CA Corp filing

I have my airplane registered in a "Mutual Benefit Corp" in the state
of California. Every year I get a nastygram from the state saying I
need to fill out some soft of tax return. They also say I'll have to
pay a percentage fine for being late (which I assume is zero since the
corp doesn't make any money, its just a holding corp for me). If
anyone else uses this set up, what FTB form to you submit?
-Robert