As you can easily see in my first picture it happens after some seconds
It's not possible to understand how much *time* the scenery takes to disappear since you started FSX, from your pictures, since it's not clear how much time it took to taxi and takeoff. Besides, the 1st picture shows your are at some distance from the airport. Of course, since you said "after some seconds", I'm assuming you are referring to a time thing.
If, instead, you were referring to a *distance*, it's an entirely different matter, but it would helped if the screenshot contained coordinates, otherwise it's impossible to precisely assess your distance from the airport, especially at that zoom factor.
The scenery obviously has a loading range, which at JFK is kept quite low at 7.0 nm, and the range has been chosen precisely to be sure not to load JFK when you are flying over KLGA, in order not to slow down fps if you are landing at KLGA. So, if your picture was taken farther than 7.0 nm from the scenery of the airport, that's not a problem of the scenery, it's just how it supposed to behave: to disappear at 7.0 nm out.
If, instead, you were closer than that, then it's something different which, as I've said, can't be judged at all by the picture you posted.
also in landing phase it is not possible to see airport buildings as soon as the plane is into the airport.
You 2nd screenshot doesn't show anything unusual, since you have your buildings. Of course, not all buildings are displayed in the distance, the smallest one will only appear when going closer. Do you mean that the buildings that are shown there, will disappear entirely if you go *closer* ?
If yes, this has already been discussed on the forum, and it's due to the fact that you may have set FSX to run into a "XP Compatibility" mode under Vista or Win7. This is not how FSX install itself, and it's wrong, because the program is fully compatible with Vista/7 and the XP Compatibility mode, other than slowing it down, will create lots of side effects. If this is the case, turn the Compatibility mode OFF, which is how FSX install itself by default.
Can you provide me the procedure to reinstall them? I'll do a try.
There's no special procedure, other then uninstall the scenery, like any other Windows program, from the Windows Control Panel, then install it using the current installer. However, check the Compatibility thing first.