Questions on 787 Undercarriage Doors and Humidity
 

Recent took my first pax trip in 787 and was lucky enough to chat with captain, a very approachable guy, in rear galley.
In the conversation he mentioned that the U/C doors automatically open after lift off and stay open for 60 seconds ready for gear retraction. I understood him to say that although there was a slight drag penalty, this is a performance requirement which I did not understand, but did not want to belabour. Could somebody please explain?
Also when discussing the cabin altitude he seemed to be saying that the aircraft carried water to raise the humidity in the cabin, did I understand this correctly?

One of the 787's USPs is the increased cabin humdity to improve passenger comfort (6-7% instead of the normal 2%).

I wonder what the Captain was doing in the rear galley...

The 787-9 MLG doors open at lift-off and close after gear retraction or 30 seconds (?) whichever is first. It's a way of reducing the time taken for the gear to retract in the event of EFTO, the thereby reducing the time spent in the 'first segment'.

The 787-10 has the same design. You can see this in the numerous videos of the -10's recent first flight. The doors open immediately and then close again after a few seconds (I think it's 30, not 60). As it's a first flight, the crew deliberately leave the gear down.

The 787-8 does not have this feature.

Some of the benefits of a composite structure include:
Lighter, so lower fuel burn;
Stronger for the same weight, so bigger windows possible;
Similarly, higher diff pressure possible;
No corrosion problems so more moisture possible (higher humidity)

Interesting, is an engine failure after take off that critical they'd need this unusual feature ?

Thread drift but to answer the question: No reason why he/she shouldn't be in the rear galley in the right circumstances -e.g a long flight where there might be a relief pilot and therefore each pilot gets some "time away from task" .

I certainly sometimes head there on the 777 if if I'm on a break, good for the circulation and also to see how it's going with the rear galley crew.

From what I hear, yes, if you’re using a double derate (fixed + AT)...

That doesn't make much sense, why use a reduced thrust setting or derate so 'large' that you'd compromise performance to the extent you need the above 'fix' ?

At a performance limited TOW (say due to an obstacle in the takeoff cone) then it's going to buy a little more weight that can be lifted...

What about wind shear after takeoff? The whole reason you don't raise the gear in windshear (even though it's creating a load of drag) is because first of all the gear doors have to open temporarily increasing drag. So I guess their procedure is different. Might as well raise the gear as the doors are already open?

Although that effect may be noticeable on some airframes, it’s really because in severe windshear you might get ground contact. If that happens with the gear retracted or in transit, the outcome is generally not as good as with the gear down and locked.

Considering that in a windshear encounter you’ll have TOGA power on all engines, a little bit of extra drag is probably not that relevant...