can you only descend at double speed while flying with the Fly spell?

[Seraphimpunk]

the fly spell has the bit about being able to descend at double speed. but the fly skill doesn't, and nothing gets mentioned about it in the universal monster rules for flight.

is there anything in the rules that lets creatures with a fly speed descend at double speed ? or is it just the magic of the spell that lets casters get away with it?

[Purple Dragon Knight]

Double speed dive while flying has been around forever. Pathfinder may have dropped it in the 3.5 transition. Look up eagles hawks and such... some of these critters might still have that "flying charge dive" wording. ....