PA28 PLT MAKES EMER LNDG AFTER RECOVERY FROM ACFT LOSS OF CTL DUE TO ICING.

Narrative

UPON DSNDING FROM 7000 FT TO 6000 FT; A SLOW BUILDUP OF MIXED ICE OCCURRED ON THE THERMOMETER PROBE. ICE WAS NOT VISIBLE ON WING OR TAIL; BUT WAS ASSUMED TO BE PRESENT. BASED ON PRIOR EXPERIENCE IN TYPE; ACCRETION RATE WAS SLOW ENOUGH THAT A NORMAL LNDG AT ZZZ1 (36 MI AHEAD) WAS NOT THOUGHT TO BE IN JEOPARDY. AFTER ABOUT 5 MINS AT 6000 FT; THE INDICATED AIRSPD BEGAN TO FLUCTUATE. PITOT HEAT WAS VERIFIED AS ON AND FUNCTIONAL USING THE SWITCH AND AMMETER; SO AN AERODYNAMIC PROB WAS ASSUMED. DSCNT CLRNC TO 5000 FT WAS OBTAINED AND THE AUTOPLT WAS SET TO A NORMAL 500 FPM DSCNT. AS THE ACFT BEGAN TO PITCH DOWN; THE INDICATED AIRSPD DROPPED TO ZERO. AIRSTREAM NOISE VERIFIED THAT ACTUAL SPD WAS ABOVE CRUISE AND INCREASING. I ASSUMED A TAIL PLANE STALL AND DISENGAGED THE AUTOPLT. THE ACFT SUDDENLY PITCHED SEVERELY DOWNWARD (BEYOND 30 DEGS NOSE DOWN) AND AIRSTREAM NOISE INDICATED A RAPID SPD BUILDUP. NORMAL STALL RECOVERY PROCS WERE NOT ALTERING THE NEGATIVE PITCH. WITH A DSCNT RATE OF GREATER THAN 2000 FPM (THE VSI LIMIT); AND A CRASH IMMINENT; I FELT I HAD TO IGNORE THE DANGERS OF AIRFRAME G-FORCE OVERLOAD AND A POSSIBLE SECONDARY STALL. I THROTTLED TO FULL PWR AND ABRUPTLY PULLED THE WHEEL TO ITS FULL REAR STOP. THE ATTITUDE INDICATOR SLOWLY RETURNED TO THE HORIZON AFTER SEVERAL SECONDS. I WAS THEN ABLE TO HOLD A LEVEL ATTITUDE; BUT WITH GREAT DIFFICULTY. I TURNED TOWARD ZZZ; JUST 5 MI AWAY; FOR AN EMER LNDG. BASED ON WX OF 1900 FT AGL OVCST AND MY GOOD KNOWLEDGE OF THE TERRAIN; I ELECTED TO DSND BELOW THE MOCA AND MAKE A VISUAL APCH IN ORDER TO LAND ASAP. DURING THIS; AN EMER WAS DECLARED. A SAFE LNDG WAS MADE AT ZZZ.