From Sickboy:
In air unit initialization field:
Code:
nul = this spawn { while {alive _this} do { _this setFuel 1; sleep 60 } };
Unless you require the handling to be actually checked for amount, then you could do something like this:
Code:
nul = this spawn { waitUntil { if !(alive _this) exitWith {}; if (fuel _this < 0.25) then { _this setFuel 1 }; false } };
Change 0.25 to the level you want it to refill to 100%.
You can add a sleep before ; false to make it less resource intensive (for as far as it could be 😛