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Volaris A321 loses both ELACS

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  • #31
    Originally posted by 3WE View Post
    2. Sorry, I have not memorized cryptic references such as that.
    It's the part of the FAR that requires the plane to behave longitudinally stable, that is, things like to require a pull on the column to slow down, a push to speed up, and to return to the trim speed if you let go on the stick. That is, all the things that the Airbuses flagrantly fail to comply, EXCEPT when in DIRECT law.

    It is extremely likely that Airbus used the envelope protection as an alternative means of compliance giving "at least equal levels of safety" than the required stability. Unfortunately, when the system degrades from direct law to alternate law it keeps the lack of longitudinal stability but it looses a good bunch of envelope protections (especially if it reverts to abnormal alternate law, as would happen during a UAS event).

    That's why in the AF thread I was ranting that the airplane should revert from some law to either other law that keeps the envelope protections or to direct law where the airplane becomes longitudinally stable and behaves... well, like an airplane, and that a mode that eliminates the protections but retain the indifferent stability (stick-on-G) should be a no-no and should not have been approved.

    That was before we learnt that the pilot was "pulling up all the time" so they would have equally efficiently stalled a Piper Cub.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #32
      Originally posted by Evan View Post
      I don't think you're quite getting it.

      1) Read the FAR Title 14 § 25.171 and 25.173 and 25.175 for that matter.
      2) Read the A320 FCOM Chapter 1.27
      3) Realize, "Hey, according to the FAR's this thing ain't even airworthy!"
      4) Then read the history of how Airbus certified an aircraft that did not meet the criteria as described in the FAR's by offering alternate criteria.
      5) Notice that in the scenario where both ELAC's have failed and most of the envelope protections are lost, the aircraft no longer meets the alternate criteria either. Hence, from an airworthiness certification standpoint, it is now unsafe to operate as a transport category aircraft and must be landed ASAP.
      6) Tell me that is not a serious situation that should really never happen.
      7) Ask yourself, "so how the hell did this happen?" Then we're on the same page.
      Evan, don't exaggerate.
      Loosing an engine in a twin (which encompass 99.9% of the transport category planes today) also render the plane not airworthy and requires landing at the first suitable field. Is it serious? Sure. Serious is a relative word and you make it sound much more serious than it is. Do you know how many engines fail or are preemptively shut down in flight per week? Want to compare with the double ELAC fail or, for the matter, all the failure modes, combined, that make an Airbus revert to alternate law?

      I've never heard you cry so loud for engine failure, and boy that can also increase the workload and the probability of mistakes, especially if the crew is not very well trained.

      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
      --- Defend what you say with arguments, not by imposing your credentials ---

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      • #33
        Originally posted by Gabriel View Post
        Evan, don't exaggerate.
        Loosing an engine in a twin (which encompass 99.9% of the transport category planes today) also render the plane not airworthy and requires landing at the first suitable field. Is it serious? Sure. Serious is a relative word and you make it sound much more serious than it is. Do you know how many engines fail or are preemptively shut down in flight per week? Want to compare with the double ELAC fail or, for the matter, all the failure modes, combined, that make an Airbus revert to alternate law?

        I've never heard you cry so loud for engine failure, and boy that can also increase the workload and the probability of mistakes, especially if the crew is not very well trained.
        Gabriel don't exaggerate.
        I'm not 'crying so loud.' I'm simply saying this is a serious incident that deserves some attention. Losing an engine, while also serious, is to be expected and can usually be easily explained. What about losing two engines, because that is the analogy if you are trying to make one. Did both engines fail by coincidence? Is it wise to dispatch with one engine on the MEL? Or is there a common design vulnerability that could cause both engines to fail?

        The major difference between a control law degradation to alternate law in a scenario like UAS and this scenario is the permanent loss of autoflight. WIth UAS, you lose normal law protections for the remainder of the flight but after the speeds come back (a minute or two later) you can get the autoflight back and fly and land normally. Control law is irrelevant under autoflight and in the flare you are in direct law anyway. But here, you have a scenario where you must fly and land manually without protections, without speed stability 'feel' and without ailerons for that matter (which might alter the stick response at low speeds, I don't know). That's a serious situation, not a dire or unmanageable one, but ripe for degraded situational awareness and pilot error.

