[Smokeynz]

My initial thought is that there is no association between the about to spawn aircraft and old aircraft even though they possibly are the same type.

Many solutions probably.
I'd attempt,
Store the onactorcreated mission number, shortname and actor to a list, then test the list against play for true/false if they are alive or landed and then if alive destroy.

You are trying to test the about to spawn against those ingame, they are different so always false.

Also if you have a list, you could look for how many left and spawn in replacements of the missing or landed.