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  • Gabriel
    replied
    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.

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  • Evan
    replied
    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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  • Gabriel
    replied
    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.

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  • ATLcrew
    replied
    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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  • Evan
    replied
    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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  • ATLcrew
    replied
    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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  • Evan
    replied
    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).

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  • Gabriel
    replied
    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)

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  • Evan
    replied
    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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  • ATLcrew
    replied
    Originally posted by Evan View Post

    The law should remain in Normal Law as long as the RAT-powered blue system is functioning.
    Incorrect. If we're on RAT power only, we're in alternate law. (Ref AFM, EMER-24, 1/4, revision 1/19/2021).

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  • Evan
    replied
    (Also, in the case of Cactus 1549, I think one engine was still running and providing hydraulics and electrical.)

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  • Evan
    replied
    Originally posted by Gabriel View Post
    But then it will aslo provide hydro power to the elevator, rudder and ailerons, which ARE essential.
    Yes, but the RAT already has those things covered, so the APU is not needed there.

    Or did it? [Vsauce mystery music]

    Will the plane maintain normal law with RAT essential electric / hydraulic power only?
    Normal law might have been quite a factor in effecting the the 'miraculous' outcome of Cactus 1949.
    The law should remain in Normal Law as long as the RAT-powered blue system is functioning.

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  • Gabriel
    replied
    Originally posted by Evan View Post
    Of course the APU will provide electrical power which will power the electric hydraulic pumps on the blue and yellow systems, and that will provide things like additional spoiler panels, the THS, reversers, parking brake... things that aren't going to be helpful in the situation we are discussing
    But then it will aslo provide hydro power to the elevator, rudder and ailerons, which ARE essential.

    The point being: starting the APU did not effect the 'miraculous' outcome of Cactus 1949.
    Or did it? [Vsauce mystery music]

    Will the plane maintain normal law with RAT essential electric / hydraulic power only?
    Normal law might have been quite a factor in effecting the the 'miraculous' outcome of Cactus 1949.

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  • Evan
    replied
    Originally posted by Gabriel View Post

    I agree with that.

    (I thought that APU provided also hydraulic power, either directly with a hydraulic pump or indirectly via an electrical backup pump, I know that to be the case in some types. Engines and APU can be easily lost in a single-point-of-failure, like stupidly running out of fuel like Air Canada, running out of fuel do to a leak like I think it was Air Transat, or fuel contamination. But all engines can also fail due to a common point of failure leaving an operable APU, like volcanic ashes, bird ingestion, uncontained failure on one engine hitting the other one, or shutting down the wrong engine, so I think that not giving the APU the capability to provide hydraulic power directly or indirectly is an unnecessary weakness. You said "Lose both engines, the APU and the RAT and you are officially having the worst day ever in the history of automated flight", but losing both engines and the RAT and having a perfectly working APU doesn't leave you in a much better day according to your description, you cannot still control the plane at all, not even emergency backup, until you can bring the APU online and succeed at relighting one of the failed engines, which is many times impossible because typically whatever made them fail in the first place it is still there).
    Yes, I meant the APU does not have an engine-driven hydraulic pump. Of course the APU will provide electrical power which will power the electric hydraulic pumps on the blue and yellow systems, and that will provide things like additional spoiler panels, the THS, reversers, parking brake... things that aren't going to be helpful in the situation we are discussing, as well as full AC power for non-essential stuff. The RAT and the batteries will provide all the AC and DC power you will need for flight control. Actually, the batteries alone provide AC power via a static inverter although I doubt it is enough to power the electric hydraulic pumps. But like I said, not needed, and that would be the worst day ever.

    The point being: starting the APU did not effect the 'miraculous' outcome of Cactus 1549.

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  • Gabriel
    replied
    Originally posted by Evan View Post
    This is why future aircraft should move to EHA or EMA flight control and away from centralized hydraulic systems. Any fully-automated transport aircraft I could even begin to consider would have to have redundancy involving autonomous flight control actuators.
    I agree with that.

    (I thought that APU provided also hydraulic power, either directly with a hydraulic pump or indirectly via an electrical backup pump, I know that to be the case in some types. Engines and APU can be easily lost in a single-point-of-failure, like stupidly running out of fuel like Air Canada, running out of fuel do to a leak like I think it was Air Transat, or fuel contamination. But all engines can also fail due to a common point of failure leaving an operable APU, like volcanic ashes, bird ingestion, uncontained failure on one engine hitting the other one, or shutting down the wrong engine, so I think that not giving the APU the capability to provide hydraulic power directly or indirectly is an unnecessary weakness. You said "Lose both engines, the APU and the RAT and you are officially having the worst day ever in the history of automated flight", but losing both engines and the RAT and having a perfectly working APU doesn't leave you in a much better day according to your description, you cannot still control the plane at all, not even emergency backup, until you can bring the APU online and succeed at relighting one of the failed engines, which is many times impossible because typically whatever made them fail in the first place it is still there).

    Leave a comment:

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