Q:

Why do (some) aircraft shake at low speeds with flaps, slats extended?

I landed a couple of times as a passenger with the Embraer 190 (I like Embraer very much) on runways with a length below 2 km.
I noticed that the plane started to shake or vibrate quite strongly once reducing its speed prior to landing. The flaps during this time were fully extended. How can one explain these vibrations? I have to note though, that I often sit in the back of the fuselage, where vibrations tend to be more pronounced.

A:

There are many phenomena which can cause vibrations. Without knowing further details like the frequency, starting moment or the weather conditions on landing it is difficult say. Common causes are:
Stall buffet/ self induced turbulence Normally when a plane lands it flies with low velocity and a high lift coefficient, near to stall. The flow detaches much earlier and there is more turbulence behind the main wing. The tailplane has to fly through this turbulent zone and hence you feel the vibrations. Sometimes deploying the spoilers, even if you're not flying at stall speed is enough to create turbulence which will hit the tailplane. T-tail airplanes are less affected because they're further away from the air zone influenced by the main wing.
Atmospheric turbulence Can be created due to thermals (in that case it should be sunny) or wind hitting ground structures. If you have landed a couple of times and always felt the vibration it probably isn't the answer.
Wake turbulence Shouldn't be your case either, separation minima exist to avoid this problem and it would probably feel different every time you land.

A:

The answer is quite simple: Altitude. Air near the ground, as a general rule, is more turbulent and unpredictable than air aloft. Buildings create mechanical turbulence and there are pockets of updrafts from surface heating (especially over black asphalt runways).
You may also be feeling mechanical vibration from the gear extending, or maneuvering causing airframe oscillations.
Flaps and slats increase the surface area of the wing, which reduces wing-loading, which reduces wake turbulence. The aircraft creating turbulence for its own tail is likely not the answer, and in extreme situations this is actually referred to as a "deep stall" which is quite dangerous.

