one of doing would be to have two triggers, set them both to repeatedly
1.
Condition : planename ammo "su25missles" < 1
On Activation : planename land "land"
2.
Condition : speed planename < 4
On Activation : planename addmagazine "su25missiles"
planename is the name of your su25, and "su25missiles" is the name of the missiles on the su25 which i have forgotten what they are
sorry if this doesnt make much sense i did type in a rush.