On several B77 photos, I notice that flaps just behind the engines are not always coordinated with the others flaps. On this photo flaps are extended but not behind the engine whereas on this photo all flaps are extended. Given the position of this particular flap, I suppose there is something to do with the interaction between flaps and engine. Given the difference between the two photos, it may also have something to do with spoiler extension.

EDIT: OK the two photos are not the same plane, but I find a video where it is obvious this piece of flap is not always coordinated with the rest of the flaps. When looking at the video I first thought it was a flaperon, but after the flaps are extended, it deflects upward several times without making the plane banking.

  • 2
    $\begingroup$ they are different models of planes, in the table below the first is a B777-333/ER while the second is a B777-222/ER, $\endgroup$ Commented Sep 30, 2014 at 10:10

3 Answers 3


It is called "inboard aileron" or "high-speed aileron" and is actually a flaperon. It will deflect with the flaps but not extend backwards. At high speed, the outboard ailerons are locked and all roll control is achieved by deflecting the inboard ailerons (and spoilers, if needed). This helps to reduce wing torsion and twist.

Background: Since the trailing edge's sweep angle has a break at the wing station of the engine, and since in Boeing aircraft the flaps extend perpendicularly to the hinge line, there must be a gap between inboard and outboard flaps, otherwise they would collide during extension. This gap is best placed in the station of the engine blast, and is filled with a simple hinged flap. Note that Airbus aircraft have rear-sliding flaps, which stay at a constant span location and can be adjacent to each other even at the wing station of the sweep change.

BAe Boeing 767 from below with flaps extended for take-off

Boeing 767 from below. Picture by Adrian Pingstone (source)

The picture is of a Boeing 767, but shows the flap arrangement nicely. Note the offset of the forward and rear part of the Küchemann bodies which house the flap tracks.

  • $\begingroup$ I found other videos where we can see both this flaperon and the outboard ailerons. Indeed it is coordinated with other roll control surface (aileron and spoilers) $\endgroup$
    – Manu H
    Commented Oct 4, 2014 at 15:10
  • $\begingroup$ I disagree with that statement. The 737 has flaps that extend to be completely flush with each other, despite the fact they have non co-linear hinge lines. Why not use the same design for the flaps as a 737 and place the flaperon more outboard to achieve better roll control? $\endgroup$
    – Jason
    Commented Jul 12, 2019 at 17:07
  • $\begingroup$ aviation.stackexchange.com/questions/66523/… $\endgroup$
    – Jason
    Commented Jul 12, 2019 at 17:22
  • $\begingroup$ You might find some answers on that here: aviation.stackexchange.com/questions/43093/… . $\endgroup$
    – Bram
    Commented Jul 12, 2019 at 17:27
  • $\begingroup$ What actually flaperon is used for? Some says that it is used for low speed (rather than high speed). Or, is it possible used for both? $\endgroup$ Commented Mar 17, 2022 at 2:54

To expand slightly on ratchet freak's comment, they're different models of plane, with different engines. The first photo is a 777-300ER with General Electric engines (the only kind fitted on -300ERs); the second is a 777-200ER with Pratt and Whitney engines (one of three engine options). Looking at other photos, it appears that the flaps directly behind the engines of -300ERs don't extend nearly as far as those on the rest of the wing.

And note that there are significant differences between the -200ER and -300ER. For example,

  • the fuselage of the -300ER is 10m longer and the wingspan is 4m longer;
  • the -300ER seats 70+ more people;
  • the maximum take-off weight of the -300ER is about 40 tonnes higher.

That small part on the 777 is called a flaperon because it behaves like an aileron and a flap. It is particularly active during the landing phase where it is controlled by a function called "LAM", Landing Attitude Modifier. During other phases of flights, it stabilizes the plane in the roll axis, this method is more efficient because it creates less drag, making the plane more fuel-efficient. When the flaperon are not sufficient to maintain roll spoiler panels are then commanded by the flight computer (PFC). That feature is a patent https://patents.google.com/patent/EP0809165A1 It is inactive during take-off below a certain speed (that I will not disclose as proprietary information).

  • $\begingroup$ You can tell us the speed, we won't tell anyone. $\endgroup$
    – Jpe61
    Commented Feb 12, 2020 at 14:26
  • $\begingroup$ What's his behaviour when inactive? All the way down due to its weight? Free-floating int the airstream? $\endgroup$
    – Manu H
    Commented Feb 12, 2020 at 14:37
  • $\begingroup$ When inactive on the ground it just hangs down due to weight, the connected hydraulic actuator does not receive any pressure at all. $\endgroup$
    – Houba
    Commented Feb 13, 2020 at 19:04

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