A B737 FLT CREW RPTED AN ENCOUNTER WITH A HELI WHILE ON LGA'S RIVER VISUAL AND NOTED THAT THEY THINK THE USE OF THE RIVER VISUAL KEEPS CREWS TOO HIGH TO MAKE A STABLE APCH.

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WE WERE CLRED FOR THE VISUAL APCH TO RWY 13 LGA; AND TOLD TO MAINTAIN 2000 FT UNTIL ESTABLISHED ON FINAL (RIVER VISUAL). I TUNED IN THE ILS FOR RWY 13 FOR ADDITIONAL GUIDANCE. PUBLISHED PROC SAYS TO MAINTAIN 2000 FT OR ABOVE AS LONG AS POSSIBLE; REMAIN OVER THE HUDSON UNTIL STARTING TURN TO FINAL N OF THE CENTRAL PARK. PRIOR TO TURN TO FINAL; WE OBSERVED TFC BELOW US; SBOUND; OVER THE RIVER. AT THE APPROPRIATE LEAD POINT; I STARTED R TURN TO FINAL AS THE LOC HAD CASE BREAK. I THEN BEGAN TO DSND TO CAPTURE A NORMAL GLIDE PATH TO RWY. AT APPROX 1600 FT; THE TWR CALLED SAYING WE HAD DSNDED TOO EARLY FROM 2000 FT AND THEY GOT A CONFLICT ALERT WITH THE TFC THAT WE SAW EARLIER PASS WELL BELOW AND BEHIND US. HOWEVER; WE WERE CLRED FOR THE VISUAL APCH. I INITIATED TURN AT NORMAL LEAD POINT FOR 180 KTS GND SPD (1.8) NM AND AFTER CASE BREAK ON THE LOC. I AM CONSIDERED ON COURSE FOR FINAL. I THEN DSNDED FROM 2000 FT WHICH ALREADY PLACED US HIGH ON GLIDE PATH. AT 4.9 NM FROM THE RWY YOU ARE ALREADY 600 FT HIGH FOR NORMAL GLIDE PATH. IF YOU WAIT UNTIL ROLLOUT ON FINAL; AS THE TWR WOULD HAVE US DO; YOU WOULD BE AT 3.2 DME AND 2000 FT; OR OVER 1040 FT HIGH; REQUIRING EXCESSIVE RATE OF DSCNT WITH IDLE PWR AT LOW ALT AND AN UNSTABLE APCH. ATC SHOULD LOOK INTO THIS PROC AS IT SEEMS UNSAFE! SUPPLEMENTAL INFO FROM ACN 628671: OVER THE HUDSON; WE ENCOUNTERED A LOW LEVEL HELI THAT WAS NOT POINTED OUT TO US; BUT SEEN BY US ON TCAS AND VISUAL. HE PASSED WELL BELOW US AND WAS HEADED IN THE OPPOSITE DIRECTION. SOON AFTER WE TURNED FINAL AND BEGAN A DSCNT VISUALLY AND GOT DOWN TO THE GS; THE TWR INFORMED US OF A TFC CONFLICT WITH A HELI OVER THE RIVER. HE SAID IT WAS NOT A FACTOR AND WAS BEHIND US. HE SAID WE WERE TOLD TO HOLD 2000 FT UNTIL ESTABLISHED. WE SAID WE WERE ESTABLISHED WHEN TURNING FINAL AND LOC MOVEMENT WE WERE CLRED FOR THE VISUAL.