FROZEN STATIC SOURCE CAUSES LIGHT TWIN TO CLB ABOVE ASSIGNED ALT AND THEN DSND BELOW IT.

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FILED FL220 FROM MLB TO LEX. GIVEN FL210 AS FINAL. STARTED PICKING UP LIGHT RIME ICE. REQUESTED AND GOT FL250 WHICH TEMPORARILY PUT ME ABOVE CLOUDS (LONG ENOUGH FOR ICE TO SUBLIMATE). SOON BACK INTO CLOUDS AND AGAIN PICKING UP ICE. ADVISED CTR OF DEVELOPMENT AND THAT I WOULD NEED LOWER IF WORSENED. CONTINUOUS MILD TO MODERATE TURB WHICH AT TIMES WAS SEVERE. (ALL AIRLINES WERE COMPLAINING TO ZTL ABOUT TURB FROM FL200 TO FL370 AND PERHAPS HIGHER). CLRED DIRECT LEX AND SHORTLY THEREAFTER TURB INCREASED REQUIRING BOTH HANDS ON YOKE TO CTL ACFT (HAND-FLY ONLY; AUTOPLT WORSE). PITOT HEAT ON FROM BEGINNING. OUTSIDE AIR TEMP MINUS 25 DEGS C. PICKED UP CONSIDERABLE INTERMITTENT INDUCTION ICE; (EVEN WITH ALTERNATE AIR ON BOTH ENGS) WITH SURGING OF ENGS. SUDDENLY; ON SCAN; IAS AT 240 KTS INCREASING (WAS AT 155- 160 KTS IN CRUISE). ALMOST SIMULTANEOUSLY ALTIMETER WAS SEEN TO BE RAPIDLY DSNDING AND WAS AT 24300 FT. I PULLED UP ELEVATOR (BACK); AS MY INITIAL THOUGHT WAS I WAS IN A DIVE. ATTITUDE INDICATOR INDICATED SAME INSTRUCTION (FLT DIRECTOR). IAS NEEDLE WENT TO AND BEYOND RED LINE (244 KTS). AT THIS TIME; I WENT TO ALTERNATE STATIC SOURCE. ALTIMETER THEN INDICATED I WAS AT 25700 FT. ASI REMAINED THE SAME. I IMMEDIATELY DSNDED TO FL250. ZTL ASKED WHAT MY ALT WAS AND I TOLD THEM 25000 FT; AND ALSO ABOUT THE PROBS I HAVE DESCRIBED AND THAT I THOUGHT EVERYTHING WAS UNDER CTL. THEY SAID NO PROBS. PRESSURIZATION GAUGES INDICATED CORRECTLY. NEVERTHELESS; I WENT ON OXYGEN AS A PRECAUTIONARY MEASURE AND ASKED FOR AND WAS GIVEN A DSCNT IN STAGES TO 11000 FT. IAS DID NOT RETURN UNTIL PASSING THROUGH 17000 FT; WHERE OUTSIDE TEMP WAS MINUS 20 DEGS C (MUST BE CONSIDERABLE MOISTURE IN BOTH STATIC SYS; WHICH I AM GOING TO HAVE INVESTIGATED). REMAINDER OF FLT WAS ESSENTIALLY NORMAL. I HEARD LATER THAT ATL CLOSED BECAUSE OF BLOWING SNOW.