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  • #46
    Originally posted by ATLcrew View Post

    Incorrect. If we're on RAT power only, we're in alternate law. (Ref AFM, EMER-24, 1/4, revision 1/19/2021).
    I'm trying to figure out why this causes this reversion. On the face of it, only things that affect the integrity of the air data, IR data or logic should cause a loss of AOA protections. On closer inspection, losing the G & Y systems leaves you without any yaw damper (although the blue system still powers the rudder). I'm still not quite clear on why that should affect AOA protections.

    Normal law might have been quite a factor in effecting the the 'miraculous' outcome of Cactus 1949.
    Though not providing thrust, at least one engine should have been powering the G or Y system enough to keep Normal Law. If not, you will get the 'reduced protections' version of Alternate Law, which still provides low speed protections and provides a synthetic "STALL" warning.

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    • #47
      Originally posted by Evan View Post
      Though not providing thrust, at least one engine should have been powering the G or Y system enough to keep Normal Law. If not, you will get the 'reduced protections' version of Alternate Law, which still provides low speed protections and provides a synthetic "STALL" warning.
      Even with the APU running? (and no engine providing any kind of power)

      --- 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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      • #48
        Originally posted by Gabriel View Post

        Even with the APU running? (and no engine providing any kind of power)
        I suspect that the reason for the FBW law degrading to Alternate with only the blue system is that they felt they needed BOTH full logic redundancy and full control surface power redundancy to allow AOA protections. The reason for this probably goes very deep into system interdependencies and interactions. Perhaps the yaw damper is needed. I don't know.

        But the APU adds only electrical power and pneumatics to the situation. So, in theory, if that APU electrical power keeps you from losing the G or Y hydraulics, then it should keep you in Normal law. However, it's important to consider that you can't even start the APU until 45 seconds after the engines are lost and IDG's go offline, and during that time with no engine pumps running the G and Y systems will lose power and thus the law should revert to Alternate before the APU can be started. And once in Alternate, I don't think you can recover Normal law in the air.

        So I still don't see any benefit that an APU start would bring to the situation we are discussing (where both engine are too damaged for relight).

        Comment


        • #49
          Originally posted by Evan View Post

          And once in Alternate, I don't think you can recover Normal law in the air.
          You can, depending on what got you in Alternate law in the first place. The only law that is not "recoverable" is Abnormal Attitude law.

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          • #50
            Originally posted by ATLcrew View Post

            You can, depending on what got you in Alternate law in the first place. The only law that is not "recoverable" is Abnormal Attitude law.
            ATL, maybe you should answer Gabriel's question. The scenario is Cactus 1549-esque, both engines damaged beyond the possibility of relight, both IDG's are lost. What, if any, benefit will come from starting the APU that might effect the outcome of a controlled ditching?

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            • #51
              Originally posted by Evan View Post

              ATL, maybe you should answer Gabriel's question. The scenario is Cactus 1549-esque, both engines damaged beyond the possibility of relight, both IDG's are lost. What, if any, benefit will come from starting the APU that might effect the outcome of a controlled ditching?
              Well, I'd always much rather be on the APU than the RAT. At least all the screens will come back as will some more hydraulics. So, at least better situational awareness and better controllability, but as far as the physics of the ditching, I don't think it would help much.

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              • #52
                And, again, the plane remained in direct law, something that it would not have done if both engines were totally off and the APU was not online by then.
                Perhaps one of the engines kept producing enough hydraulic and electrical power, but even in that case you would not know if it would keep doing it all the way until touchdown.

                So, between keeping the full panel working on both sides of the cockpit, keeping full hydraulics for extra control and improved redundancy, and keeping direct law, can influence the outcome of the ditching as a whole (not just the touchdown). Starting the APU increases the chances of a good outcome.

