Narrative:

Upon descending 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 landing at ZZZ1 (36 mi ahead) was not thought to be in jeopardy. After about 5 mins at 6000 ft, the indicated airspeed began to fluctuate. Pitot heat was verified as on and functional using the switch and ammeter, so an aerodynamic problem was assumed. Descent clearance to 5000 ft was obtained and the autoplt was set to a normal 500 FPM descent. As the aircraft began to pitch down, the indicated airspeed dropped to zero. Airstream noise verified that actual speed was above cruise and increasing. I assumed a tail plane stall and disengaged the autoplt. The aircraft suddenly pitched severely downward (beyond 30 degrees nose down) and airstream noise indicated a rapid speed buildup. Normal stall recovery procedures were not altering the negative pitch. With a descent 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 power 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 emergency landing. Based on WX of 1900 ft AGL overcast and my good knowledge of the terrain, I elected to descend below the MOCA and make a visual approach in order to land as soon as possible. During this, an emergency was declared. A safe landing was made at ZZZ.

Google
 

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.