        But I'm done trying to explain why this is serious. My original intention was to speculate on how this could happen, not how serious it is. The only plausible explanations I have is a) that one ELAC was already MEL'd (that still makes this highly unlikely); or b) there is a common fault possibility in the electrical circuits.

        It also caught my attention because the AirAsia crash was at one point rumoured to involves a double FAC failure (still waiting for that report) which I also found nearly impossible unless one (or both) was manually shut down (and why would anyone do that?). Or was that flight dispatched with a single functioning FAC? It's all food for thought. And discussion, should that ever happen here...

        Comment


        • #34
          Originally posted by Gabriel View Post
          1. It's the part of the FAR that requires the plane to behave longitudinally stable

          2...some other stuff similar to what ELAW said...

          3....Evan, don't exaggerate...
          1 & 2: What did I miss from Eric's comments? The plane becomes "unstable"...unstable means a LOT of things, though...I'm reading that if you keep it well within a healthy envelope (cue my known, healthy attitude-speed-power comments), it remains fairly controllable.

          I get the fear...don't get near the edge of the envelope (you guys keep dropping that 'envelope' term) where any number of bad things might happen, and I would certainly be more comfortable knowing that HAL and his silicone friends are making a million mathematical calculations a minute to keep the plane straight and in the envelope, instead of that Mark IV carbon computer that is much slower at math, but can grasp that Pitch + Power = Performance concept and can resolve how to steer a DC-10 with no hydraulics...

          But again, "unstable" means so many things- Dutch roll that pilots can't stop, but which doesn't usually crash an airliner if the yaw damper is off...phugoids that gradually get worse (even though the pilot can hold the nose on the horizon to make it stop), planes where the turn develops into a spiral dive (even though the pilot can level the wings)...It also means that tail draggers want to ground loop (even though pilots can usually keep the plane straight)

          Unstable doesn't necessarily mean that the plane flips backwards and upside down a second after the control computer goes off and the plane gets a tiny bit sideways...

          I guess Evan has that fear that the plane becomes a rattlesnake ready to tumble out of control, and ATL-crew (and perhaps you) knows (feels) that it just means you have to work a little more at keeping the plane pointed the right way and that you probably do want to keep things in the fat, dumb, happy and healthy envelope (which is an envelope of some size and not a coffin corner)?

          Do I have it, or not Gabe/ATL/Eric?
          Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

          Comment


          • #35
            Originally posted by Evan View Post
            Gabriel don't exaggerate.
            I'm not 'crying so loud.' I'm simply saying this is a serious incident that deserves some attention. Losing an engine, while also serious, is to be expected and can usually be easily explained. What about losing two engines, because that is the analogy if you are trying to make one. Is it wise to dispatch with one engine on the MEL?
            That is a discussion stopper for me. I can't challenge that, or better, someone who says that.
            Ok Evan, you are neither exaggerating nor crying so laud. Loosing both ELAC is comparable with lossing bothe engines, and dispatching a plane with an ELAC out of service is comparable to do so with an engine out of service. If you say so.

            Have a good day.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

            Comment


            • #36
              Originally posted by 3WE View Post
              1 & 2: What did I miss from Eric's comments? The plane becomes "unstable"
              First of all, replace "unstable" for "indifferent longitudinal stability", that means neither stable or unstable.

              Second, it is not that it becomes stability indifferent. It IS stability indifferent in normal law.

              Longitudinal stability is defined in terms of the tendency of the plane to return to its trimmed speed and AoA.

              The FAR requires that, if you are trimmed for 200 kts, you need to keep a pull up force to fly slower (without changing the trim) and you need to keep a push force to fly faster. Farther, it requiree that, in both cases, the plane returns to its originally trmmed speed if you let go on the controls.

              Indirectly, this also means that the plane must pitch down by itself if losses speed and pitch up by itself if it gains speed.

              The Airbus, as any commercial airliner (and 99.99% the rest of the planes too) complies with that from an aerodynamic point of view. But Airbus enured to spoil that by introducing a computer that activelly controls the plane to behave in a different way.

