A PA44 CFI AND STUDENT; ON AN IFR FLT PLAN; ENCOUNTERED ICING IN CLOUDS WHICH CAUSED THE STATIC PORT TO FREEZE WHICH RESULTED IN AN ALT OVERSHOOT.

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ON A TRAINING FLT; WE WERE ON AN IFR FLT PLAN FROM SAC TO RAL. WHILE FLYING OUR RTE; WE WERE GOING THROUGH SMALL AMOUNTS OF CLOUDS; BUT WERE VMC MOST OF THE TIME. WE DID PICK UP SOME TRACE ICE (MIXED ICE). THE ALTIMETER AND VSI STARTED TO JUMP AROUND. THE VSI JUMPED AND STUCK AT A 2000 FPM DSNDING; WHILE THE ALTIMETER STAYED AT 8900 FT. THEN; THE VSI STARTED COMING BACK TO ZERO; AND THE ALTIMETER JUMPED TO 8500 FT. WE DECIDED OUR STATIC PORT WAS PLUGGED WITH ICE. THE PITOT HEAT WAS ON AND DIDN'T STOP IT. AS WE WERE TRYING TO OPEN THE ALTERNATE AIR; APCH CALLED AND ASKED US OUR ALT. I TOLD HIM WE WERE INDICATING 8500 FT AND OUR STATIC PORT WAS FROZEN AND CORRECTING. HE TOLD US TO REMAIN AT 8500 FT OR ABOVE. I ALSO ASKED HIM WHAT ALT HE SHOWED FOR US AND HE SAID 8500 FT. AS SOON AS I OPENED THE ALTERNATE AIR; WE WERE AT 9000 FT. THE PITOT (AIRSPD INDICATOR) WORKED OK THE WHOLE TIME. THE PITOT HEAT WAS ON BEFORE ENTERING THE CLOUD. WHEN THE INSTS WERE ACTING UP; WE WERE VMC. I THINK MOISTURE WAS IN THE STATIC LINES AND FROZE. IT ALSO HAPPENED ON OUR FLT HOME TO SAC; BUT JUST AS THEY STARTED JUMPING AROUND; I OPENED THE ALTERNATE AIR. ON THE GND CHK; PITOT HEAT WORKED. I THINK WE WERE AT 9000 FT MSL THE WHOLE TIME.