Why do most Boeing aircraft and their corresponding Airbus counterparts have different flaps and slats mechanisms (747 and A380, 737 and A320)? I'm assuming a lot of these mechanisms are patented. For example, the 747 is equipped with Krueger flaps while the A380 has the leading edge droop as slats.

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  • $\begingroup$ If I had an answer for you, I'd be happy to do so. I had to go look up Kreuger flaps and droop and slats, so I added links in while I was at it. Honestly, I'm still debating whether that's a droop or Krueger flap in your picture - my reading least me to believe it's a flap, but the shape of the leading edge says droop (to me, at least). $\endgroup$
    – FreeMan
    May 20, 2015 at 13:13
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    $\begingroup$ Its actually the Krueger flap of a 747. $\endgroup$ May 20, 2015 at 13:20
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    $\begingroup$ I'm not sure if there's a good answer to this because it's like asking why BMW and Audi use different components. It's just what those companies decided to use as part of their overall design: there's more than one way to implement most things. $\endgroup$
    – Pondlife
    May 20, 2015 at 19:36
  • $\begingroup$ @Pondlife, I completely agree that there's more than one way to implement things. But there is always a reason for doing/implementing something. For example, there must be a reason for the 747 having Krueger flaps, and the A380 having drooping slats. I'm not sure what the actual reason may be, but possibly because one creates more lift on an airfoil than the other. $\endgroup$ May 21, 2015 at 19:04
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    $\begingroup$ It appears to me that the US calls the moveable leading edge "slats" and the Europeans "a droop," just like the US calls the wheel assembly's "landing gear" and the Europeans refer to it as the"undercarriage." Dave Dietle, MSgt USAF Retired Heavy Aircraft Mechanic/Crew Chief $\endgroup$
    – user17772
    Oct 27, 2016 at 17:42

2 Answers 2


While the aircraft you mention would appear to be similar, this is not really entirely true. Specification wise yes, but the period it was designed in has a huge impact. For instance:

  • The first flight of the Boeing 737 was in 1967, with the Airbus A320 following considerably later in 1987. (20 years later)
  • Boeing 747 first flew in 1969, the Airbus A380 followed in 2005. (36 years later)

Each redesign is expensive and time-consuming so if there is no conceivable benefit it is not done. There is no clear cut equation on when a certain type of slat and flap mechanism is chosen:

  • The operating environment changes over time. One important consideration is the landing speed (and corresponding runway length). This for instance limited the Boeing 707 greatly, and it was likely that Boeing might not wish to run into the same problem with the B747 to enable it to serve as many markets as possible at a time when airports were not so advanced. For this reason they put in the very complicated triple-slotted flaps. (For comparison, look at the A380 which has caused headaches as airport operators have to modify and/or rebuild facilities to accommodate it)

  • Aircraft have become lighter over the years with the introduction of new materials, production techniques and better optimized engineering.

  • Technology has become better at modelling, so we can be less conservative in design choices as there is a better idea of the end result. This applies not only to the structure but also aerodynamics.

The bottom line is that you take the values and look at a few different designs. You choose the one that best fulfills your criteria (not only in performance but also what you can build) and go with it, and this choice would change over time. If the Boeing 737 would be built today, the details most probably be fairly different.

What does this mean?

  • The A380 for instance only has single slotted slats. When the B747-8i recently received a new wing design, it shifted from triple to double and single slotted flaps.

  • Both the A350 and B787, designed roughly around the same period, both have the rear single slotted flaps and (to large extent) the same slat design.

  • $\begingroup$ Most recent aircraft use a flap/slat design which is as simple as possible to reduce the maintenance cost. The tripple slotted flaps seen on the 747 look awesome but are a pain to service. $\endgroup$ Mar 16, 2018 at 8:30

Generally, a series of related aircraft uses the same wing: Take the A318, A319, A320 and A321: All use the same wing with the same airfoil, but the much lighter A318 can get away with single-slotted flaps while the heaviest, the A321, needs two flap sections for a double-slotted flap to tease out every bit of lift in order to keep the increase in take-off and approach speeds small. Since the flaps are the easiest part of the wing to adjust its lift capabilities to the needs of the aircraft, you will see a lot of variations in them.

The leading edge devices don't show so much variation: Inboard, a Krüger flap is most common, and outboard a slat is used. While a slat will always have a slot between itself and the wing, a Krüger flap can be slotted or unslotted. When extended, both will show a very similar shape to the airflow, and only their movement is different: While the slat moves forward with a small downward rotation, the Krüger flap swings around a hinge close to the wing's leading edge. Both have either a fully retracted or a fully extended position, while trailing edge flaps can be set in steps to adjust them for moderate lift / low drag increase during take-off, or high lift / high drag for landing.

The complex hinge mechanism in your picture allows the Krüger flaps of the Boeing 747 to increase their camber when extended. This is actually unique among airliners.

The Krüger flap is the newest of them and was invented in 1943. All patents on them expired long ago, and only individual details like the mechanism might be covered by patents today.

  • $\begingroup$ IIRC A321 actually has slightly (4 m²) larger wing than the three smaller variants. $\endgroup$
    – Jan Hudec
    May 31, 2015 at 20:28
  • $\begingroup$ @JanHudec: Interesting - the Wikipedia link gives the same 122.6 m² for all four, but lists a smaller tail for the A318. Airbus itself does not even list such mundane numbers as wing area. $\endgroup$ Jun 1, 2015 at 8:41
  • $\begingroup$ I remember reading it somewhere, but I can't find it now either. I have downloaded some PDFs from Airbus (Airplane Characteristics for Airport Planning; separate document for each type) that have more dimensions, but don't have the wing area either. But they do have different wing span for A318/A319/A320 and A320, 34.10m and 34.15m respectively for the older variant with the small wing fences (for the variants with winglets it shows the same span 35.48m). A321 is the only where the winglet variant got new series number, so maybe they switched to the common wing for it? $\endgroup$
    – Jan Hudec
    Jun 1, 2015 at 9:36
  • $\begingroup$ A321-100 also has different fuel tank layout without the split to inboard and outboard and, somewhat surprisingly, slightly smaller capacity. But I only have document for A321-100, not A321-200, so I can't tell whether they switched to the common wing for NEO or not. $\endgroup$
    – Jan Hudec
    Jun 1, 2015 at 9:38
  • $\begingroup$ @Peter, from memory, A318 has larger tail (apparently due to its shorter arm). $\endgroup$
    – Zeus
    Oct 27, 2016 at 23:28

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