              In an Airbus, if you are flying straight and level and steadily at 200 kts suddenly, for any reason, it gains 50 kts, the pkane will not pitch up and slow down back to 200 kts. Instead, it will pitch dosn and hold the altitude, and the same but opposite will happen if it looses 50 kts. If you intentionally slow down (by pulling back on the throttles) you don't need to pull up to hold the slower speed. The active controls will do that by themselves. If you cut the throttles to idle, the plane will not pitch down. Onstead, it will first pitch up to increase the AoA and keep the altitude, then it will keep the pitch constant (since the control law response transitions from stick-on-G at high speed to pitch-rate-on-stick at slow speeds), and it would happily hold the pitch while loosing speed all the way to the stall if it wasn't for the slow speed and high AoA envelope protectionsm, which will first increase thrust, then if the condition keeps deteriorationg will change the control law to direct law (so now you need to keep increasingly up to keep increasing the AoA, just like in a normal plane, and then will command the elvator as necessary to avoid that you pull up past the stall AoA, even if you caressly and relentlessly pull fully up for 5 minutes.

              So, as you can see, the Airbuss don't comply with the FAR's longitudinal stabilty requirements, not in normal law at least. So they applied for an alternative means of compliance and for that they needd to demonstrate how the combination of the normal law with the envelope protections gave a level of safety that was at least as good as the required stability conditions. Which I have no doubt they do. Boy, you can't stall, you can't roll past 60 degrees, you can't overspeed, you can't overload... That is VERY safe... as long as the normal law is in place.

              When the plane looses some systems it reverts to alternate law. In this condition, depending on the kind of failures, the envelope protections are significantly degraded or eliminated alltogether. But the problem, for me unacceptable, is that the plane still lacks longitudinal stability because the basics of the normal law (stick-on-G / stick-on-pitch-rate) remain in place.

              You need to loose much more systems for the plane to revert to direct law (in this case the analog stick bypasses all the digital computers, just an analog computer, that has no input other than the stick, controls the surface controls proportinally to the stick deflections) and the plane "becomes" longitudinally stable (by removing the computers that artificially made it indifferent) and it starts to behave like a Cessna 152 (no envelope protections and your only protection against the stall are the longitudinal stability, a stall warning, and your stick-and-rudder-skills).

              Note that in alternate law, especially in abnormal alternate law, you also have the stall warning and whatever airmanship you have, but you don't have the longitudinal stability, so it becomes easier to unadvertelly stall the plane if you don't use special cautions BEYOND what is needed to avoid stalling a Cessna (for example, you don't need to puch increasingly up to keep increasing the AoA all the way to the stall, and you don't have AoA and speed force feedback on the stick).

              Loosing both ELACs puts the plane in alternate law. Hence the level of "seriousness" that Evan claims. But for me the root cause of the "seriousness" is not the loss of the ELACs but the control laws philosophy, as described above. There are A LOT of conditions that will make the plane revert to alternate law, and lossing both ELAC is not the most common of them (rather the oppsoite).
              Do I have it, or not Gabe/ATL/Eric?
              You didn't. Do you now?

              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
              --- Defend what you say with arguments, not by imposing your credentials ---

              Comment


              • #37
                Originally posted by Gabriel View Post
                ...You didn't. Do you now?...l
                Actually I think I DO get it but you (the splitter) won't be happy until I (the lumper) start splitting.


                Lump 1: All those ELAC computers and related systems keep the plane flying super nicely...

                Lump 2: Many of us assume (incorrectly) that if they computers go down you get a plane that's very hard to fly.

                Lump 3: ATL said (paraphrased) "all you had to do was land the thing, do some paperwork and then hit the coffee shop."

                More lump 3: And, in your continued attempt to get me to understand- I keep getting the lumped message that while you lose lots of fancy computer-stabilized flight inputs, you still get something that can be controlled.

                More lump 3: Eric pointed out what the FAR was about.

                Bottom line lump: Sure, it's important- I think we agree with Evan there, and maybe we don't want to stall or exceed the big $20 word "Envelope" the thing with the computers off (not that we want to stall it or exceed the "Envelope" with the computers on either)

                Briefer bottom line lump: But it sounds to me from your posts and others, that dual ELACS failure is not a NEAR TOTAL AIR DISASTER!!!!!!
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                Comment


                • #38
                  Originally posted by Gabriel View Post
                  ...Note that in alternate law, especially in abnormal alternate law, you also have the stall warning and whatever airmanship you have, but you don't have the longitudinal stability, so it becomes easier to unadvertelly stall the plane if you don't use special cautions BEYOND what is needed to avoid stalling a Cessna (for example, you don't need to puch increasingly up to keep increasing the AoA all the way to the stall, and you don't have AoA and speed force feedback on the stick)...
                  Blah blah blah...

