can understand this a little better, I am reasonably new to type and this threw up some interesting scenarios.
So if my understanding is correct, after initially carrying out the memory drills "AP/FD Disconnect, A/THR disconnect, (in this case) CLB 5" because the pitots had blocked and therefore two ADR's were giving the same but incorrect information (aircraft overspeed) the Flight control protections kicked in and tried to force the nose of the aircraft upward as designed to exit the overspeed condition, but in this unique set of circumstances led toward a stall condition. The only way to get out of this was to switch off two ADRs thus forcing alternate law where manual control of pitch could be reestablished?
Thanks!
Just so I can understand this a little better, I am reasonably new to type and this threw up some interesting scenarios.
So if my understanding is correct, after initially carrying out the memory drills "AP/FD Disconnect, A/THR disconnect, (in this case) CLB 5" because the pitots had blocked and therefore two ADR's were giving the same but incorrect information (aircraft overspeed) the Flight control protections kicked in and tried to force the nose of the aircraft upward as designed to exit the overspeed condition, but in this unique set of circumstances led toward a stall condition. The only way to get out of this was to switch off two ADRs thus forcing alternate law where manual control of pitch could be reestablished?
Thanks!
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Hi RHS,
Affirm. When we did it in the sim, we had TOGA Lock (due Alpha Floor activation) and overspeed "protection" fighting each other.
A "big red switch" would have been most welcome rather than the loss of a perfectly serviceable Altimeter Source (with 2 ADRs off) in order to regain control.
The only way to get out of this was to switch off two ADRs thus forcing alternate law where manual control of pitch could be reestablished?
A "big red switch" would have been most welcome rather than the loss of a perfectly serviceable Altimeter Source (with 2 ADRs off) in order to regain control.
Last edited by Goldenrivett; 13th Dec 2015 at 11:17.
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Originally Posted by 737Jock
We tried the same scenario without switching ADRs off just to see what could happen, outcome wasn't pretty. Would have hit the ground at 12000 ft per minute if it wasn't for sim freeze.
Originally Posted by ExV238
But so is the report into the recent RAF A330 Voyager tanker upset, caused by a loose object pushing the stick full forward. In the latter case, the captain, at a loss to understand the behaviour of the aircraft, considered switching off the ADRs. However, the high speed protection was active at that moment and protecting the aircraft. Had the ADRs been switched off, or a 'shiny red button' been used, the aircraft would almost certainly have been lost due to grossly exceeding design speeds.
Once a friend of mine took delivery in Toulouse and asked the question why didn't they install a red guarded switch to force direct law and kill any interference ?
The answer was : "Some pilots may use it"
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PUD SOD
Pitot blocked speed under reads in the descent. Thus over reads in climb. So the higher you climb the greater the over speed error until the Airbus normal law over speed corrections kick in and pitch up the aircraft until it stalls. Usual scenario in SIM is on take off.
I love the QRH statement "level off for troubleshooting".
In my mind if the protections stop you levelling off to troubleshoot turn 2 ADR,s off set a sensible pitch/pwr setting. CONF 1,2,3.... 7.5/57% but if CONF 1+ F pitch about 3.5. That's what I have in the back of my head.
Easier said than done as startle factor huge.
Pitot blocked speed under reads in the descent. Thus over reads in climb. So the higher you climb the greater the over speed error until the Airbus normal law over speed corrections kick in and pitch up the aircraft until it stalls. Usual scenario in SIM is on take off.
I love the QRH statement "level off for troubleshooting".
In my mind if the protections stop you levelling off to troubleshoot turn 2 ADR,s off set a sensible pitch/pwr setting. CONF 1,2,3.... 7.5/57% but if CONF 1+ F pitch about 3.5. That's what I have in the back of my head.
Easier said than done as startle factor huge.
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Originally Posted by JosuaNkomo
Pitot blocked speed under reads in the descent. Thus over reads in climb. So the higher you climb the greater the over speed error until the Airbus normal law over speed corrections kick in and pitch up the aircraft until it stalls.