ONE FLAP MECHANISM ON A C402 BROKE FREE CAUSING A SEVERE ROLL MOTION AT 500 FT ON APCH. THE PLT LANDED SAFELY WITH FULL YOKE DEFLECTION.

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ON 4.5 MI; JUST OUTSIDE THE SHORELINE; AN IN-RANGE 'FLOW' WAS COMPLETED DURING WHICH THE FLAPS WERE SET AT 15 DEGS. AT THIS TIME THE ACFT WAS AT AN ALT OF 1500 FT MSL AND AT AN AIRSPD OF 160 KTS. THE ASSIGNED RWY WAS RWY 33. AT THE SHORELINE A BEFORE LNDG 'FLOW' WAS COMPLETED; DURING WHICH THE LNDG GEAR WAS LOWERED. AS ALWAYS AFTER COMPLETING THE 'FLOWS' CHKLISTS WERE COMPLETED. AT THIS POINT EVERYTHING APPEARED TO BE NORMAL. WHEN THE GEAR WAS EXTENDED THE ACFT BEGAN TO SLOW. AT 140 KTS; AND AN ALT OF 900 FT FLAPS WERE SET TO 45 DEGS. THE ACFT BEGAN TO SLOW TO THE APCH SPD OF 120 KTS. THE FLAPS LOWERED NORMALLY AND THE ACFT WAS ON A STABILIZED APCH AND I HAD BEEN CLRED TO LAND. AT 500 FT MSL A LOUD; DULL BANG CAME FROM THE R SIDE OF THE ACFT. IMMEDIATELY AFTER THE NOISE THE PLANE ENTERED AN ABRUPT R TURN. I IMMEDIATELY REGAINED CTL OF THE ACFT TO WINGS LEVEL. I KNEW THAT I DID NOT HAVE MUCH TIME TO TROUBLESHOOT THE PROB BECAUSE OF MY LOW ALT SO I OPTED TO CONTINUE TO APCH TO AN IMMEDIATE LNDG. I SCANNED THE ENG INSTS AND EVERYTHING WAS OPERATING NORMALLY. I THEN LOOKED AT THE WINGS FOR DAMAGE AND SAW NONE. (AT THIS POINT IT IS IMPORTANT TO NOTE THAT A CESSNA 414 IS EQUIPPED WITH SPLIT TYPE FLAPS THAT ARE NOT VISIBLE TO THE PLT IN FLT.) BY THIS TIME I WAS AT 200 FT AGL WITH NEARLY FULL L DEFLECTION OF THE YOKE AND ABOUT 1/2 R RUDDER TO MAINTAIN DIRECTIONAL CTL. I ALSO INCREASED PWR TO ASSIST IN MAINTAINING SPD. THE ACFT WAS IN A SAFE POS TO LAND ON RWY 33 AND I REDUCED PWR AND EXECUTED AN UNEVENTFUL LNDG. DUE TO THE LOW ALT OF THE EVENTS I DID NOT HAVE TIME TO ALERT THE CTL TWR OF THE PROB; OR DECLARE AN EMER AS I WAS BUSY FLYING THE PLANE. I TAXIED INTO THE TERMINAL AND OFF LOADED THE 3 PAX ON BOARD THE SCHEDULED 135 PAX FLT. AFTER UNLOADING THE PAX THE PRESIDENT OF THE COMPANY; WHO IS ALSO A LINE PLT WAS NOTIFIED BECAUSE HE WAS THE ONLY MEMBER OF MGMNT AVAILABLE IN PERSON. HE THEN CONTACTED THE DIRECTOR OF MAINT; TO WHOM I SPOKE WITH; FULFILLING THE REQUIREMENTS OF THE GOM (GENERAL OPERATING MANUAL) FOR CONTACTING COMPANY PERSONNEL. WHEN TAXIING IN; I LEFT THE AIRPLANE IN THE CONFIGN THAT IT WAS IN DURING THE LNDG SO THAT I COULD AID MYSELF AND MAINT IN DETERMINING WHAT CAUSED THE ROLL. THE INDICATION THAT FIRST LED ME TO BELIEVE THAT IT WAS A FLAP FAILURE WAS THAT THE FLAP INDICATOR WAS SHOWING ZERO DEGS AND THE FLAP HANDLE WAS STILL SET AT 45 DEGS. ON POSTFLT INSPECTION I DISCOVERED THE R FLAP MOVED UP FREELY FROM THE DOWN POS; WHILE THE L FLAP SHOWED NORMAL RESISTANCE. I BELIEVE THAT WITH MORE ALT I WOULD HAVE BEEN ABLE TO DIAGNOSE THE PROB INFLT; HOWEVER WHEN PROBS OCCUR AT LOW ALT A PLT MUST ALWAYS FLY THE PLANE FIRST; WHICH IS EXACTLY WHAT I DID. MAINT LATER TOLD ME THAT THEY FOUND THE R FLAP MECHANISM BROKE FREE ALLOWING THE R FLAP TO COME UP IN FLT WHILE THE L FLAP REMAINED DOWN; CAUSING THE ROLL AND THAT THE REASON THE FLAPS WERE SHOWING A ZERO DEG INDICATION IS BECAUSE THE INDICATION IS BASED OFF OF A READING FROM THE R FLAP. I AM GRATEFUL THAT THERE WAS A SAFE OUTCOME TO THE SITUATION; HOWEVER I ALSO BELIEVE THAT THERE WAS NOTHING I COULD HAVE DONE TO PREVENT IT.