MDT DEICED PRIOR TO DEP HAS RAPID ICE BUILDUP; ENG FIRE WARNINGS; POOR CTL. RETURN LAND.

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I WAS THE REPLACEMENT CAPT TO FLY FLT FROM FWA-ORD IN ACFT. FREEZING RAIN WAS FALLING AT THE TIME OF PAX BOARDING. PREVIOUS CAPT BRIEFED ME ON ICING; FREEZING LEVEL 3000 FT MSL; AND MINOR ENG INDICATION WITH #2 ENG TORQUE METER. I HAD THE ACFT DEICED BEFORE GATE DEP INCLUDING ALL WINGED SURFACES; PROPS; SPINNERS; AND NOSE SECTION. FUSELAGE WAS NOT DEICED. TAXIED TO RWY 5; 8 MINS FROM GATE DEP TO TKOF ROLL. ALL ENG ANTI-ICING AND DEICING WERE ON; INCLUDING CONTINUOUS IGNITION. TKOF CLRNC WAS RECEIVED; CLB TO 3000 FT RWY HDG. I INITIATED THE TKOF KEEPING AN EYE ON #2 ENG TORQUE; FO CALLED V1; VR; V2; I INITIATED PITCH ATTITUDE TO 6 DEGS ON ADI; ACFT FELT SLOW AND SLUGGISH EVEN THOUGH AIRSPD INDICATED V2 PLUS 10 AND INCREASING. I REMARKED TO FO ACFT FELT SLOW AND I DELAYED FLAP RETRACTION A FEW HUNDRED FT (USUALLY 400 FT). TWR TURNED US TO 360 DEGS AND CONTACT DEP. DURING L TURN IT WAS VERY DIFFICULT TO ROLL ACFT L; I IMMEDIATELY LOOKED AT SCS (STANDBY CTL SYS) FOR WARNING LIGHTS. AILERONS WERE VERY HVY TO THE L; NORMAL TO THE R. I HAD FO FLY THE ACFT TO SEE IF HE COULD CONFIRM MY CTL FEEL; WHICH HE DID. I TOOK CTL BACK AND ROLLED OUT OF TURN ON 360 DEG HDG. (FO WAS WITH PREVIOUS CAPT AND FLEW ACFT FROM ORD-FWA.) ACFT HAD JUST COMPLETED MAJOR WING DAMAGE REPAIR TO R WING; PREVIOUS DAY.) ON ROLLOUT; FIRE WARNING SYS FOR L ENG ILLUMINATED. INITIATED MEMORY CHKLIST ITEMS; INCLUDING PWR REDUCTION ON L ENG. I HESITATED INITIALLY ON FEATHERING ENG BECAUSE THE ICING CONDITIONS. I HAD FO DECLARE AN EMER AND RECEIVED HDG TO DOWNWIND LEG FOR RWY 5. I ELECTED TO FEATHER ENG AND FIRE BOTH FIRE BOTTLES; FIRE LIGHTS AND WARNINGS REMAINED ILLUMINATED. I TOLD FLT ATTENDANTS WE WERE RETURNING TO FWA WITH AN END FIRE WARNING; MADE SHORT AND BRIEF SPEECH TO PAX; FO COMPLETED CHKLISTS FOR LNDG. ACFT WAS ACCUMULATING FREEZING RAIN ICE AT A VERY RAPID RATE. CTL WAS HVY AND SLUGGISH DURING CIRCUIT. WENT FROM MAX CONTINUOUS PWR TO MAX CONTINGENCY ON THE BASE LEG WITH GEAR UP AND FLAPS AT APCH. I MADE A SLOW TURN TO FINAL; TOUCHDOWN WAS NORMAL; TORQUE WAS NEAR MAX AT TOUCHDOWN (APPROX 99 PERCENT). I LET ACFT DECELERATE; WITHOUT REVERSE AND LIGHT BRAKING (RWY CONDITION). CFR CONFIRMED NO FIRE. WE TAXIED BACK TO GATE; AND DEPLANED PAX. DURING POST-FLT; ACFT WAS 1 LARGE ICE CUBE; I HAVE NEVER HAD THAT MUCH ICE ON AN ACFT IN ALL MY FLYING CAREER. ACFT HAD 60 PAX; 4 CREW; 5000 POUNDS OF FUEL; UNKNOWN BAGS; AND THIS WAS NEAR OUR MAX GROSS OF 50550 POUNDS. THE ACFT HAS HISTORY OF FALSE FIRE WARNINGS; ESPECIALLY AFTER DEICING PROCS. MECHS FOUND CHAFFED FIRE WIRES. THIS CAPT WILL NO LONGER OBSERVE DEICING FROM COCKPIT AND WILL PERFORM A FULL AND OBSERVED CTL CHK OF ALL CTLS. WE SPECULATE THAT 1 OR BOTH AILERON SPRING TABS FROZE; DEICING FLUID MAY HAVE BEEN FORCED INTO PITOT SYS OR STATIC SYS WAS FROZEN OVER. I DIDN'T NOTICE STANDBY INST READINGS AT THE TIME. THEIR STATIC SYS IS HEATED; ONCE AIRBORNE; BOTH AIR DATA COMPUTERS FOR PRIMARY EFIS DISPLAYS ARE NOT HEATED. IN THEORY PLACEMENT OF STATIC PORTS ON FUSELAGE PREVENTS ICE PROBLEMS. I'M NOT SURE IF THIS IS TRUE DURING FREEZING RAIN. DEICING FLUID MAY HAVE PLACED A MINOR ROLL; FLUID WAS NOT AT NORMAL TEMP (160-170 DEGS) FOR DEICING. FREEZING RAIN CAME EARLY; OPS WAS NOT READY. MODERATE TO SEVERE ICE WAS RPTED BY SEVERAL ACFT IN AREA AFTER OUR INCIDENT.