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 Screenshot from the linked video with Krueger flap circled.


2 Answers 2


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.

enter image description here



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.

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


Nield, B. N.

other side
Jerry Pang

  • $\begingroup$ The part about "each [engine] manufacturer had a unique Krueger [flap]" - interesting! (But surely that adds complexity to the design and manufacture of this aircraft.) $\endgroup$
    – pr1268
    Mar 9 at 21:04

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