ACR X NON ADHERENCE TO ATC CLRNC EXCURSION FROM ASSIGNED ALT.

Narrative

ZOB CLRED US TO DSND AND MAINTAIN 12000 FT ON THE SPICA 2 ARR AS EXPECTED. ON INITIAL CONTACT WITH DTW APCH; WE REQUESTED THE VERNR (FMS) TRANSITION. AFTER A LITTLE HESITATION WE WERE CLRED THE VERNR TRANSITION AND SELECTED 5000 FT ON THE MODE CTL PANEL; THE FINAL ALT ON THE TRANSITION. WE WERE LATER TOLD TO SLOW TO 250 KTS. THE AIRPLANE; ON AUTOPLT; BEGAN A DSCNT ON THE FMS TRANSITION AS IT IS PREPROGRAMMED TO BE AT ROBBI AT 5000 FT; THE NEXT CONSTRAINT. APCH THEN INQUIRED ABOUT OUR ALT AND ADVISED US TO MAINTAIN 7000 FT. I THINK WE WERE PASSING ABOUT 9000 FT AT THE TIME. THE FMS TRANSITION NEEDS AN ALT CONSTRAINT AT THE BEGINNING. ALL OTHER KNOWN FMS TRANSITIONS OR PROFILE DSCNTS HAVE THIS FEATURE ENABLING SELECTION OF FINAL ALT OF THE PROC WHEN CLRED. THE VERT PROFILES PREPROGRAMMED NEED TO MATCH THE ALTS THE CTLRS ARE LIKELY TO WANT. THEY THEN MUST SPECIFICALLY RESTRICT US TO ANY OTHER ALT NECESSARY. IF THE ENTIRE PROC IS ONLY A GND TRACK (NO VERT NAV) IT MUST BOLDLY STATE 'ALTS AS ASSIGNED.'