"ligne de vol" = "level attitude" (and nothing more)
As HazelNuts rightly points out, there was extensive discussion of this phrase in Part 1 of this thread (back in early July 2010).
For anyone who has not seen that discussion I suggest a quick look at these posts:
#3300 (8 July page 165) NOTanAM
#3301 (8 July page 166) Lemurian
#2808 (3 July page 141) Lemurian
(And thanks to all the regular, expert, posters for such an interesting thread).