                  Yeah, it's nice to have my 172 trimmed for healthy flight so that it "fights me" if I try to stall it, and there's the evil insidious Airbus that will pitch up, but not give me tactile feedback until the stall warning goes off. Shall I place links to ITS going over the top with trashing Airbus and Frenchmen?

                  So, I'm back to one of my endless, repetitive rants:

                  Select known, healthy power, attitude and speed settings and (do I have to say this- because it WAS implied) keep monitoring the airspeed and attitude and power (see footnote) to see that the airspeed and attitude stay at those healthy settings ...

                  ...and then you will catch the plane when it starts going to the wrong attitude even though the plane doesn't give tactile feedback (and even though a Cessna would not go to the wrong attitude without tactile feedback)

                  Footnote: To the best of my knowledge, most aircraft provide gauges (or flat panel readouts) to tell you about the air speed and the attitude and the power settings.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                  Comment


                  • #39
                    Originally posted by Gabriel View Post
                    That is a discussion stopper for me. I can't challenge that, or better, someone who says that.
                    Ok Evan, you are neither exaggerating nor crying so laud. Loosing both ELAC is comparable with lossing bothe engines, and dispatching a plane with an ELAC out of service is comparable to do so with an engine out of service. If you say so.

                    Have a good day.
                    I get the sense there are two opinions in Evan's world. One is his, and the other is wrong. I don't see how else to explain it when a guy current and qualified in the type in question tells him a dual ELAC failure is not that big a deal, he drones on with pagefuls of "yes, it is!"

                    Comment


                    • #40
                      Originally posted by ATLcrew View Post
                      I get the sense there are two opinions in Evan's world. One is his, and the other is wrong. I don't see how else to explain it when a guy current and qualified in the type in question tells him a dual ELAC failure is not that big a deal, he drones on with pagefuls of "yes, it is!"
                      I disagree and think you are downplaying the paperwork and I wish to have a flame war with you on the subject!
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                      Comment


                      • #41
                        Originally posted by Gabriel View Post
                        Ok Evan, you are neither exaggerating nor crying so laud. Loosing both ELAC is comparable with lossing bothe engines, and dispatching a plane with an ELAC out of service is comparable to do so with an engine out of service. If you say so.
                        What if I didn't say so? I said that losing both engines is as unlikely and hard to explain as losing both ELAC's. I never said it was as serious. Go back and read the post. It's pretty clear what I meant. Jesus, Gabriel, you were one of the few here that didn't twist my words around.

                        But enough on that. It's serious and you have admitted this as well. Done on that.

                        Now to untangle that Gabrielesque post...

                        In an Airbus, if you are flying straight and level and steadily at 200 kts suddenly, for any reason, it gains 50 kts, the pkane will not pitch up and slow down back to 200 kts. Instead, it will pitch dosn and hold the altitude, and the same but opposite will happen if it looses 50 kts. If you intentionally slow down (by pulling back on the throttles) you don't need to pull up to hold the slower speed. The active controls will do that by themselves. If you cut the throttles to idle, the plane will not pitch down. Onstead, it will first pitch up to increase the AoA and keep the altitude, then it will keep the pitch constant (since the control law response transitions from stick-on-G at high speed to pitch-rate-on-stick at slow speeds), and it would happily hold the pitch while loosing speed all the way to the stall if it wasn't for the slow speed and high AoA envelope protectionsm, which will first increase thrust, then if the condition keeps deteriorationg will change the control law to direct law (so now you need to keep increasingly up to keep increasing the AoA, just like in a normal plane, and then will command the elvator as necessary to avoid that you pull up past the stall AoA, even if you caressly and relentlessly pull fully up for 5 minutes.
                        No. Wrong.

                        Once you have lost Normal Law, AoA protections and Alpha Floor...

