Just to give a little background to this question, I am the DM that ruled that you cannot use Tumble while flying, Poor little borc killer is mad at me hehe. but here are my reasons what do you think?
Here is why I have a problem with PCs or NPCs getting fly cast on them and being able to Tumble.
Couatl - CR10 - Fly 60ft (good) - Tumble +15 - pg 37 MM3.5
Pit Fiend - CR20 - Fly 60ft (Average) - Tumble +31 - pg 57 MM3.5
Eladrin – CR6 – Fly 100ft (Perfect) – Tumble +13 – pg 93 MM3.5
The creatures listed above also have listed ground movement speeds as well.
Those are the ONLY creatures in the whole book that have fly as a listed speed and have Tumble as a skill. I just find it hard to believe that a PC can, when he has fly cast on him, fly better then any other creature in the book that has been ‘flying’ its whole life. It is just ludicrous to me that all a rogue needs is a 3rd level spell cast on him and he can maneuver better then an Air Elemental, or any other flying creature with Perfect flying movement.
Sure you can say that “ohh but I have been practicing my tumbling my whole life”, but tumbling is NOT based on movement through air it is movement based on resistance, and you learned tumbling by rolling on the ground, doing flips on the ground, or just maybe you learned to do acrobatics through doing the flips on the bars. At any rate the skill Tumble in the PHB only refers to acts of tumbling as it relates to movement on the ground or to grant a soft landing on the ground.
Do I have any written game rules to back up my argument? No, it is all interpretation just as you have interpreted what you see, however I believe that giving any creature the ability to tumble in the air is granting way too much power to that creature, it just has no interpretation to a real life situation as described in the PHB and MM. Because really if you could tumble in the air, then why wouldn’t every creature that can fly have tumble as a skill if it is to be considered their “aerobatics”.
I guess what it comes down to is that while you are granted the ability to fly at a movement of 60’ at a good rate, it is not a natural movement; it is a magical movement rate. While it takes no concentration to use the effects of the spell you are not used to it mentally so how would you know exactly what kind of a tumbling move would be effective in combat if you had never flown or had very little flying experience before hand. I think it just comes down to the fact that if there should be a tumbling skill for flying creatures it should NOT be interchangeable between flying tumbling and ground tumbling.
Borc