Do NOT arm the autobrake before ditching?
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Joined: Jun 2009
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From: Worldwide
Do NOT arm the autobrake before ditching?
Here's a tricky one for the 747-400 whisperers:
Checklist item 4 in the DITCHING checklist in the B747-400 QRH states "DO NOT ARM THE AUTOBRAKE."
Does anyone have any idea for which technical reason this is stated in the checklist? On the face of it one would think that the autobrake setting is irrelevant in a ditching with the gear up, but if Boeing puts it in the checklist there must be a good reason for it.
Checklist item 4 in the DITCHING checklist in the B747-400 QRH states "DO NOT ARM THE AUTOBRAKE."
Does anyone have any idea for which technical reason this is stated in the checklist? On the face of it one would think that the autobrake setting is irrelevant in a ditching with the gear up, but if Boeing puts it in the checklist there must be a good reason for it.
Joined: Jan 2003
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From: Seoul/Gold Coast.....
I assume that most NNC are written by pilots/engineers then vetted by lawyers to make sure that it will stand up in court during the enquiry/inquest, to eliminate any doubt they might insert a statement which although obvious to most in the profession, could be manipulated in court to apportion/direct blame to innocent parties.
Joined: Feb 2001
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From: UK
Maintaining hyd pressure?
Without digging out the QRH I seem to recall there’s something in there about preserving hydraulic pressure system so that the gear doors stay closed which would help in a ditching.
Happy to be corrected if I’ve completely misremembered this! If in doubt just do what the QRH says as there may be system interactions that are not obvious. A lot of thought goes into these checklists.
Happy to be corrected if I’ve completely misremembered this! If in doubt just do what the QRH says as there may be system interactions that are not obvious. A lot of thought goes into these checklists.
Joined: Jan 2013
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From: Seattle Area
The presumption is you are ditching with no engine power, therefore no normal electric power, and therefore no hydraulic power generation as you drop below flight speed. You don't want to waste the remaining stored hydraulic energy on unneeded brake actuation when you may need it for flight control movement after the first bounce or two off the wave tops, or to try to keep the gear doors shut as stated by BBK above. I suspect the gear doors get ripped off pretty early in the process, though, in an open ocean situation with waves.
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From: unknown
The presumption is you are ditching with no engine power, therefore no normal electric power, and therefore no hydraulic power generation as you drop below flight speed. You don't want to waste the remaining stored hydraulic energy on unneeded brake actuation when you may need it for flight control movement after the first bounce or two off the wave tops, or to try to keep the gear doors shut as stated by BBK above. I suspect the gear doors get ripped off pretty early in the process, though, in an open ocean situation with waves.
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From: Seattle Area
Joined: May 2019
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From: UK
The presumption is you are ditching with no engine power, therefore no normal electric power, and therefore no hydraulic power generation as you drop below flight speed. You don't want to waste the remaining stored hydraulic energy on unneeded brake actuation when you may need it for flight control movement after the first bounce or two off the wave tops, or to try to keep the gear doors shut as stated by BBK above. I suspect the gear doors get ripped off pretty early in the process, though, in an open ocean situation with waves.
Does arming the autobrake system consumes hydraulic power?
Joined: Jan 2013
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From: Seattle Area
Arming the system does not use significant hydraulic power, and they would be armed on approach while the engines were still windmilling enough to produce enough hydraulic power for adequate flight control (if you were ditching with engines not running).
The autobrakes would not actuate unless a bunch of gear and wheel sensor mayhem occurred upon striking the water, but I believe this unlikely scenario is what Boeing was accounting for with that advice/step in the procedure. Given the logic for autobrake actuation, I agree it's very unlikely for autobrakes to be triggered.
The autobrakes would not actuate unless a bunch of gear and wheel sensor mayhem occurred upon striking the water, but I believe this unlikely scenario is what Boeing was accounting for with that advice/step in the procedure. Given the logic for autobrake actuation, I agree it's very unlikely for autobrakes to be triggered.
Joined: Dec 2006
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From: The No Transgression Zone
The presumption is you are ditching with no engine power, therefore no normal electric power, and therefore no hydraulic power generation as you drop below flight speed. You don't want to waste the remaining stored hydraulic energy on unneeded brake actuation when you may need it for flight control movement after the first bounce or two off the wave tops, or to try to keep the gear doors shut as stated by BBK above. I suspect the gear doors get ripped off pretty early in the process, though, in an open ocean situation with waves.
Joined: Jul 2013
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From: Everett, WA
The windmilling hydraulic pressure is small - barely enough to control the aircraft at approach speeds (particularly on the 747-400 - as the bypass ratio of the engines increase, the windmill N2 decreases everything else being equal). Slower speeds mean slower windmill mean lower hydraulic pressure. Hence anything you can do to reduce hydraulic demand during a dead stick is goodness.
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From: unknown
The windmilling hydraulic pressure is small - barely enough to control the aircraft at approach speeds (particularly on the 747-400 - as the bypass ratio of the engines increase, the windmill N2 decreases everything else being equal). Slower speeds mean slower windmill mean lower hydraulic pressure. Hence anything you can do to reduce hydraulic demand during a dead stick is goodness.