                --- 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


                • #53
                  Originally posted by Gabriel View Post
                  Starting the APU increases the chances of a good outcome.
                  Of course. That is why it is part of the ENG 1 + 2 FAIL procedure (after relight attempts fail). But I don't see how it is critical to a good outcome. Sully ditched as PF. PNF was along for the ride at that point. RAT provides LH displays and powers critical control surfaces. More power is better, of course, but that isn't the discussion here.

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                  • #54
                    I just thought of another one. They extended the slats/flaps allowing for a slower touchdown. Would they have been able to do it with just the RAT and how long it would have taken?
                    And I am still wondering what would have happened if normal law had been lost.

                    --- 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


                    • #55
                      Originally posted by Evan View Post

                      There is also a 45-second delay from the time the IDG's go offline until the APU can be started...
                      Out of curiosity, where did this come from? There is nothing in my manuals that says that. Now, it takes 50 seconds to a minute for the APU to actually spin up, but I see nothing that says I have to wait 45 seconds to initiate the APU start sequence.

                      Comment


                      • #56
                        Originally posted by Gabriel View Post
                        They extended the slats/flaps allowing for a slower touchdown. Would they have been able to do it with just the RAT and how long it would have taken?
                        Yes, and longer than normal.

                        Comment


                        • #57
                          Originally posted by Gabriel View Post
                          I just thought of another one. They extended the slats/flaps allowing for a slower touchdown. Would they have been able to do it with just the RAT and how long it would have taken?
                          Ok this...

                          After the 'Miracle on the Hudson' there was a lot of forum discussion about how the APU start was needed to provide flaps.

                          First of all, Cactus 1549 lifted off with flaps 2 and ditched with flaps 2. No flap changes were made or needed. Therefore, the APU was not critical to the outcome.

                          Secondly, more obscurely, is the issue of flap operation with the RAT alone.
                          - According to my A320 FCOM and every other one I've seen, the RAT powers the Blue system and the Blue system also powers the flaps. Normally, the Blue system is electrically powered by the engines and provides 3000psi. WIth the RAT alone, it still provides 2500psi.
                          - According to the NTSB final report, the flaps are only powered by the Green and Yellow systems.

                          So which is true. It's hard to imagine different A320 builds would vary on this basic architecture.

                          My FCOM suggests that the RAT can provide flap extension, albeit slower to transit and will shed slats below a low pressure threshold via a priority valve (the A330 has a LAND RECOVERY switch to recover the shed slats shortly before landing).

                          Perhaps ATL can shed some light on this....

                          Comment


                          • #58
                            Originally posted by Evan View Post


                            My FCOM suggests that the RAT can provide flap extension, albeit slower to transit and will shed slats below a low pressure threshold via a priority valve (the A330 has a LAND RECOVERY switch to recover the shed slats shortly before landing).

                            Perhaps ATL can shed some light on this....
                            There is nothing in my manuals re slat shedding or priority valves.

                            Comment


                            • #59
                              Originally posted by ATLcrew View Post

                              Out of curiosity, where did this come from? There is nothing in my manuals that says that. Now, it takes 50 seconds to a minute for the APU to actually spin up, but I see nothing that says I have to wait 45 seconds to initiate the APU start sequence.
                              Unfortunately, I think that came from the FCOM I accidentally deleted while running a cleaner app on my very ill computer. It was deleting duplicate files so I must still have a copy but I also had to disable the finder indexing to keep it from overheating. I'm replacing it next week so hopefully I can find that FCOM again.

                              The issue had to do with electrical switching and some safeguard that prevent the APU generator from coming online for 45 secs. It's sort of redundant here anyway because if you follow procedure (Sully did not) and attempt a relight for 30 secs, wait 30 secs, and then attempt a second one before starting the APU, that's already well over 45 secs.

                              Comment


                              • #60
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                                Originally posted by ATLcrew View Post

                                There is nothing in my manuals re slat shedding or priority valves.
                                Is it possible that Airbus has changed the hydraulics architecture?

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