i dont think you can detect units with the z axis. however, if you only have one or two types of air unit which you want to be exempt from the trigger then you can put a check in the script you are calling which goes something like:
_type = typeof _unit
? _type= "AirUnit1Type" OR _type!= "AirUnit2Type": goto "end"
;stuff you want to do
#end
exit
where _unit is the unit activating the trigger. not sure how you would be able to put the variable into the script from the trigger though.