Well you could use the nearestObject command to grab the aircraft closest to the unit, but it will only work for vehicles in a 50m perimeter. I guess it'd be much simpler to make a list of all aircraft available, i.e.
_aircraft= [aircraft1,aircraft2,...];
then loop through the list and see which one is closest to _unit:
_minseed=9999
; Initialize with any unit just to make sure OFP allocates variable globally,
; not just within the loop's scope
_daOne=player
"if (_x distance _unit<_minseed) then {_minseed=_x distance _unit;_daOne=_x}" foreach _aircraft
Now _daOne should be the closest aircraft.