B744 Ditching Checklist Autobrakes OFF?
 

Can anyone enlighten me why Boeing wants the Autobrakes OFF for a ditching on the B747-400?

The Ditching checklist in the QRH specifically requires the Autobrakes NOT to be armed (checklist item 4), and in the Deferred Items the Autobrakes need to be set to OFF. The ditching itself should be done with the gear up.

Anyone?

Perhaps they don't want water in the system..... :p

My guess. This is something to do with a system logic that is triggered by activation of auto braking this logic then being prohibitive towards a successful ditching.

That's me done.

I'd imagine it's more to do with certification. The autobrakes were only tested to a certain depth of water. I doubt wherever you opt to ditch was within the test parameters.

To prevent aquaplaning.

I’ll get my coat.

Let me reverse your question.
Why would you want the auto brakes ON? You wouldn’t.
Leaves the mystery as to why it’s even mentioned.
Not my best effort I admit but I’m guessing it has to do with air-ground logic.
The autobrake system allows a specific deceleration rate with info coming from the IRS....
Now I need to open a book ;)

Like some of the other checklists, the ditching checklist has deferred items on the "Descent Checklist" part. Since it takes the place of the normal descent checklist, they have to show something for an autobrake setting to fill in that spot to keep things the same. So "OFF" just take the place of whatever setting you might otherwise consider or reply to in normal operations.

In terms of system operations, I don't think that it makes a difference at all, if you were to select a brake setting.

I would agree, if it weren't for checklist item 4, which is NOT in the Deferred Items part. Number 4 specifically states "Do NOT arm the autobrake."

Why would Boeing stress that fact if it didn't make a difference systemwise?

I know the descent "PRALA" items are standard across most of the Boeing fleet, and since such different aircraft most likely utilize differing system logic, the setting of AUTOBRAKE OFF could be to keep the descent checklist consistent as Bigduke suggests.

bigduke6 is absolutely correct. There's no system consequence by selecting something other than OFF, it's simply there for checklist consistency.