FLC OF LGT ALLEGEDLY EXCURSIONED FROM ASSIGNED ALT.

Narrative

WE WERE PROCEEDING TO THE TAMPA ARPT FROM THE S ON AN ASSIGNED HDG OF 360 DEG AT 5000 FT. THE CAPT WAS FLYING. WE WERE TOLD TO SLOW TO 210 KTS. WE SAID WE WERE ALREADY AT 180 KTS. APCH SAID; CALL THE FIELD IN SIGHT FOR A VISUAL TO 18L. AT THE TIME WE WERE 3 MI E OF 18L; WE CALLED THE FIELD IN SIGHT AND THE CREW UNDERSTOOD WE WERE CLRED FOR THE APCH. AT 2500 FT AND STILL HDG 360 DEG THE CTLR ASKED OUR ALT. WE GAVE IT AND HE GAVE US A 260 DEG HDG AND CALLED TFC AT 1 O'CLOCK WHICH WE SAW AT ABOUT 3 MI. WE WERE GIVEN A 220 DEG HDG AND SWITCHED TO TWR. TWR CLRED US TO LAND. I TALKED WITH TRACON AFTER LNDG. THEY SAID WE WERE NOT CLRED FOR A VISUAL APCH AND WE DEVIATED FROM OUR ASSIGNED ALT OF 5000 FT. THE PROBLEM SEEMS TO BE USE OF NONSTANDARD INSTRUCTIONS. THE CALL THE FIELD IN SIGHT FOR A VISUAL TO 18L AND THEN NO INSTRUCTION TO MAINTAIN ALT AFTER; THE FIELD IN SIGHT; WAS CALLED CREATED THE SITUATION. IF THE INSTRUCTION 'PLAN A VISUAL APCH TO 18L' OR 'MAINTAIN 5000' WOULD HAVE BEEN GIVEN THERE WOULD HAVE BEEN NO SITUATION.