N181 is taken. It's currently assigned to a 1941 Boeing A75N1 registered in Chandler, AZ.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N181 has been worn before. Past registrations from FAA deregistration records:
Tails one nudge away from N181 that aren't assigned or reserved: