AN MD80 CAPT EXPERIENCES AIRFRAME ICING DURING A LOW ALT ARR IN STL. THE HORIZ STABILIZER WAS AFFECTED ADVERSELY AND CAUSED AUTOPLT INPUTS THAT ROLLED THE STABILIZER TRIM UPWARDS PAST THE 11 PT 0 INDICES. A GAR WAS INITIATED TO CLB INTO WARMER AIR FOR DEICING PURPOSES.

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UPON DSCNT INTO ST LOUIS; WE NOTED THAT THE TEMP AT 10000 FT WAS +11 DEGS C. ST LOUIS WX WAS RPTED WITH CLOUD BASES AT APPROX 600 FT AND ZERO DEG SURFACE TEMP. STL APCH CTL CLRED US TO MAINTAIN THE HEADING OFF THE VANDALIA ARR TO INTERCEPT THE ILS RWY 24 FOR CONVERGING APCHS. UPON LOC INTERCEPTION WE ENTERED CLOUD TOPS AT APPROX 3800 FT. AIRFRAME ICE EQUIP HAD BEEN TURNED ON WELL BEFORE ENTERING THE CLOUDS WITH ANTICIPATION OF POSSIBLE ICING. AFTER PASSING THE FAF INBOUND; DUE TO A CONFLICT; WE WERE DIRECTED TO BREAK OFF THE APCH AND CLB TO 3000 FT WITH SUBSEQUENT VECTORING FOR A L DOWNWIND FOR AN ILS RWY 30. JUST PRIOR TO BASE VECTOR; I ADVISED APCH CTL THAT 3000 FT WOULD NOT BE A GOOD ALT FOR ANY PERIOD OF TIME AS AIRFRAME ICE ACCUMULATION APPEARED TO BE RAPID. ON BASE VECTOR I INITIATED THE TAIL DEICE CYCLE NOTING APPROX 3/4 INCHES OF ICE ACCUMULATION ON THE WINDSHIELD WIPER NUT. UTILIZING THE AUTOPLT WITH NORMAL LOC AND GS CAPTURES; AND LNDG CONFIGN WITH FLAPS 40 DEGS HAD BEEN SELECTED. UP TO THIS POINT EVERYTHING APPEARED NORMAL UNTIL WE RECEIVED 3 AURAL TRIM ANNUNCIATIONS AND NOTING TRIM INDICATOR PASSING 11.0 INDICES. I DISCONNECTED THE AUTOPLT WITH A VERY OUT OF TRIM ACFT REQUIRING VIGOROUS FORWARD YOKE PRESSURE AND A SUBSEQUENT SLUGGISH; ACFT PITCH-UP TO ABOUT 10 DEGS. ACFT HANDLING CHARACTERISTICS WERE THE SAME I WOULD EXPECT OF AN IMMINENT STALL; ALTHOUGH WE WERE STILL APPROX REF SPD PLUS 20-25 KTS. I INITIATED A GAR MANEUVER DIRECTING FLAPS 15 DEGS WHEREAS THE ACFT RAPIDLY REGAINED NORMAL FLYING CHARACTERISTICS. WE REQUESTED A CLB TO AN ALT WITH WARM AIR TO REMOVE ALL AIRFRAME ICE. IT SUBSEQUENTLY TOOK OVER 10 MINS AT +8 DEGS C IN CLR AIR TO CLEAN THE AIRFRAME OF ICE THAT WE COULD SEE. A SUBSEQUENT APCH AND LNDG WERE NORMAL. CONCLUSIONS AND QUESTIONS: I HAVE EXTENSIVE EXPERIENCE WITH DC9 EQUIP IN DIFFERENT AIRFRAME ICING SITS THAT HAVE ALWAYS APPEARED TO BE HANDLED ADEQUATELY. WITH THIS EXPERIENCE I'M NOT SURE THE MD80 IS AS CAPABLE AND MAY HAVE SOME SERIOUS ANOMALIES WHEN COMBINED WITH AUTOPLT USAGE. I AM CONCERNED THAT WITH WHAT APPEARS TO HAVE BEEN ICE CONTAMINATION OF THE TAIL AND CONTINUED AUTOPLT USE COULD HAVE PRESENTED AN UNRECOVERABLE SIT; HAD WE ALLOWED THE AIRSPD TO DECREASE CLOSE TO REF. IN ADDITION; CREWS THAT HAVE BEEN INVOLVED IN TRAUMATIC EXPERIENCES; SUCH AS THIS ONE; SHOULD EVALUATE THEIR ABILITY TO CONTINUE THEIR TRIP PAIRING. IN THIS CASE THE COCKPIT CREW MISSED A SUBSEQUENT CLRNC TO CROSS AT A SPECIFIC ALT ON THE NEXT LEG DUE TO THEIR PREOCCUPATION WITH THEIR EXPERIENCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT AFTER INITIATING THE GAR MANEUVER WITH 15 DEG FLAPS; THEY CLBED BACK UP TO WARMER AIR AND REMAINED THERE APPROX 10 MINS UNTIL ALL ICE HAD DISSIPATED FROM THE AIRFRAME. THE SECOND APCH WAS MADE WITHOUT ANY FURTHER DELAY OR ACCUMULATION OF ICE.