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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Originally posted by Gabriel View PostStarting the APU increases the chances of a good outcome.
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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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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?
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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.
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Originally posted by Gabriel View Post
Even with the APU running? (and no engine providing any kind of power)
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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Originally posted by Evan View PostThough 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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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).
Normal law might have been quite a factor in effecting the the 'miraculous' outcome of Cactus 1949.
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(Also, in the case of Cactus 1549, I think one engine was still running and providing hydraulics and electrical.)
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Originally posted by Gabriel View PostBut then it will aslo provide hydro power to the elevator, rudder and ailerons, which ARE essential.
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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Originally posted by Evan View PostOf 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
The point being: starting the APU did not effect the 'miraculous' outcome of Cactus 1949.
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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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).
The point being: starting the APU did not effect the 'miraculous' outcome of Cactus 1549.
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Originally posted by Evan View PostThis 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 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).
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