Thanks, guys
Flaps and slats:
Max operating altitude with slats and/or flaps extended is 20 000 ft.
Now re-reading I see that it is crystal clear: you cannot operate the airplane above 20,000 ft with slats or flaps extended.
and now the next question:
what about landing gear?
Can you extend it below 25,000 and 250 kt and then climb to FL280 with the gear down?
Here the english is not so clear to me:
Maximum altitude at which the landing gear may be extended............................................... 25 000 ft
I'll take a look at that thread, J.T, thanks