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(wikimedia.org) Airbus A350.

There aren't many wide-bodies out there. Whether it's Douglas, Boeing, or Ilyushin, they all have the high-speed aileron interrupting the flaps. Whereas Airbus uses uninterrupted flaps in all their fly-by-wire aircraft with the two ailerons outboard.

After Boeing has adopted fly-by-wire, it is still sticking with its flaps design in the 777 and 787. I'm aware Boeing now uses flaperons, but as the image below shows, its deflection is not all the way to avoid the jet blast, but how does Airbus get away with it?

For the uninterrupted, here are some reasons I can think of but can't link:

  • Smaller deflection for takeoff and landing is better for noise and fuel, but may incur a weight penalty because the flaps are in the jet blast and need to be built stronger.
  • A smaller overall wing is better for weight reduction, but worse for wing loading.

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(wikimedia.org) Boeing 777.

Why did Airbus choose this design (apart from its A300/A310)? They must have a good reason. Equally for Boeing for not using them with its FBW aircraft. Unless it doesn't matter, or a patent is involved.


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2 Answers 2


The two designs are indeed based on compromises.

Earlier Boeings were designed for high-Mach cruise, which means high sweep and the need for elaborate flaps when flying at approach speeds. The 'thrust gate' reduces the thrust/flap interference and drag, especially in a go-around. It is also a good choice when more powerful engines will be offered as the MTOW increases.

Since the A320, Airbus have used high-lift planform layouts, which allow the continuous single-slotted flaps to work. An A380's approach speed is slower than a 747 or a 737, despite having single-slotted flaps. Owing to its high-lift wing.

When Airbus designed the A330/A340, it helped that they launched the bigger variants first. A targeted (tailored) design is not good when the aircraft would later be stretched, which would require a re-design of the flaps, i.e., if the shorter variant comes first, then 'over-designing' the flaps and having a thrust gate is a good choice.

Interestingly for the A350-1000 (a stretch of the -900), Airbus had to increase the wing area by 4% by modifying the wing trailing edge to maintain the same approach speed (flightglobal.com).

Losing the gap does save weight, complexity, and by extension, maintenance. But the limitations are:

  • Slower cruise speeds (not a problem since it's by design, flaps follow wing).
  • Thinner spoilers due to thicker flaps, they have to be made stronger.
  • Must demonstrate controllability during a go-around.
  • When stretching a variant, a flap redesign will be needed (ala A350).
  • Limits the flap span to allow for the innermost aileron to be inboard enough to not cause aileron reversal (lots of wind-tunnel and CFD models help fine tune this).

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Own work based on scaled drawings from boeing.com and airbus.com.

Comparing the wings of the A330-200 and 777-200(ER):

  • The maximum operating Mach numbers (MMO) are 0.86 (A330) and 0.89 (777). Note the higher sweep on the 777.
  • The MTOW and MLW of the 777-200ER are 55 and 31 tonnes heavier, further evidence to the higher wing loading of the 777, which would indeed need more elaborate flaps.
  • Keeping in mind in the development cycle, the 777 was stretched, while the A330 was shrunk.

In short:

Boeing tends to prefer higher MMO, higher MTOW, and to offer the base-variant then scale it up. All of which would benefit from the 'thrust gate' and the more elaborate flaps. The overall trend however shows Boeing going for simpler designs:

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This answer is based on a publication by an Airbus engineer:


One data point which shows that interrupting flaps does indeed cost a lot of lift is the comparison of the maximum lift coefficients of an Airbus A320, where the fuselage causes a wide gap between the left and right wing flaps, with the high wing BAe 146 where the gap between the flaps is filled by the fuselage. Even though the BAe 146 has no slats, its landing lift coefficient is 20% larger than that of the A320. The lower sweep of the 146 wing helps here, but most of the difference is caused by the fuselage. If the BAe 146 had slats, the difference would be even larger.

  • $\begingroup$ Is it because of vortex drag? $\endgroup$ Commented May 9, 2020 at 6:58
  • $\begingroup$ @ABJX: Yes, that is part of it. Lift drops in the gap and causes some vorticity to be shed. $\endgroup$ Commented May 9, 2020 at 7:02

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