Amount of ammo is an Arena Setting, which is always set to 1.0 multiplier.
There is no way to rearm a plane in flight, only by reupping/rearming.
In OFFLINE play, you can change the Arena Settings as you want them.
To do that, open your clipboard and go to OPTIONS > ARENA SETUP > ENVIROMENT > Arena Settings. Find
PlaneAutoAmmoMult and
PlaneGunAmmoMult in the list. Change them to 100,000 (i know it says you cant do it, but it will do it anyway) to move your ammo counter OOR (out of range) for unlimited ammo.
While youre there, you can also change the FuleBurnRate (or something like that). Put at 0, your plane wont consume any fuel. Short of getting a fuel leak, you can fly forever.
If you hit the SAVE button before exiting, the Settings will stay as they are untill you change them again.
AH Help File on Arena Setting