AN E120 FLC EXPERIENCED A SPLIT FLAP CONDITION. EMER DECLARED. APCH ABANDONED.

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FLT BTWN CLE AND ROC; PF; VISUAL APCH; TO RWY 28 AT ROC. UPON ENTERING DOWNWIND AT 2100 FT MSL; ASKED FOR GEAR DOWN AND FLAPS 15 DEGS AT 170 KIAS. IMMEDIATELY BEGAN TO NOTICE ROLLING MOTION TO R. AT THE SAME TIME WE HAD A FLAP AURAL WARNING AND ANNUNCIATOR. CAPT INFORMED ME WE HAD A FLAP DISAGREEMENT AND I CALLED FOR QRH. ROLL TO R INCREASED; WHICH I CONTINUED TO COUNTER WITH L AILERON AND L RUDDER. I TOLD HIM IT WAS GETTING MUCH HARDER TO CTL AND HE INFORMED ME THAT WE NOW HAD AN ASYMMETRY INDICATION AND TO USE SPLIT PWR IF NECESSARY TO CTL ROLL. APCH WAS ABORTED AND EMER DECLARED. CAPT INFORMED ME L OUTBOARD FLAPS WENT TO 45 DEGS; R OUTBOARD TO 10 DEGS AND ALL OTHERS TO THE SELECTED 15 DEG POS. QRH FOR ASYMMETRY WAS USED; ENABLING US TO DRIVE R OUTBOARD FLAP DOWN; USING MANUAL OVERRIDE. THIS FLAP WOULD ONLY GO TO 25 DEGS. THIS INCREASED CONTROLLABILITY FOR LNDG. WE DETERMINED TO PROCEED TO ARPT FOR LNDG AND USED THE LONGER RWY 22. FLT ATTENDANT WAS NOTIFIED; PAX IN 'BRACE' POS. LANDED WITHOUT FURTHER INCIDENT. LATER MAINT CHANGED THE FLAP CTL UNIT AND A TEST FLT SHOWED OPS CHKED GOOD. THERE IS A DEFINITE DESIGN FLAW WITH THIS SYS. NO MECHANICAL INTERLINK EXISTS; THE SYS RELIES ON COMPUTER INPUT TO AVOID ASYMMETRY. IT'S JUST A MATTER OF TIME UNTIL THIS SYS CAUSES AN UNCONTROLLABLE AND UNRECOVERABLE ROLL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE E120 FLAP SYS IS CTLED BY A FLAP CTL UNIT. FLAP SELECTION IS ELECTRONIC -- THE FLAP COMPUTER SENDS A MESSAGE TO THE PRIMARY FLAP AND SUBSEQUENT FLAP 'STEPDOWN' IS ACCOMPLISHED THROUGH MESSAGE INTERACTION BTWN THE INBOARD; NACELLE; AND OUTBOARD SECTIONS. THE QRH DIRECTS SEPARATE; MANUAL OVERRIDE OP FOR AN ASYMMETRIC FLAP CONDITION. THE FAILED FLAP IS DRIVEN TOWARD THE NON FAILED FLAP BY A GUARDED TOGGLE SWITCH (ONE EACH -- FOR INBOARD; NACELLE AND OUTBOARD). WHEN THE CAPT SELECTED THE OUTBOARD MANUAL OVERRIDE IN THE UP POS; HOWEVER; THE 10 DEG FLAP MOVED TOWARD 5 DEGS; CAUSING A GREATER SPLIT AND CTL PROB. WHEN THE SWITCH WAS MOVED TO THE DOWN POS; THE SPLIT WAS REDUCED TO A 20 DEG DIFFERENCE; MAKING THE ACFT MORE MANAGEABLE. AN EMER WAS DECLARED; AND THE FAA MET THE FLT. SINCE THIS RPT WAS SUBMITTED; THE RPTR STATES THAT THE ACFT HAS BEEN IN MAINT FOR A FLAP CTL FAULT (THE COMPUTER WAS NOT GIVING CORRECT INFO; SO IT 'FROZE'); AND TRACK AND ROLLER CLEANING; AND A LOOSE CANNON PLUG. WHEN THE RPTR AND SAME CAPT FLEW THE SAME ACFT; ON THE SAME LEG; A FEW DAYS LATER; THEY EXPERIENCED A FLAP DISAGREEMENT AND THE ACFT WAS AGAIN RETURNED TO MAINT. THE RPTR DOES NOT KNOW THE OUTCOME OF THE LATEST MAINT. RPTR STATES THAT THIS IS NOT THE FIRST TIME THAT ANYONE HAS EXPERIENCED THIS PROB; IT ALSO OCCURRED AT ANOTHER AIRLINE.