Originally Posted by flint-wwrr

D6 is definitely within the range of the flute, but I wouldn't recommend writing it for anyone less than a professional... it's extremely difficult to get the note out.

Personally, I can play up to an E6, but I immensely dislike doing so. Eb6 and E6 are a full order of magnitude harder to play than the D6. The D6 (and C#6) are much more difficult than the C6.

As a rule of thumb, C6 should be the upper limit unless you a) know the flutist, b) are a flutist yourself, or c) can truly justify having to exceed the practical range of the flute (i.e., it's part of the line or it fits the music)