Do NOT arm the autobrake before ditching?

[NGjockey]

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.

:confused:

[victorpapa]

Wet contaminated ‘surface‘ to prevent hydroplaning 😉
Sorry being silly I would like to know reason too

[B-HKD]

It is solely mentioned to provide consistency with the checklist format.

[flightleader]

Pretty obvious, no reason to arm the autobrake as you would ditch with landing gear up mate.

[lomapaseo]

Pretty obvious, no reason to arm the autobrake as you would ditch with landing gear up mate.


How does arming it caused harm?

[zlin77]

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.

[Compton3fox]

How does arming it caused harm?
We don't know but that, I think, was the purpose of the question.

[BBK]

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.

[Dave Therhino]

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.

[tcasblue]

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.

The can definitely be reasons for ditching with engine power, so best not to presume that.

[Dave Therhino]

The can definitely be reasons for ditching with engine power, so best not to presume that.

I was talking about the engineering basis for that step in the procedure, not presuming that all ditchings are engine out

[medod]

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.

Why would the brakes start actuating with the gear up and locked?

Does arming the autobrake system consumes hydraulic power?

[Dave Therhino]

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.

[Pugilistic Animus]

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.

What about hydraulic pressure from windmilling, in order to keep the flight control circuits pressurized?

[tdracer]

What about hydraulic pressure from windmilling, in order to keep the flight control circuits pressurized?
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.

[Pugilistic Animus]

Thanks TdRacer for clarifying that for me. I think that I read Dave Therino's post wrong also but now it's completely clear to me
thanks Gentlemen

[tcasblue]

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.

I feel a little silly for not knowing this but.....how does arming the autobrake increase hydraulic demand? Locked wheel protection?