ACR X NON ADHERENCE TO ATC CLRNC EARLY DSCNT WHILE ON VISUAL APCH. ALTDEV.

Narrative

ACR X WAS APCHING LGA FROM BOS ON A VISUAL APCH TO RWY 13 WITH A R TURN TO LAND ON RWY 13. WE WERE ON A HDG OF 220 DEGS AT 3000 FT AND WERE CLRED FOR A VISUAL AND INSTRUCTED BY NEW YORK APCH TO MAINTAIN 2500 FT TILL OUR BASE TO FINAL TURN. THE HDG WE WERE ON WOULD CAUSE US TO INTERCEPT THE EXTENDED CTRLINE OF RWY 13 APPROX 3-4 MI FROM THE APCH END AND AT THAT DISTANCE WE SHOULD BE AT 1200 TO 900 FT AGL. IT WAS CLRLY IMPOSSIBLE TO MAINTAIN 2500 FT TILL THAT POINT AND EXECUTE A NORMAL OR EVEN SAFE APCH. WE THEREFORE TURNED L SLIGHTLY TO INCREASE THE DISTANCE FROM THE END OF RWY 13 BEFORE WE STARTED OUR TURN TO FINAL. WE THEN BEGAN A SHALLOW TURN TO FINAL AND A SLOW DSCNT. LGA TWR AT THIS TIME QUERIED US IF WE WERE INSTRUCTED TO MAINTAIN AN ALT TILL BASE TO FINAL TURN AND WE RESPONDED THAT WE WERE IN A SLOW TURN AND AT 1900 FT DSNDING. WE ALSO TOLD TWR THAT THE CLRNC AS PREVIOUSLY ISSUED PRECLUDED A SAFE APCH. THIS CLRNC IS COMMON AT LGA AND CTLRS SHOULD BE ADVISED OF THE 3:1 RULE IN DSCNTS.