When an aircraft is sent to some destination, there is no check
for sufficent fuel.
If an aircraft is sent on a second mission shortly after returning
from a first mission (without enough time to refuel), it can happen
that it has to return in mid-flight before reaching its second target.
I suggest a fuel-check and a dialogbox with options "Fly anyway" /
"Wait for complete refuel" / "wait for minimum refuel".
