AN ACR ATR42 FLYING AN INST APCH INTO STL; HAS A SUDDEN LOSS OF CTL DUE TO ICING. FLC RECOVERS ACFT FROM LOSS OF ALT AND LANDS SAFELY.

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ON THE ILS RWY 12R APCH TO STL; FLT ENCOUNTERED UNEXPECTED SEVERE ICING WHICH SUBSEQUENTLY CAUSED THE ACFT TO DEPART CTLED FLT. THE STALL/DEP INDICATIONS WERE CONSISTENT WITH A 'TAILPLANE STALL.' THIS RESULTED IN AN UNCTLED DSCNT BELOW THE APCH GS WHILE THE CREW WORKED DILIGENTLY TO RECOVER THE ACFT. THE CREW RECOVERED CTLED LEVEL FLT AT 1600 FT MSL AND MAINTAINED LEVEL FLT (ABOVE THE LOC MDA OF 960 FT MSL) TO A GS INTERCEPT. PRIOR TO GS INTERCEPT; THE CREW PERFORMED A NOMINAL CTL CHK AND FLEW A NORMAL LNDG AT VFE 30; 150 KIAS. DEP/RECOVERY: AS THE FLAPS EXTENDED TO 30 DEGS THE ACFT DISPLAYED A DRAMATICALLY INCREASING CRESCENDO OF HIGH FREQ CTL FLUTTER AND LOW FREQ AIRFRAME BUFFETING. AS THE FLAPS REACHED 30 DES; THE ACFT DECELERATED ABRUPTLY AND DEPARTED CTLED FLT. AT DEP; THE ACFT ROLLED L WING DOWN AND PITCHED DOWN 4 DEGS. (THE ESTIMATED 15 DEGS PER SECOND ROLL WAS UNCOMMANDED -- OPPOSITE THE 30-40 DEGS R CTL WHEEL INPUT. THIS ROLL WAS ARRESTED AT 10-12 DEGS L WING DOWN BY POSITIVE FORWARD STICK INPUT.) THE CREW RECOVERED THE ACFT BY APPLYING BASIC AIRMANSHIP: SIMULTANEOUSLY RESELECTING FLAPS 15 DEGS (CLB FLAPS); INCREASING PWR (+/-90% Q); AND APPLYING POSITIVE FORWARD STICK (+/-5 DEGS DOWN). CONCERNED THAT THE ROLL MAY BE TORQUE RELATED; THE CREW APPLIED MAX PWR (100% Q X 100% NP) AT ABOUT 165-170 KIAS. THE CAPT MAINTAINED R ROLL CTL WHEEL INPUT FROM THE ONSET UNTIL THE WINGS RESPONDED TO THE CTL WHEEL (APPROX 175 KIAS). AT 175-180 KIAS THE FO CHKED LEADING EDGES (GOOD AND IMPROVING) AND THE WINGS MATCHED THE WHEEL SO THE CAPT LEVELED THE ACFT VERY GRADUALLY (APPROX +1 DEG PER SECOND) FROM 1700-1600 FT MSL. INITIALLY THE ACFT REQUIRED 100% Q FOR LEVEL FLT AT 180 KIAS BUT AS THE ICE PROTECTION SYS FUNCTIONED; THE CAPT WAS ABLE TO RETARD PWR TO MAINTAIN 180 KIAS. THROUGHOUT THE RECOVER; THE CAPT'S CTL INPUTS WERE SMOOTH; STEADY AND STABLE WHILE THE ACFT RESPONSE WAS SLOW AND VERY SLUGGISH; ESPECIALLY ROLL AXIS; WITH THE PITCH SLOWLY OSCILLATING UNTIL LEVEL FLT WAS ACHIEVED. THE ENCOUNTER WITH SEVERE ICING WAS OF AN EXTREMELY BRIEF DURATION (ESTIMATED LESS THAN 1 SECOND) AND THE CREW HAD NO PRIOR AIRFRAME INDICATIONS (SIGNIFICANT SIDE WINDSHIELD SPATTERING OR PROP SPINNER ACCUMULATIONS GROWING AFT) NOR OTHER WARNINGS (ATIS; RADAR OR PIREP) THAT SEVERE ICING COULD BE EXPECTED OR MAY BE ENCOUNTERED. THE ACFT HAD BEEN CONTINUOUSLY OPERATED WITH ALL ANTI-ICE/DEICE SYS ON (LEVEL 3) DURING FLT IN ICING CONDITIONS. THE CREW HAD EXPERIENCED NO PROBS WITH ICING PRIOR TO THE UNEXPECTED ENCOUNTER WITH SEVERE ICING. THE FLT HAD CONDUCTED 1 PRIOR ILS RWY 12R APCH TO A MISSED APCH (FOR SEQUENCE). DURING THIS PREVIOUS APCH; THE CREW BRIEFLY OBSERVED MODERATE ICING AT 3000 FT MSL. ON COMMENCING THE SECOND APCH; THE ACFT HAD BEEN IN ICING CONDITIONS FOR APPROX 15-25 MINS AND HAD ACCUMULATED A TOTAL OF 1 INCH OF MIXED RIME. THE WINGS WERE CLR AND CLEAN PRIOR TO THE APCH. THE CREW WOULD LIKE TO CREDIT THE TRAINING DEPT FOR FORGING COCKPIT TEAMWORK DURING THE ANNUAL CHKRIDES; AND FOR INCLUDING PRACTICE STALLS DURING THE SVT. IN PARTICULAR; THANKS TO TRAINING FOR SYSTEMATICALLY REVIEWING WINTER OPS PROCS (INCLUDING TAILPLANE STALL INCIDENTS AND RECOVERY PROCS); DURING OUR ANNUAL ORAL EXAMS. THEIR VIGILANCE AND GOOD COACHING KEPT ATR; J32; AND J41 PLTS SHOP-TALKING ABOUT AND THUS AWARE OF ICING UPSETS AND RECOVERY. THE CREW WOULD ALSO THANK STL ATC WHO PASSED THE PIREPS; OBTAINED THE INFLT WX CONDITIONS AND GAVE US AN ALT ABOVE ICING CONDITIONS ON VECTORS TO OUR SECOND APCH. ON ANY GIVEN DAY; THEIR VIGILANCE AND ABSOLUTE PROFESSIONALISM IS AN AWESOME VIRTUE. ON THIS DAY; OUR PROBS WERE SUDDEN AND UNEXPECTED. THE CREW DIDN'T HAVE TIME TO DECLARE AN EMER OR TO COMMUNICATE MUCH ABOUT THE NATURE OF OUR CONDITION UNTIL THE ACFT WAS SAFE ON DECK. JUST TOO BUSY; WE COULDN'T COMMUNICATE MUCH. BUT STL ATC WAS PULLING ON THE SAME SET OF OARS WITH US; AND THAT WAS VERY GOOD HELP TO HAVE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SPOKEOF A FLUTTER IN THE CTLS AND A BUFFETING IN THE AIRFRAME AS THE FLAPS WERE EXTENDED TO 30 DEGS. AT THAT TIME THERE WAS A HIGH TWISTING ACTION. AS THE ACFT ROLLED L; THE NOSE WENT DOWN. IT TOOK FULL R AILERON INPUT TO STOP THE ROLL ALONG WITH NOSE DOWN INPUT ON THE STICK. 500-600 FT OF ALT WERE LOST AT THE TIME. AS SOON AS THE FLAPS WERE MOVED TO 15 DEGS THE CTL FLUTTER DECREASED AND PITCH CTL RETURNED TO NORMAL WITH ADDITION OF PWR. THE SIDE WINDOW HAD SUDDENLY GONE OPAQUE FROM ICE ACCUMULATION. ENOUGH VISIBILITY TO THE SIDE ALLOWED THE FLC TO VIEW THE WINGS; DETERMINING THAT THEY WERE CLR OF ICE. EACH PLT CAN SEE 2/3 OF THE WING. THE FLUTTER IS A RESULT OF ICING ON THE ELEVATOR. AS FLAPS GO TO 15 DEGS; THE FLUTTER STOPS. THIS WAS REINFORCED TO THE FLC DURING THEIR UPSET TRAINING BY THE ACR. THE FLC MISSED THE PRIOR APCH AND FLEW FOR 15-20 MINS IN IMC ICING CONDITIONS BEFORE BEGINNING THE SECOND APCH. CONDITIONS WERE RIGHT FOR PICKING UP ELEVATOR ICING. THE FLC SPOKE WITH THE NTSB FOR 3 HRS REGARDING THIS INCIDENT.