What is the point of having lots of different flaps settings?

Question

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?

Answer 1

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.

3. 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

Answer 2

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.

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.

Answer 3

Airbus aircraft have five flap settings. They are:

• Flaps 1.
• Flaps 1+F.
• Flaps 2.
• Flaps 3.
• Flaps FULL.

The setting for flaps 1 and 1+F are controlled by one position of the flaps lever. When flaps 1 is selected on the ground, the slats and the first stage of flaps extends. Hence the name 1+F. The 1 stands for the slats and the F stands for the flaps. In flight, things are different. If you extend flaps 1 in air, you will only get the slats. The flaps start moving out only when you set the flaps lever to 2. This I believe is same for the Boeing 777. Moving the flaps lever to 1, in the 777 only moves the slats. This makes the flaps 1 position of the aircraft redundant on the ground because you cannot take off with slats alone. Airbus on the hand changed the slat and flap computer logic so that the same flap lever position demands two separate settings depending on the phase of flight. So, it is a bit of clever design.

As for why Airbus designed flaps lever like so is because they wanted commonality across their aircraft. I fly the A320 and the A321. The latter is 15 tons heavier than the other but my call outs for both the aircraft for flap selection remains the same. It is Flaps 1,2,3 and FULL. It is the same call out for the A330, A340, A350 and even that massive A380. The flap slat angles are of course different for each of these aircraft. The flap lever positions are just settings. They do not really correspond to the actual flap or slat angle.

There are also many Airbus operators that opt for mixed fleet flying where A320 pilots also fly the A330. Having similar call outs and operating procedures helps the pilots in flying two different aircraft at the same time. And also, because Airbus aircraft have so much in common, the type rating time frame is reduced if a pilot already flies an Airbus aircraft. For example, to go from flying an A320 to an A330 takes just 3 simulator sessions and some ground classes. The course will usually be done within a week. If you compare that to a normal type rating course it is four times shorter.