                        If you gain enough airspeed to exceed VMO/MMO the nose will come up. This is overrideable with increased stick force. This is true in Alternate law with reduced protections. This is true when you lose both ELAC's.

                        If you lose enough airspeed to approach stall (5-10 kts above stall warning), the nose will lower. This is overrideable with increased stick force. This is true in Alternate law with reduced protections. This is true when you lose both ELAC's.

                        Neither the certification authorities nor Airbus are the insane, irresponsible folks you make them out to be. That's what I'm getting at. But that certification was predicated on having certain systems, such as an ELAC, in working order.

                        UAS is a different animal. Without airspeeds you can't have ANY speed protections and that is far more immediately dangerous. That is why the procedures exist to maintain the airspeed envelope. That is why one of them has the distinction of being a memory item (except at Air France). That is why the first order of business when speeds return is to get it back on the automation. I have already agreed with you that UAS is a more dangerous situation on a airplane with automatic pitch trim and thus no out-of-trim 'feel' for speed changes. But UAS is one of those situations technology cannot yet contend with. It requires disciplined, well-trained pilots to fly safely through it.

                        What baffles me is that you find the control laws philosophy alarming but not the fact that you can apparently lose both ELAC's in flight to a common fault. But that is because you have the control laws philosophy wrong...

                        Comment


                        • #42
                          Originally posted by Evan View Post
                          What baffles me is that you find the control laws philosophy alarming but not the fact that you can apparently lose both ELAC's in flight to a common fault.
                          How do you know they were lost due to a common fault? There's not even a prelim on this out yet.

                          Comment


                          • #43
                            Now..., back on the subject of how and why, after finding some time to do a bit of research I found this on an incident report for a Tiger Airways A320 that experienced a ELAC 1 fault back in 2009:

                            An intermittent fault in ELAC1, though it could not be reproduced on the incident aircraft, was known by the aircraft manufacturer which caused the transfer of primary control from ELAC1 to ELAC2. A non-mandatory service bulletin (SB) had been issued in June 2006 providing a solution to the problem. This SB had not been incorporated in VH-VNC.
                            The incident became more serious because one of the ELAC2 back-up servos was mis-adjusted and caused serious vibration due to aileron occillation. When ELAC2 took over control, the problem became apparent. THe crew cycled ELAC 1 and got it back online and the problem thus stopped.

                            Possibly the Volaris aircraft was also not modified. But that doesn't explain the loss of ELAC2 unless there was also a latent mechanical issue that would cause it to go offline. An Alitalia A320 had this happen in 2013 when a stablizer jam (common fault) occurred.

                            I also found this: A320, 2011:

                            We experienced a double elevator and aileron computer (elac) failure on the arrival phase. Approximately 8 miles west of the airport descending to 6;000 ft; the autopilot kicked itself off. As the flying pilot; I took control of the aircraft and began to manually fly. We then realized that the failure was due to the elac ECAM. The first officer read the messages off the ECAM. We were in 'alternate law' without an autopilot we had an elac fault and an elac pitch trim failure in both elac's. Note: the ECAM did not give us the 'blue to do items' it just gave us the problem; vref + 10; direct law with gear down info. The first officer requested an extended downwind and declared an emergency [note: I think we can assume that they considered this a serious situation]. The irregular procedure in the flight manual was then referenced and read by the first officer. When the elac switches were cycled we regained the functionality of both elac's. We then finished up the action items per the manual and took a turn back toward the airport. Landing and taxi in was uneventful.
                            The Volaris report does not indicate that they were able to get the ELAC's back despite working the checklists.

                            I've also found that a common control surface mechanical issue can cause ELAC PITCH FAULT 1+2 messages. The AV Herald report might have misinterpreted these for full system faults. Also voltage spikes due to wear in the sidestick can cause intermittent ELAC faults but not necessarilly throw them offline.

                            Comment


                            • #44
                              Originally posted by ATLcrew View Post
                              How do you know they were lost due to a common fault? There's not even a prelim on this out yet.
                              Well, the alternative is a coincidence, right? That's a hell of a coincidence, but it's not entirely impossible....

                              Comment


                              • #45
                                Another alternative was that one was already MEL'd, remember? You know, the thing you just wrote two pages of bloody murder on?

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