Why such a small Krueger flap on 777-300?

Question

Why is there such a small Krueger flap just inboard of the engine pylon on the Boeing 777-300, when the rest of the wing (including the further-inboard section of the wing) has leading edge slats?

I don't see an equivalent Krueger flap outboard of the pylon, unless there is one but can't be seen in this video There isn't a Krueger flap outboard of the pylon, according to this image from Wikipedia.

Refer to this video (flap extension beginning at 2:35).

Screenshot from the linked video with Krueger flap circled.

Answer 1

Due to wing sweep the engine nacelle blocks airflow that would feed a slot in that location, so the Kreuger flap is used instead. Outboard the pylon there is no such problem.

ref

Answer 2

It has to do with the slat–strut sealing; a photo from the other side shows the issue better (images below).

A normal slat would create a hole since the 777 engines are "close coupled"; this does not affect the lift (since there's a chine), but it affects the drag.

Boeing's solution to this is that tiny Krueger, which they call a "Seal Krueger" as it seals the hole, and with three engine manufacturers for the first generation 777, each manufacturer got a unique Krueger for a perfect fit.

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Source:
Horse's mouth (Boeing's engineers):
- Nield, B. N. "An overview of the Boeing 777 high lift aerodynamic design." The Aeronautical Journal 99.989 (1995): 361-371. p. 367. https://doi.org/10.1017/S0001924000028670

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Images:

Nield, B. N.

Jerry Pang