MD80 CREW IS CLRED FOR AN ILS APCH TO RWY 4 AT LGA; CIRCLE TO LAND RWY 31. FO WAS REQUIRED TO ADD THRUST TO OVERCOME A NORMAL LOW ALT XING AT THE OM.

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FLEW ILS RWY 4; CIRCLE LAND RWY 31. BRIEFED AND FLEW GND TRACK FOR EXPRESSWAY VISUAL RWY 31. WERE ALSO FORCED TO FOLLOW GND TRACK EXPRESSWAY VISUAL RWY 31 TO FOLLOW ACR AHEAD. USED EXPRESSWAY VISUAL RWY 31 FOR CIRCLE PORTION OF APCH. NORMAL EXPRESSWAY VISUAL RWY 31 THAT WE FLY OFTEN STARTS AT TANKS AT 2500 FT AND IS A LOW PWR DSCNT ALL THE WAY TO LNDG. ILS RWY 4 HAD US IN THE VICINITY OF THE TANKS AT 1400 FT. FO; ME; FLEW APCH AND BEGAN CIRCLE AT LOW PWR AND FULLY CONFIGURED FOR LNDG. ACCUSTOMED TO LOW PWR DSCNT ALONG EXPRESSWAY; WATCHING TFC AHEAD; AND TRYING TO KEEP LGA IN SIGHT WE WENT BELOW 1000 FT ALONG THE EXPRESSWAY. CAPT NOTED ALT AND CALLED FOR CLB BACK TO 1000 FT. HIGH PWR SETTING IN LNDG CONFIGN MAY HAVE SET OFF NOISE MONITORS NEAR LGA. WE SHOULD HAVE CONSIDERED THE DIFFERENCES BTWN A CHARTED VISUAL AND A CIRCLING APCH. ESPECIALLY MANEUVER ALT AND DSCNT PROFILE DIFFERENCES; ESPECIALLY AT NIGHT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CREW RPTED WX TO BE 2000 FT OVCST; VISIBILITY 4 MI. COMPANY MINIMUMS FOR CIRCLING APCH ARE 1000 FT OR CHARTED MINIMUMS; WHICH EVER IS HIGHER. PLT FELT LGA WAS JAMMING APCH IN AND FORCING AN 'EXPRESSWAY VISUAL' TO BE FLOWN WHEN BELOW MINIMUMS FOR THAT APCH.