Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Aircraft; Procedure; Weather
Right downwind ORD 27R descending in heavy icing conditions. Aircraft encountered heavy ice in the descent between 8000 and 4000 ft. Engine and Wing anti-ice had been selected 'on' before entering icing conditions appoximately 5 minutes prior to event. Tail deice had not been activated at this point. Aircraft was level at 4000 ft and 250 kts.ATC issued us to slow to 210 kts and a 90 degree right turn to heading 180. The combination of flaps being extended; a 25 degree bank turn and ice on tail prevented the slow speed trim from keeping up with nose down tendencies. Aircraft started to sink rapidly in the turn.  Upon realization that the autopilot was not satisfactorily flying the airplane; the autopilot was disconnected as well as autothrottles. The aircraft was in a extreme out of trim nose down situation.Corrective inputs were applied and required aggressive correction to stop the sink. By the time the aircraft was recovered and climbing; we had lost 400 ft. The tail deice was applied and the autopilot was engaged for a normal ILS approach. In heavy icing conditions; the tail deice needs to be run more often.
[Report narrative contained no additional information].
MD-83 flight crew reported encountering tail ice on approach to ORD resulting in a 400 FT altitude deviation.