ACR LGT LOSES 8 FT OF 'FORE LAP' OFF R WING DURING VISUAL APCH IN A NIGHT OP.

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IN DSCNT INTO LGA AREA; WE WERE CLRED TO DSND FROM 8000 FT TO 4000 FT. WHEN THE VECTOR WE WERE GIVEN INDICATED THE PROBABILITY OF A R TURN TO LGA RWY 31; I DISCONNECTED THE AUTOPLT AND MANUALLY FLEW THE ACFT. IN ORDER TO GET PROPER SPACING FOR OUR R TURN; APCH CTL ASKED US TO KEEP OUR SPD UP. WE MAINTAINED APPROX 240 KIAS AT THIS POINT. WE CONTINUED THE DSCNT AS DIRECTED TO 3000 AND THEN TO 2500 FT. I BELIEVE APCH CTL THEN ASKED US TO REDUCE TO 180 KTS. WE DID SO; EXTENDING 2 DEGS OF FLAPS AT ABOUT 220 KTS; 5 DEGS AT 205 KTS. WITH THE PRECEDING ACFT IN SIGHT; AND HAVING BEEN REQUESTED TO SLOW TO APCH SPD; THE LNDG GEAR WAS LOWERED; THE FLAPS WERE THEN EXTENDED TO 15; 25; AND 30 DEGS. IN ORDER TO GAIN A LITTLE MORE SEPARATION ON THE ACFT IN FRONT OF US; AND WITH THE SPD BACK TO 140 KTS; THE FLAPS WERE EXTENDED TO 40 DEGS. I HEARD OR FELT A 'THUMP' AT THIS POINT; AND THE ACFT STARTED A DUTCH ROLL TYPE WALLOW. IT FELT LIKE WAKE TURB; AND WHEN I MADE SOME COMMENT ABOUT IT; THE FO SAID HE THOUGHT I WAS IN HIS (THE OTHER ACFT'S) WAKE. I SAID I DIDN'T THINK SO SINCE I THOUGHT IT WAS A COMMUTER IN FRONT OF US AND THAT WOULD BE AN ACFT MUCH SMALLER THAN OUR LGT. HAVING ATTAINED THE DEGREE OF SEPARATION DESIRED; AND WITH THE PREVIOUS ACFT NO LONGER A FACTOR; I CALLED FOR THE FLAPS TO BE RETRACTED 30 DEGS FOR A MORE NORMAL LNDG. THE WALLOWING STOPPED; THE REMAINDER OF THE APCH AND LNDG WERE NORMAL. AS WE CLRED THE RWY; THE FLAP HANDLE WAS MOVED TO THE UP POS; THE SPD BRAKE HANDLE WAS MOVED FULL FORWARD. WE NOTED THE FLAPS WHERE EXTENDED 2 DEGS. I ACCOMPANIED THE SO ON HIS POST-FLT INSPECTION. WE SAW NO APPARENT DAMAGE ON THE L WING. INSPECTION OF THE R WING SHOWED TORN METAL HINGES; AND A SECTION OF THE 'FORE-FLAP' APPROX 6 TO 8 FT LONG WAS MISSING. SUBSEQUENT TO REPORT: A PORTION OF THE R FORE-FLAP; APPROX 8 FT LONG; WAS FOUND IN AN EMPTY LOT SOME 3 MI FROM THE END OF RWY 31.