A DC9-50 ON APCH IN MODERATE TO SEVERE ICING CONDITIONS WITH AIRFOIL DEICING SWITCHED ON THE ACFT LOST 500 FT OF ALT WITH FLAP EXTENSION. RECEIVED GPWS WARNING AND EXECUTED A GAR.

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UPON DSCNT INTO MSN WE ENTERED THE CLOUDS AT 3700 FT MSL AND STARTED TO PICK UP A MODERATE COAT OF ICE. OUR ENG AND WING ICE PROTECTION SYS WERE BOTH ON AND FUNCTIONING NORMALLY. AT 2700 FT WE INTERCEPTED THE GS FOR RWY 36. FOLLOWING COMPANY PROCS WE EXTENDED THE LNDG GEAR AND STARTED TO DEICE THE TAIL. NEXT; WE EXTENDED THE FLAPS TO 25 DEGS AND I NOTICED A SLIGHT PITCH DOWN MOVEMENT; NOTHING EXCESSIVE. JUST PRIOR TO XING MONAH; THE OM; WE EXTENDED THE FLAPS TO 40 DEGS AND IMMEDIATELY LOST APPROX 500 FT OF ALT SHORTLY FOLLOWED BY THE GND PROX SOUNDING 'WHOOP WHOOP GS PULL UP.' AS DIRECTED BY COMPANY PROCS WE EXECUTED A MISSED APCH AND A GAR. DURING OUR RETURN FOR LNDG WE PICKED UP ADDITIONAL ICING; WHICH I RPTED AS MODERATE TO SEVERE. WE DEICED THE TAIL A SECOND TIME AND THE ACFT STABILIZED ON THE SECOND APCH AND WE MADE AN UNEVENTFUL LNDG. THE DISCONCERTING THING OF ALL THIS WAS THAT WE FOLLOWED COMPANY PROCS TO THE LETTER; AND STILL THE ICE WAS BUILDING UP TOO FAST AND COULD NOT BE REMOVED EFFECTIVELY. ALSO; THIS INCIDENT OCCURRED LATE IN THE EVENING AFTER A FULL DAY IN IFR CONDITIONS WITH A BRAND NEW FO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE AIRFOIL ANTI-ICE SYS WAS SWITCHED ON AND OPERATIVE AND WAS WORKING NORMALLY AND ALL COMPANY PROCS WERE FOLLOWED; BUT THEY ENCOUNTERED MODERATE TO SEVERE ICING. THE RPTR SAID THE ICE WAS BUILDING UP TOO FAST TO BE REMOVED EFFECTIVELY.