FLC OF ACR MLG EXPERIENCED A POSSIBLE FALSE GPWS AND AN OUT-OF-TRIM EMER ON CLB. NEITHER ELECTRIC TRIM OR MANUAL INPUT ON CTLS WOULD AUTO-DISCONNECT THE AUTOPLT.

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APPROX 20 NM N OF KSWF; BEING VECTORED (HDG 180) FOR DSCNT (4000 FOR 3500 MSL) FOR APCH RWY 27 KSWF. CONDITIONS OF FLT WERE NIGHT; IMC; AND ICING CONDITIONS. WE WERE LEVELING OFF AT 3500 MSL AND WERE BEING GIVEN A HDG CHANGE TO 200 DEG FOR A DIFFERENT APCH (ILS 9; CIRCLE TO LAND RWY 27) WHEN GPWS AURAL WARNING ANNOUNCED A REPEATED/CONTINUOUS 'TERRAIN; TERRAIN....' SINCE WE WERE IMC AND COULD NEITHER CONFIRM NOR DISCOUNT THE VALIDITY OF THE WARNING; A PULL-UP MANEUVER WAS INITIATED BY PF (FO) IMMEDIATELY; WHILE PNF (CAPT) ADVISED ATC OF OUR ALTDEV AND OUR INTENTIONS. AFTER AN INITIAL NOSE UP PITCH AND A MOMENTARY POSITIVE CLB RATE ON VERT SPD INDICATOR; I NOTICED THE CLB RATE WENT FROM A POSITIVE RATE TO 0 AND THEN TO A DSCNT. BELIEVING WE HAD ENCOUNTERED A DOWNWARD SHEAR; I BEGAN PULLING BACK ON THE CTL WHEEL MORE AND MORE UNTIL I WAS PULLING WITH ALL MY MIGHT AND WAS STRUGGLING TO MAINTAIN ALT. I GLANCED AT THE HORIZ STABILIZER TRIM POS INDICATOR AND NOTICED IT WAS FULL NOSE DOWN. THINKING I HAD A RUNAWAY TRIM AND STRUGGLING TO HOLD ALT; I CONTINUED TO TRIM NOSE UP AND PULL BACK AS HARD AS I COULD. AT THE TIME THIS WAS OCCURRING; THE CAPT NOTED THE LACK OF CLB AND AGAIN COMMANDED A PULL-UP. AS HE SAID THIS; HE SAW THE DIFFICULTY AND STRAIN I WAS ENCOUNTERING. I EXPLAINED TO HIM MY DIFFICULTY IN MAINTAINING ALT AND THE POS OF THE HORIZ STABILIZER TRIM (FULL NOSE DOWN). HE THEN PUSHED THE AUTOPLT DISCONNECT TO CONFIRM DISENGAGEMENT; AND BEGAN ASSISTING IN THE PULL-UP. HE THEN DECLARED AN EMER. BTWN THE TWO OF US ;WE WERE ABLE TO CLB TO 6000 MSL WHICH GOT US ABOVE THE CLOUDS AND OUT OF ICING CONDITIONS. THIS TOOK APPROX 10 TO 15 MINS. AS WE BEGAN OUR LEVEL OFF AT 6000; THE CTL FORCES BEGAN TO SLOWLY OSCILLATE FROM A NEAR NORMAL FEELING TO NOSE DOWN. EVENTUALLY CTL FORCES RETURNED TO NORMAL. WE REVIEWED THE PROBLEM AND COULD NOT DETERMINE THE CAUSE AND REVIEWED OUR COURSE OF ACTION SHOULD THE PROBLEM RECUR. THE APCH AND LNDG EXECUTED WITHOUT FURTHER INCIDENT. IMMEDIATELY AFTER THE INCIDENT WE CONTACTED OUR MAINT DEPT WHO SUSPECTED THAT THE ICING CONDITION CAUSED THE PROBLEM SINCE AN EXTREMELY SIMILAR INCIDENT HAD PREVIOUSLY OCCURRED ON 1 OF OUR OTHER MLG'S WHILE IN ICING. AFTER REPEATED DISCUSSIONS AND REVIEW OF THE PLT OPERATING HANDBOOK (POH)/ACFT MANUAL; WE; THE CREW; BELIEVE THE FOLLOWING MAY HAVE OCCURRED: IT IS POSSIBLE THE FO INITIATED