Let say we have a flying creature (60' fly speed) with flyby attack.
Can it charge "swooping down" for 120' and making his attack anywhere during the move or since its a charge it as to be at the end of the 120' move?
Honestly under the present rules i have difficulty big time to imagine a flying creature that is literatly swooping down from sky at his foe for a "awsome" 60' move!
Are there any rules im not aware of or the rules are simply anti-climax*?
* don't get me wrong i'm not a hater, i actually think that pathfinder is the best thing that could have happend to D&D post 3.5. It's just that some of my few "gray zone" are in regard with flying and big creatures "snatching" tinyer ones with beaks and all the confusion about grappling.
Flyby Attack lets a creature take a Move action, and a Standard action at any point along that movement. Charging is its own Full-round action, as is running.
So a swooping Wyvern can use a Move action to fly 60ft, and using its Flyby Attack Feat can use a Standard action at any point along that movement to make a bite attack. If the bite hits it can use its Grab ability to automatically make a grapple check (against a medium-sized or smaller opponent) - either a regular one (which ties up the Wyvern with the grappled condition as well) or a special one at -20 CMB (which means the wyvern itself isn't considered grappled, only the thing it's grabbed). With only a +16 grapple CMB this second tactic is difficult for the Wyvern to pull off, but it can be done (especially against smaller opponents who have really low CMBs & CMDs).
You cannot charge while using Flyby attack, as they both have incompatible actions (a full round action VS a standard and move action, respectively).
However, you could dive down (using your Fly 60' example) 60 feet, expending only 30 feet of movement while doing so, use your standard action (like an attack or spell), and then fly upwards and away another 30 feet (or 15 feet if you are using a fly spell).
In general, you can move double your speed while diving, and ascend at your normal speed (or half your speed if you are using fly spells).
Please note, however, that Flyby Attack is NOT like Spring Attack, Shot on the Run, or Ride-by Attack. It does NOT protect you from attacks of opportunities. In my above example, if you dived in and pulled out of the melee reach of an enemy, he might knock you around a bit for your trouble.
On the plus side, since you can use ANY standard action during your move, this feat is far more versatile (you can come out from behind a pillar, cast a spell, and move back to total cover).