Why are the outboard Krueger Flaps retracted before the others on the B747?
This is from memory and applies to 747-100/200 aircraft, which I last flew in 1999. The video shows, I think, a -400. However, to the best of my knowledge, it has the same flap settings.

The flap selection handle detents and resulting device positions are:


0&deg; &mdash; leading and trailing edge retracted
1&deg; &mdash; leading edge inboard of the engines extended, trailing edge up
5&deg; &mdash; all leading edge extended, trailing edge 5&deg;
10&deg; &mdash; all leading edge extended, trailing edge 10&deg;
20&deg; &mdash; all leading edge extended, trailing edge 20&deg;
30&deg; &mdash; all leading edge extended, trailing edge 30&deg;


At least one carrier, TWA, but maybe others, ordered aircraft with additional setting. I seem to remember 15&deg; and 40&deg;, but the above was the 'standard'.


  Why the staggered retraction?


On takeoff, it's all about changing the wing from a low-speed wind to a high-speed wing. Normal takeoff flap setting for the -100/200 was 10&deg;. I think it's 20&deg; for the -400 but I never flew them. As you retract the flaps (both leading and trailing edge), the minimum safe airspeed increases approximately 20 knots for each decrease in flap extension. That gave you a safe bank angle of 15&deg;. Add 10 more knots and you could, as I remember, bank 30&deg;.

So, in the video the airplane is accelerating, and flaps are being retracted in stages as it reaches the minimum safe airspeed for that stage.

I'm not knowledgeable enough to authoritatively explain why, if you want to change part of a swept wing to a lower-lift wing while leaving part of it as a higher-lift wing, you need to make the change to lower-lift from outboard to inboard, but I do know, at least I think, that you must avoid the possibility of the outboard wing stalling while the inboard is still flying. You need to retain outboard aileron control as long as possible. Thus, retract the outboard leading edge devices before the inboard devices.


  Does each leading-edge flap configuration have its own unique setting on the pilot's flap-control lever?


No, the leading edge flaps do not have a separate selection. They're tied to the trailing edge flap setting. Note that flaps 1&deg; doesn't actually put any trailing edge flaps down (although I seem to remember it took them out of the retract position and moved them backwards, but I may be wrong), but extends only two of the four leading edge flap sections.


  On approach, are the leading-edge flaps extended in a reverse staggered order?


Yes. On approach you're slowing, and you must not slow below the minimum safe speed for your flap setting. Those speeds will be lower than the respective takeoff speeds because you're lighter. I just looked up the fuel for a -400 that I have on my weight and balance web page, and it shows 359415 max fuel. Thus it's not unreasonable that you might be as much as 340000 lbs lighter than at takeoff and the minimum safe airspeed for a given flap setting could conceivably be in the range of 40 to 50 knots slower than at takeoff.

For those interested in more detail about the -400 leading edge devices, go to this incident report detailing an un-commanded retraction of the leading edge devices inboard of the engines. The single paragraph that most caught my attention is:


  The serious incident involved the un-commanded retraction of the automatic
  Group ‘A’ leading edge flaps on rotation for a period of about 23 seconds.
  Subsequent to the initiation of the retraction of the Group ‘A’ leading edge flaps, the aircrew was faced with unexpected stall warnings. The pilot flying was able to prevent the aircraft from stalling, with support from the other crew members and to keep the aircraft flying until the leading edge flaps re-extended and normal performance capability returned.