THE PULL-UP WHILE THE AUTOPLT WAS STILL ENGAGED. THAT; IN TURN; CAUSED THE TRIM TO RUN FULL NOSE DOWN. FURTHERMORE; IT IS POSSIBLE THAT WHEN THE CAPT PUSHED THE AUTOPLT DISCONNECT TO CONFIRM DISENGAGEMENT; THAT WAS THE FIRST DISENGAGEMENT OF THE AUTOPLT. THAT WOULD EXPLAIN WHY THE FO ENCOUNTERED SUCH STIFF RESISTANCE OF THE FLT CTLS AND WHY HE WOULD NOT SEE ANY RESULTS FROM PULLING BACK AND TRIMMING NOSE UP WAS INEFFECTIVE SINCE THE CTL WHEEL TRIM SWITCH IS BYPASSED DURING AUTOFLT (SINCE THE FO BELIEVED HE WAS HAND FLYING). HOWEVER; ONCE WE KNOW THE AUTOPLT WAS DISENGAGED; WE ARE UNABLE TO SPECULATE WHY REPEATED ATTEMPTS TO MOVE THE STABILIZER TRIM WITH THE TRIM SWITCHES AND THE TRIM WHEEL WERE INEFFECTIVE. IN RETROSPECT; NEITHER PLT REMEMBERS SEEING A RADIO ALT READING; AND WE SUSPECT THE GPWS WARNING MAY HAVE BEEN FALSE. THE TERRAIN WARNING OBVIOUSLY BEGAN THE CHAIN OF EVENTS. IT IS MY UNDERSTANDING THAT OUR COMPANY IS HAVING NUMEROUS RPTS OF FALSE/NUISANCE GPWS WARNINGS ON THE MLG. WHILE I AM NOT TRYING TO LAY BLAME ELSEWHERE; I FIND IT INCREDIBLY HARD TO BELIEVE THAT THE AUTOPLT WOULD CONTINUE TO TRIM THE HORIZ STABILIZER OPPOSITE PLT TO A FULL NOSE DOWN POS AND WITH SUCH GREAT FORCE BEING APPLIED TO THE CTL COLUMN. IN OTHER ACFT I HAVE FLOWN; SUCH ACTION EITHER: 1. DISCONNECTED THE AUTOPLT OR 2. STOPPED MOVEMENT OF THE TRIM. WHILE I BELIEVE THIS TO BE A FLAWED DESIGN; I SUPPOSE THE ACFT MANUFACTURER WOULD CALL IT AN 'INSUFFICIENT UNDERSTANDING OF THE ACFT'S SYS.' NONETHELESS; IF THIS IS THE RESULT OF COUNTERING THE AUTOPLT; I BELIEVE GREATER EMPHASIS SHOULD BE PLACED ON THIS RATHER THANTHE CASUAL STATEMENT IN THE POH WHICH STATES 'IF AN ATTEMPT TO HAND FLY THE ACFT IS MADE WHILE THE AUTOPLT IS ENGAGED; THE TRIM WHEEL WILL MOVE OPPOSITE TO ANY PITCH COMMANDS OF THE PLT.' AGAIN IN REGARDS TO ACFT SYS; THIS IS THE ONLY ACFT I HAVE FLOWN IN WHICH THE AUTOPLT DOES NOT DISCONNECT WHEN THE CTL WHEEL TRIM SWITCHES ARE USED. I BELIEVE THE FAA SHOULD REQUIRE GPWS/TERRAIN AVOIDANCE TRAINING TO BE MANDATORY IN THE SIMULATOR WITH ESTABLISHED GUIDELINES TO BE FOLLOWED MUCH LIKE THEY HAVE ON WINDSHEAR RECOVERY TECHNIQUES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHEN THE CAPT MANUALLY DISCONNECTED THE AUTOPLT; THE TRIM WOULD NOT BUDGE OUT OF THE FULL NOSE-DOWN SETTING; THROUGH APPLICATION OF EITHER ELECTRIC TRIM; OR BY (THE CAPT) ATTEMPTING TO USE THE MANUAL TRIM WHEEL. THE RPTR FURTHER INDICATED THAT IN A SUBSEQUENT DISCUSSION WITH A PLTS UNION SAFETY COMMITTEE REPRESENTATIVE; A SIMILAR STUCK NOSE-DOWN TRIM PROBLEM HAD OCCURRED ON ANOTHER COMPANY ACFT.