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This question is about the number of flaps settings on different airliners. One can notice that this varies greatly between models and manufacturers. On a Cessna 172, you usually only have 4 or 5 settings. But on airliners you have usually more, especially on Boeing.

So I did a quick search and found the following (links point on the image I used):

  • B777: 7 settings: 0-1-5-15-20-25-30
  • B737-900: 9 settings: 0-1-2-5-10-15-25-30-40
  • B787-8: 7 settings: 0-1-5-15-20-25-30
  • B787-9: 10 (!) settings: 0-1-5-10-15-17-18-20-25-30

On the other hand, Airbus seems to stick to the same scheme for its models:
the A320/A330/A350/A380/A330 all have 5 settings: 0-1-2-3-full

The questions are:

  1. When you have 9 or 10 different settings, what is the point of having so much ? Is that really useful ? Does the crew really incrementally uses all of them on approach and landing ?
    I mean, if they only step, say from 0 to 5 on approach, then 5 to 15 on long final, and then 15 to 30 on short final, then the manufacturer could have saved some metal.

  2. Boeing planes have more settings than Airbus. Is there any particular design reason behind that ?

  3. Airbus doesn't designate the different positions as angles, but as incremental numbers. Anybody knows why they did that design choice?

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  • $\begingroup$ Why Airbus only uses 0-1-2-3-FULL: I think they do this because they are wise and want to support pilots in their work. Its the same with the ECAM they invented. Genius, awesome inventions that makes life for Pilots so much easier and safer. $\endgroup$ – Noah Krasser Jan 8 '17 at 17:29
  • $\begingroup$ @NoahFisher Sorry but this sounds like a rant: I don't see the relationship between using sequential numbers instead of angular positions and supporting the pilots in their work. The question is not about which design is better (if there is any that would qualify as so). $\endgroup$ – kebs Jan 8 '17 at 18:47
  • $\begingroup$ @Noah what do Boeing EICAR and Airbus ECAM have to do with the question? Note: flaps don't just rotate, they also extend, and there often is more than one per wing. The flap "angle" on Boeing airliners is just an arbitrary number that incompletely (and inaccurately) describes the flaps actual position. $\endgroup$ – RedGrittyBrick Jan 9 '17 at 10:32
  • $\begingroup$ The Airbus scheme is 0–1–1+F–2–3–Full. The lever position 1 selects different configuration depending on other conditions. $\endgroup$ – Jan Hudec Jan 9 '17 at 18:17
  • $\begingroup$ @kebs Yes, it is a rant too. I don't like Boeing. But he also asked if there is a particular design reason for the Flaps Position Levers. I am not an engineer at Airbus but I think that they want to keep it simple. As everything in this amazing airbus planes. $\endgroup$ – Noah Krasser Jan 9 '17 at 19:36
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1a. On take off, with a light airplane you may select less flaps to reduce the additional drag and save some fuel. Max takeoff flaps are reserved only for short runways and high loads. Fuel savings are very important nowadays.

1b. On approach, less flaps produce less noise and may also allow a higher approach speed (VFE is the maximum flaps extended speed and may be different for different flap settings). Maximum flaps are set only later in the approach when the plane has stabilized at the final approach speed.

1c. The flap setting has an effect on the airplane's pitch angle. A certain flap setting may help to minimize the tailstrike risk. The longer B787-9 has nine settings, while the shorter 787-8 has six.

  1. I learned the Boeing numbers are not true angles either. For example, settings 1 and 5 may only affect the leading edge slats. At higher settings they approximate the actual angles (which are anyway somewhat misleading in the case of today's multi-part Fowler flaps, which move both down and backwards).

https://www.flightglobal.com/news/articles/farnborough-aero-secrets-of-boeings-new-dreamliner-401784/

http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_08/pdfs/AERO_Q408_article05.pdf

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  • $\begingroup$ On point 3, neither the Airbuses position are related to flaps angles ONLY, for instance the second flap position on the A300/310 ("15/0") means 15° slats, kruger flaps extendend, 0° flaps and 7° ailerons droop. Source: A310 FCOM. $\endgroup$ – Marco Sanfilippo Jan 11 '17 at 11:51
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Airbus doesn't designate the different positions as angles, but as incremental numbers. Anybody knows why they did that design choice?

Probably because flap angles are basically lies.

Airliner wings don't just have one flap that only rotates. They have multiple flats and slats, the flaps extend as well as rotate. The pilots don't need (or want) to know the full multi-dimensional details of the positions of all the individual elements.

The same is true for other control surfaces. Particularly as they can vary dynamically to even out loads.

enter image description here

Position    Slat     Flap
0           0        0
1           18       0 when selected above 100kts. (approach)
1           18      10 when selected below 100kts. (takeoff)
2           22      15
3           22      20
FULL        27      35

The actual angles depend on what engine the aircraft has (e.g. CFM vs RR) and may be different angles for different variants of the aircraft.

The inner and outer flap elements are not mechanically coupled with each other and are controlled independently of each other for the purpose of setting the span load distribution. The system also comprises a flap control that controls the angle of the outboard flap element, independent of the inboard flap element, for optimizing a lift to drag ratio during aircraft takeoff and landing.

Airbus patent

There's something to be said for not confusing pilots cross training onto a new aircraft type or minor variant. Maybe having 0 1 2 3 FULL be used the same way across a large number of aircraft makes pilot's lives easier and makes errors less likely.

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