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I keep seeing pictures of A320/321 planes with slats or some other form of leading edge high lift devices. However, on all my flights in the last years with A320s/321s I never noticed anything changing on the leading edge during approach (from my pax window point of view).

So my question is: Is that an option airlines can choose, if they want to have leading edge high lift devices? Or are they in all aircraft of the family? If so, are operators flexible in using them for approaches?

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Unless something broke mid-flight -- which you would have probably been told about as a passenger because of the faster landing speed -- leading edge devices (slats) are used for takeoff, approach, and landing on the A320-family.

For takeoff the slats are put in position 18 or 22, and for landing in position 22 or 27, as the table below from an A320-family flight crew operating manual shows:

enter image description here

If you are sitting well ahead of the wing, you may not notice a change if you don't know what to look for. Here are the 4 positions:

enter image description here
(YouTube) Slats 0, 18, 22, and 27. Also note the arrow showing the 3 extended position markers.

One of the few jetliners built without slats is the DC-9 Series 10.

The Series 10 was unique in the DC-9 family in not having leading edge slats. The Series 10 was designed to have short takeoff and landing distances without the use of leading edge high-lift devices. Therefore, the wing design of the Series 10 featured airfoils with extremely high maximum lift capability in order to obtain the low stalling speeds necessary for short field performance.

The rationale would have been weight and maintenance cost saving, which was helped by the short range of the plane. For faster cruise speeds and longer range, high-speed tailored wings with slats for low-speed are better than low-speed tailored wings.

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    $\begingroup$ The more common jet is the CRJ100/200. Like the CL-600 it came from, and many bizjets in general, there's no LE devices due to cost. The result is that a CRJ200's takeoff run can rival a 757. $\endgroup$ – user71659 May 9 '18 at 20:17
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Most Airlines have slats, as a matter of fact, even smaller jets have them. Slats have to do with swept wings which allow to fly faster. Manufacturers use this method to delay the onset of relative wind on the upper side becoming supersonic. The trade off for this is (amongst other things)a higher stall speed and a higher angle of attack required at approach speed or close. Slats together with flaps help circumvent this problem allowing the aircraft to fly slower. On the Falcon 2000 (my current A/C) the slats and flaps come together and we can’t control one or the other ( unless in an emergency) it’s both at the same time i.e slats extend when the first stage of flaps is selected i believe it’s the same on all airliners including the A320.

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